tuning a let

[pdc78502]

LET vs XE cams

The stock LET cams are a milder version of the XE items. They have shorter duration and lower valve lift (8.5mm instead of the XE's 9.5) meaning that the gas volume that can leak during overlap is even lower.


LET 'race' cams

At some point I came across a 'competition spec' LET cam setup. Here's how it was:

IN:17/70 (267 degree angle) with 10mm lift

EX:52/16 (248 degree angle) with 9mm lift

Overlap is 33degrees. Less than stock. Hmm....let's see what they did:
The inlet has slightly more duration, and a lot more lift (1.5mm extra)
The exhaust has 20 degrees duration LESS, and is advanced by 10degrees
It's not the exact figures that are important here, but the general direction. After all, in the race version the cylinder head would be gasflowed and the turbo trim would also be different.

Basically they tried to enhance the intake without affecting the angle - and they advanced the exhaust, shrinking it's angle at the same time (20degrees less!). Technically they increased the lobe separation. To keep the exhaust flow unchanged, they gave it a touch more lift. These people really tried to keep overlap to a minimum.

It means that even in a competition environment (a lot more full throttle than road-spec) reversion is an issue at high revs. Fuel consumption is not their priority, max power is - and it's reversion that robs more power, not boost leaking straight to the exhaust. Let's keep this in mind when trying to optimise the cam timing for a high-powered LET!



LET Cam Playground Basics

We always refer to crank degrees. You shouldn't be reading this page if you don't know this, but for every two crank revolutions, the cams turn only once. Therefore, a full four-stroke cycle has 720 crank degrees. If our cam pulley has 46 teeth, each one corresponds to 720/46 = 15.6 crank degrees.

Vernier wheels are a useful tool for fine-tuning if one is prepared to experiment a little and optimise the timing. The alternative is to move the pulleys a full tooth (15.6 degrees). That can be a bit drastic though, as we'll probably need them shifted just by a few degrees.

Below is a stock pulley versus a 'vernier' equivalent. Strictly speaking it is simply 'adjustable'. No big deal in real life, as the finest discrete steps are no smaller than 2 degrees anyway.



Beware, early and late LETs have pulleys with different pattern teeth: early ones are rounded at the base of the tooth, late ones (low noise) are square (as in the pics). The diameter of the later ones is also different, and the number of teeth is 46, compared to 42 of the early ones. You cannot mix'n'match!
They are the same as XE pulleys of similar age, totally interchangeable. Be careful not to get XEV ones from the Ecotec.

The pulley shown above has been adjusted for 7degrees retarded timing. The fact that 5 bolts hold the two pieces together is a good sign that enough clamping force will be applied (we don't want these bolts to get loose under operating conditions, so some threadlock wouldn't go amiss)

In any case the engine will have to be manually turned fully (2 crank revolutions, or 720 degrees) to make sure that nothing touches. Always turn the LET engine clockwise by the way - never backwards.

Even so, it could be that the pistons are still too close for comfort, and at 7000rpm they MIGHT still touch (expansion, inertia, what have you). Remember that this is an interference engine (i.e. try to open fully all valves with the piston at TDC and you'll hear something!). So if I want the engine to run with the inlet cam advanced by one tooth, I'd like to test-crank it (gently!) with a 2-tooth advance. If that goes well, then I'd say it's safe to run it with one tooth advance. Similarly, if it's half a tooth I want it retarded, I'd crank it with one tooth retard and see if it hurts.
That covers interference then.



LET Cam Timing Optimisation

For starters I would keep the LET exhaust cam in place. My experience has shown that turbos respond mainly to the inlet cam timing - it's indirectly linked to the exhaust timing via the turbo (the equivalent of loose coupling in systems design). OEM implementations of variable cam timing of turbos seem to confirm this theory. Another reason to leave it alone, is that the ignition timing on the LET is linked to the exhaust cam, yet another complication if it's to be meddled with.

Here is an attempt to measure the cam durations. It's not very straightforward because the head is optimised for size, so the tops of the hydraulic lifters are almost completely covered by the cam lobes - leaving not much space for the dial gauge to work with







As a result, the margin of error is larger than it could have been, but repeated measurements produced fairly consistent results:

[Note: the absolute degree figures are different from the others, because my trigger point was different. This means that it would be incorrect to compare them to those of other people, although it's fine for comparisons among themselves]

setting 1 (stock LET)

IN: 7/64, i.e. duration 251

EX: 50/20 i.e. duration 250, overlap 27 degrees



With the XE intake cam retarded by a tooth the figures become:

setting 2 (XE inlet retarded by a tooth)

IN: -3/78, i.e. duration 255

EX: 50/20 i.e. duration 250, overlap 17 degrees

The XE inlet cam has a bit more duration, and more importantly 1mm more lift (9.5mm instead of 8.5mm).



If it were to slot right in, bang on the marks, it would be

setting 3 (XE inlet spot on marks)

IN:13/62, duration 255 again, this wouldn't change of course.

EX: 50/20 i.e. duration 250, overlap 33 degrees

This would increase overlap a lot (6 more degrees and more lift as well).



Setting 2 (or even better XE inlet retarded by 5 - 8 degrees) has the following cumulative advantages:

There is better filling of the cylinders as the intake cycle now lasts a bit longer and lift is 1mm higher (more area under the curve)
There is less overlap, reducing reversion at high revs. This addresses a serious limitation of the stock KKK16 exhaust setup The graph below is from the racing version of the LET and it, too, was plagued by reversion from 4500rpm upwards. Typically this feels like the engine runs out of breath - with this mod it feels more free-revving.
The intake valves are kept open for longer during the compression stroke, 14 degrees in the extreme setting 2. This reduces the dynamic compression ratio, something quite useful for engines running high boost while keeping the stock CR. It's a good way of slightly decompressing the engine without affecting the squish band. The compression test figures (dry) were stock: 12.0, 12.0, 11.8, 11.9. With inlet cam setting2:10.4, 10.9, 10.2, 10.8. So from an average of 11.9bar it came down to 10.6 This is like fitting lower-compression pistons!
A by-product of less overlap is smoother idling. It purrs like a pussycat and the servo is happy too.

Ideally, a few runs on the dyno would be needed to find the optimum position. The first can be with the XE inlet cam right on the marks (setting 3). We assume that we already have done a dyno run with stock LET cams (setting 1) and setting 2 as well. Second attempt I would go for Inlet retarding 5 degrees and Exhaust advancing by 2degrees. Third attempt Inlet retarding 8 degrees and Exhaust advanced 3. Finally Inlet retarded 10 and Exhaust advanced 3. [The exhaust could be left alone, if we're not feeling adventurous. It's the Intake that makes most of the difference at this stage.]

The reduction in overlap is closely related to the turbine restriction. A bigger turbo, or one with a larger A/R ratio will create less backpressure at high revs, so there will be less need for reducing the overlap - if at all. The reduction of the dynamic CR is also related to the static CR and the maximum boost achieved. If boost is kept below 20psi and 8:1 pistons are used, then there may be no benefit in further reducing it via cam timing. Setting2 is meant for stock engines running high boost.

To avoid any confusion, if we want to reduce the overlap, we need to retard the IN cam and/or advance the EX cam. Therefore, if we're facing the cam pulleys and they normally turn clockwise, then we'd have to shift the IN counterclockwise and/or the EX clockwise.

A clear trend should be visible by now. If power is picking up steadily until the fourth attempt, maybe we could try retarding the inlet a couple of more degrees and advancing the exhaust a couple more. If the power has dipped after the 3rd attempt, we know that we should settle at settings somewhere between second and third.





XE exhaust cam

That's an experimentation for the more adventurous.

What the best tuners also know (and keep to themselves of course) is that symmetrical cams don't work well with turbos (The XE cams are symmetrical, they rhyme 18-62/62-18). Of course when you start advancing/retarding them, they stop being symmetrical.

In my experience with turbos:

They want a bit less lift on the exhaust side (compared to the inlet) and a bit less duration too. These valves can get red hot, and appreciate spending quality time in contact with the cylinder head, it's their main way to cool off.
Early opening of the exhaust valve eats up part of the power stroke (not good)
Late closing leads to more overlap (not good either, especially if the exhaust-side A/R ratio is below 0.6).
If you can't find a decent aftermarket exhaust cam with the stock duration and a bit more lift, better bet is to leave the stock cam in place.

Right on the stock marks, the XE exh cam opens about 12 degrees earlier. That's eating into the power stroke by almost one tooth - ouch. We need the exhaust gases to spin the turbine, but if that's at the expense of using our hard-earned power stroke, maybe it's not worth it.

To avoid this early opening, and bring it closer to the stock LET exhaust timing, we need to retard the cam by those 12 degrees. Naturally, this will result in a 12 degree increase of overlap, so it's swings and roundabouts.
A compromise would be to retard it by half a tooth, that's around 7 degrees. But that would affect the ignition timing, and that's unacceptable (except if you run aftermarket ignition timing)

In a nutshell, without an aftermarket engine management system, we'd like to use the XE exhaust cam with the settings spot on. But as we've seen, the early opening makes this a mixed blessing. It results in considerable midrange flatness as the power stroke is amputated - but after 4K it transforms completely and boost builds at a frantic pace.
For race use, or large shots of nitrous, it's well worth a try, but trading off midrange does not make the car fun to drive in everyday traffic. I've tried it and I hated it.



Handling the XE/LET cams

If you need to rotate a cam manually (or counterbalance it while undoing the pulley) use a 24mm spanner (on the body, near the pulley). Never apply force on the lobes or the bearings.

It's important for all dyno runs to be consistent, so the engine temp has to be monitored and the FMIC sprayed with water if needed. If we don't want to bother with experiments or vernier pulleys, then the XE cam is retarded by 1 tooth and we hope for the best (setting 2). Remember Forrest Gump the retard.




Intake Ports

These have to be smooth - again, not spit shine, but certainly finger smooth. Casting flaws and ridges have to be smoothed away. The entry of the ports can be made slightly larger than the manifold runners (use the amended gasket as a guide to draw the new edges). It helps prevent reversion (a bit!).

A Dremel extension would be quite handy:

If you're going to enlarge the ports, remember that most of the gains are at the TOP of the port, that's where you should focus.

If you feel that all this is totally over your head, then just cleanup and polish things, and put it back together. It's very easy to botch it up and make the head flow worse in the end! As this is non-reversible, it's best to be safe than sorry.

More on porting can be found at the bottom of this page

A good guide on head porting is here (local copy here)



Intake Valves



Run your finger down the valve stem. If you can feel any anomalies that are not due to carbon deposits, the valve (and probably the valve guide) has to go. The valves can be soaked in degreaser overnight, and have the rest of the carbon taken off by fitting it on a drill and using sandpaper to do the work. If in doubt, use the smoothest paper you can find. Do protect your eyes and lungs from all the carbon dust that will float about. Also be VERY careful not to touch the valve seat. It's sacred. If sandpaper hits it, all the grinding paste in the world won't bring it back.



That's how it should look after the operation:



The valve heads have three-angle cuts, not bad. They can be further improved by rounding up slightly the two outer angle edges (the ones further away from the valve seat). This will increase VE, by improving flow at the lower valve openings.

Now it's time to lap the valves. Look at the seats carefully, both around the valve heads and on the cylinder head. If they look fine, then only use fine grinding paste. 10 seconds lapping should do it, wipe them clean, apply a small drop of fresh paste and lap once again. If the smudge from the paste is a full circle with consistent thickness, you're done. Wipe everything clean and proceed to the next valve. Every valve will have to go back to it's old place, so keep them separate and clearly labelled.








Valve Stem Seals

If the head happens to be lying around and the valves are already out, it's a good idea to change the stem seals. They're cheap, and the engine will burn less oil. After 60-70K miles they might start leaking. The symptoms are similar to the turbo bearings going south, so with turbo cars you're never really sure if the stem seals leak or not. Change them while you're at it.
If you buy pattern ones, better order a few spares, as once they are pulled out they can't be used again. Then some may disintegrate as you push them in place (the tiny coils brake loose - don't force them, or it may happen with the engine running!).
Use a deep 11mm socket to push these seals in place, much safer than pushing them by hand!

Because of all the work involved in changing them, I would only use original Vaux seals. I've learned this the hard way...



Valve Springs

The stock valve springs are not too bad, despite what resellers of aftermarket 'uprated' items might tell you. They are the best compromise for lobe long-life, power-sapping and smooth valve operation at all revs.

The first thing to check when the head is dismantled is the 'free height' of the springs. As they age they tend to shrink, losing their elastic properties and allowing the possibility of valve float at high revs.

This is a brand new LET valve spring.

Height: 42.25mm. Note that the accuracy of the instrument is far higher than the margin of error in this case, as the spring top and bottom surfaces are not totally level.

I'd treat this as an indication that if the springs are less than 42mm tall I'd keep an eye on them, and if they are 41mm or less I'd consider replacing them all.


On high boost turbo applications certain unique conditions can occur that n/a engines never experience.

If boost pressure is high enough, it can literally force the inlet valves open. Similarly, if exhaust backpressure is high enough, it can (potentially) interfere with the exhaust valve timing. It's not always cut'n'shut because the intake/exhaust pressure ratio is also important. But to keep things simple, lets just say that normal 'race' engines need stiffer valve springs for reasons of overcoming inertia forces (revving at much higher rpm) while tuned turbos don't usually have to rev any higher than stock. So their valvetrain issues are completely different, and non-turbo experts are the wrong people to consult when it comes to such 'uprated' parts that can do more harm than good.




Spark Plug Protrusion

While the head is out, tighten the spark plugs temporarily and check that they all protrude equally in the combustion chambers. If any one is different, it has to be fixed before the head goes back on. Use this opportunity to thoroughly clean the threads. Crossthreaded plugs are not funny while the engine is in one piece.







Exhaust Valves

They have to be cleaned in a similar way to the Intake valves. Most probably they will have more carbon deposits and their seats will be in worse condition. Coarse grinding paste might be needed for all of them, but once the seats look decent, wipe it off and switch to fine paste. You don't want to overdo it and damage the seats with too much lapping, you'd need new seats then. These will have to be deeper in the head, and you only have a couple of millimetres leeway for that, or else the tappets can't follow the valves - i.e. the head is scrap.

These valves are hollow and sodium-filled (that's why you shouldn't throw them in the bin with ordinary waste). Filling the valves with sodium allows them to cool better because the sodium liquifies at operating temps and allows convective cooling to occur, as opposed to the conductive cooling of solid valves.



Exhaust Ports



Most probably they'll be full of carbon. Thoroughly clean and polish them, but do NOT enlarge them. Focus mainly on the upper part of the ports, as they are responsible for most of the airflow (it's the outer side of the turn, as the gases rush out of the chamber)



If there are sheared studs (as above) now is the time to get them out. Do not put the manifold back if there are missing studs, as they hold the weight of the whole turbocharger assembly. A leaking exhaust gasket can lead to localised overheating of the head or a burnt valve. Drilling slowly is one way to get them out (eventually). Giving them to a specialist machine shop is not as macho, but far safer. This one above didn't even need helicoil afterwards, despite the 2 (two) broken drill bits that I skilfully inserted before finally taking it to specialists...

This is the assortment of parts for the LET Coscast head (pic by Caveman)



And this is the head put together




Below is a cut-out of the LET cyl head. Gives an idea of the thickness of the aluminium walls





CosCast Vs Vaux heads

There are two kinds of cylinder heads used in LETs:

Vaux-made ones using el-Cheapo production techniques, heads tend to go porous if overheated. It's either sloppy casting, substandard alloy or just bad design. The fact is that after a number of expansion/contraction cycles a crack forms allowing pressurised oil to seep into the waterjacket. (focus on the red circle in the middle-left part of the pic. it shows where <probably> the water/oil holy union takes place in porous heads).


Nasty feeling when the coolant expansion tank has that semi-digested curry look.



Cosworth-made ones, made by people who know what they're doing. These have 'CosCast' printed on them, but it's quite hard to find with the head in place.




Head Gasket - Z20LET mod

A well known successful modification for tuned LETs is to use the head gasket from the Z20LET, which is cheaper as well.



One difference is that it has 4 rivets on the edges. Apart from the top-right one, they have to be drilled out first (easy, with an HSS drill bit).

Another difference is the diameter of the cooling holes, which is a bit smaller. This (when done in moderation) has the effect of raising the coolant pressure inside the head, something desirable on a high-performance application that has to shed more heat overall and risks more hotspots than a std-boost engine.

Thickness is essentially the same, but it has the advantage of splitting into separate layers, so a thicker one can be easily and cheaply made up. It's probably stronger than the stock item too.



Crank vent and Camcover mod

...And if you think crank ventilation is girl stuff (real men only focus on power-making mods) then take a look at the contents of this intercooler



It's the stock item from a Toyota SoarerTT, piston blowby and ineffective crank ventilation made sure it was half-full with engine oil. We don't want an oil drain plug on our intercooler, do we?



Engine Block Refurbishment

If you're feeling adventurous and want to further refresh the engine, then a new set of rings might not go amiss, especially if the engine has done 70K+ miles. Here is what to do with the block



Additional reading:

Here is how to measure the volume of a combustion chamber

Here is an alternative way of porting, by making the ports smaller. This guy swears that it gains lots of power, but I'm not convinced. It may work on ports that are too big from the factory perhaps but I don't see this happening on the LET. The article is interesting though. (Local copies here and here)

Some conventional (!) porting fundamentals from Tomorrow's Technician

Make your own Flowbench with these parts

From the same source here is a DIYFlowbench forum (ah the wonders of the internet!)




Double gaskets and other short stories
Squish Band

This is defined as the area in the combustion chamber where the piston (at TDC) comes extremely close to the head. It's usually around the perimeter of the piston, and no mixture is expected to burn there. Physical contact is the only factor determining the 'height' of the squish band, so practically no mixture will be expected to be there as the flame front moves from the spark plug outwards.

Here's how it works: the upcoming piston squishes out that mixture, forcing it to blend with the rest around the plug. This action creates extra turbulence and homogenises the resultant mixture, which means that it burns cleaner and quicker, requiring less ignition advance. As a bonus, the outmost edges of the combustion chamber are closer to the plug tip, further reducing the need for extravagant ignition advance. All this leads to efficiency, especially in off-boost situations, where the mixture is not well homogenised (there are rich and lean spots within it)

Under boost, the role of squish takes the backseat. The compressed air is already agitated enough for the squish turbulence to make any major difference. Therefore race engines designed to run mainly on-boost don't have to pay much attention to it.

Here's an interesting paper on mixture preparation strategies



Measuring the LET's squish

It comes up to roughly 650millimiters, or 0.65cc, around 2.35% of the total Chamber volume.
This means that the designer has sacrificed the burning of 2.35% of the mixture, in return of better burning of the rest. It's actually less than one would expect from such an efficient design. Probably the mixture was expected to already be quite turbulent - the air coming from the turbo would certainly help in that respect.



Thicker gaskets?

If someone fits a gasket thicker by 1mm (or steel plate to that effect) the new, deformed chamber will be sacrificing 4.16% of the volume with not much squish effect
These are static figures, would be accurate at revs while cranking the engine by hand.
In real-life rpm there's a lot of inertia involved, and the stock LET squish can easily be below 1% while the 'deformed' one over 5%. The static CR would be about 8.3:1 in this scenario.

Is that bad then?
Yes, because it takes a fixed number of milliseconds at those revs for the flame front to propagate from the spark plug to the edge of the charge. (It also depends on the strength of the mixture, effective scavenging, etc. Check out the ignition section)
The whole piston crown is NOT the bottom of the combustion chamber. The mixture is squeezed into a homogenous blob around the plug tip just before TDC

Fitting a thick gasket DEFORMS the combustion chamber shape, creating an extra 'donught' a couple of mms tall and around 8mms thick. Most of the mixture in this space will NOT be burnt - it's too far from the plug tip.
Some of it will indeed burn, but too late, after the power stroke (not good)
In the process you've lost quality of the rest of the charge that DID burn, because it wasn't stirred up as thoroughly as the designer intended it.

That's why running double (or more) the stock boost with stock CR is not how the manufacturer would have done it. There are complicated formulas to calculate *how* much the CR should be (it's not linear!) but whatever the figure, one has to make sure that the combustion chamber is not being deformed in the process. Combustion chamber efficiency is the engine's most powerful tool against detonation.



How do we reduce the Compression Ratio then?

The DIY methods of lowering the CR range from botches to acceptable. The majority is in the first category - thicker gaskets, double gaskets, steel plates, machined pistons.
The only acceptable one is to use a lower-compression piston that retains the squish band incorporated by the OEM design
Call the piston manufacturer and ask them - if they don't know what you're talking about, look elsewhere. People have been known to skim metal out of XE pistons to reduce compression. This is a serious botch, it weakens a piston that's not strong enough to begin with!

Of course there's always the cheap'n'cheerful way of altering the cam timing to achieve a similar result, as I describe in the 'cams' section.



...So is thicker gasket a no-no?

Lets not get paranoid here.
We're talking about a drop of efficiency of a few percentage points.
It could have been a lot worse, the squish area of the LET is already relatively small.
If you're hell-bent on running 2 bar on stock pistons, then you'll have to fit a 2-3mm gasket because you've got no other choice (intercooling isn't enough at this stage). In this scenario, retaining squish is a bit of a luxury. Off-boost efficiency will be sacrificed for on-boost reliability.

If, however, you're only doing up to 20psi overboost, and no more than one bar is sustained, then it might not be worth upsetting the delicate balance of a well-designed combustion chamber. For a reliable high-boost setup that is expected to survive prolonged full-throttle, a proper set of pistons will bring the static CR down to 8:1 or thereabouts.

In-cylinder cooling provided by such means as water injection or small shots of nitrous oxide can also work wonders in keeping the engine together in such conditions.





Here are some thoughts on performance head gaskets

Combustion chamber design from street rods

Even more on combustion chambers





Backpressure: Friend or Foe?

There are two schools of thought, both fiercely battling for world domination:

One says that exhaust backpressure is an 'evil thing' and it has to be eliminated here and now. The other one says that some backpressure is needed by the engine, to run smoothly and efficiently

The 'evil' dudes counterattack with the argument that 'how can it be good for breathing to have flow resistance', for that's what backpressure is. The 'moderates' reply that if you dump the whole exhaust it won't run much better, it will be lumpy and erratic.

Who's right and who's wrong? Can they both be right (or wrong?)

Of course they can - they are generalising, and it's unavoidable for both schools to be right in some cases and wrong in others.



Let's start from the beginning...

There is no single figure for exhaust backpressure. It varies dramatically depending mainly on engine speed and less on engine load. Typically it's almost zero at idle, and a fraction of a psi at midrange. As we approach high revs it shoots up quickly and at full revs it can be quite a few psi. When we refer to 'exhaust backpressure' that would imply full revs and full throttle.

On a n/a engine the intake also experiences some 'backpressure', which follows a similar pattern but is overall of much lower amplitude. "Backpressure" by the way is a catch-all term technically incorrect, but I'll use it nonetheless.

A (mechanically) supercharged engine will have an exhaust backpressure pattern similar to the n/a version, but all figures will be a bit higher, as more exhaust gases are trying to flow from the same-old exhaust.

A turbocharged engine will probably have a huge mother of a restrictor before the exhaust even starts. It's called a turbine, and it squeezes and upsets the outgoing gasflow like you'd never believe. Stock, OEM turbo designs have exhaust housings that are VERY restrictive, squeezing the air through a tiny passage, trying to make it give away all it's energy to spin the turbine. (low A/R ratios, but let's keep it simple here). If you want boost at low-mid revs, then the pre-turbine chamber must be squeezed like a garden hose trying to get the neighbours wet. That restricts flow big time, several psi worth of drop and we're still at midrange revs. At full revs this restriction is much higher, and that's even before the exhaust pipes contribute their own share of backpressure.

Race-preped turbos run high A/R housings, which means that the turbine wheel might be bigger, but the housing around it is FAR bigger, you can stick your finger in there. That improves flow immensely, especially at high revs. In fact, half of the air molecules may get through the turbo without even touching the turbine. This leads to low backpressure all right, but if they haven't tried to spin the turbine, who's gonna do it? Hence the 'race' turbos don't make boost until 5K rpm

Then we have the 'hybrids' and the modified turbos, somewhere in the middle (much closer to OEM, really)

OK, enough popular mechanics, why should we care what the backpressure is?

One issue is the interaction between the cam timing and the exhaust.

As we saw back in the "cams" section, during overlap both exhaust and inlet valves are open for a short while. This means that stuff could flow either way. We don't want it move the wrong way. We either want everything to stay in place, or move a bit towards the exhaust. That would encourage fresh (cool) mixture to wash out the remaining crap from the combustion chambers, push the old rubbish away and cool the valves a bit (at the expense of higher fuel consumption and emissions)

If it goes the 'wrong' way then some crap will remain in the chambers and some will go back to the intake port. This will preheat the ports, the intake valves and the whole chamber in general. It will also displace fresh mixture, contaminating what we're trying to burn. Preheating the area it also decreases the density of the mixture that DID make it into the chambers. This happens a lot actually, that's one of the reasons why VE is so much below 100%.

Race engines of yesteryears used to run loads of overlap. At some revs this exhaust gas reversion would be really nasty, because exhaust pressure waves would stuff the burnt gases back into the carbs. But at other revs (max torque revs) these same pressure waves would suck out the burnt gases, creating a vacuum below atmospheric, pulling in the fresh mixture in. VolumetricEfficiency at those revs was 110-120%. The ridiculous overlap wouldn't let them idle properly, and at low revs they were hopeless, but race engines are meant to be full-throttle all day long, so that wasn't an issue. Fuel consumption and emissions weren't a problem either, but they are now, and OEMs go for minimal overlaps nowadays.

Back to turbos: There's this notion that overlap lets the boost get away straight to the exhaust. This is perpetuated by 'tuners' and 'experts', so ordinary folk treat it as gospel. Some books on turbocharging also fuel this fire.



At last, let the TRUTH be told!

Here is a list of fallacies on the subject:

On n/a engines the intake is sucked in and the exhaust gases are pushed out by the explosions. That's why intake valves are always bigger.
Turbos leak boost during overlap
A totally free-flowing exhaust can only help a turbo spin faster. There cannot be any downside to this.
There are more, but you get the picture. The reason this stuff prevails is because it 'sounds right'. Hey, it's conventional wisdom, it MUST be right. That's a good excuse for Joe Public, but specialist tuners should know better than that. How many of them have actually ever measured exhaust backpressure? Ask them to show you how they did it. There are 'gotchas' when you first try it, but once the adaptor is made, it can be used time and again. If they haven't ever measured intake and/or exhaust backpressure they're just repeating the age-old crap I keep reading in books and articles allover. But because the 'tuner' said so, it gains even more credibility. He then sells you some Slick50 to eliminate any friction between you two.

Let's look at the above gospels more closely:

1. First of all there are no explosions in the combustion chambers. If there are, then we have a problem and the engine won't last long. What we strive for is controlled burn of the mixture. We want everything to happen in an orderly manner, with no hanky-panky before the spark plug fires and no rush for cover as the flame propagates. We don't want the high chamber pressures (after the burn) to be used to push the exhaust gases out of the way. We're not in the fireworks business, we want the energy to be used to push the piston downwards!

Everything has to be timed so that the last drop of energy is squeezed onto the piston. There's only one power stroke in a 4-stroke engine, let's get value from it! Neither do we want the piston to push the exhaust gases out during the exhaust stroke, because that would be energy lost from the crank. We've got enough losses as it is, everything tries to drain energy from the crank, but this is not SSEnterprise.

Therefore an optimised engine strikes a balance on when and how easily the exhaust gases will be gone. The result is that in practice the intake/exhaust pressures are fairly similar.

2. How can someone know whether a turbo leaks boost during overlap? Do they stick their hand beside the valves and feel the breeze? Sweeping statement that.

Again, it's the inlet/exhaust pressure ratio that dictates where the flow will tend to be. Measure these, and you know. If an engine sees 20psi at the intake and 30psi backpressure at the exhaust, is it gonna leak boost during overlap? I don't think so. It will leak boost when the intake shows 5psi and the exhaust 2psi. It's still the same engine, you know, just different rev/load combinations.

Below are backpressure measurements taken by username: c20let on the MIG board

Inlet manifold / Exhaust manifold

- 0,2 / 0,25
0,0 / 0,45
0,1 / 0,55
0,3 / 0,75
0,5 / 0,90
0,7 / 1,30
0,9 / 1,80
1,1 / 2,00
1,3 / 2,20
1,5 / 2,45

At high boost pressures there is almost 1 bar more backpressure than boost. That's a lot of reversion! This refers to a standard KKK and exhaust manifold, but it wouldn't be much better with a straight-through exhaust, or even a hybrid. For serious power, a larger A/R ratio is needed here.

Supercharged engines tend to have intake pressure generally higher than exhaust backpressure.



In that case you know that increasing overlap will shove boost straight through the exhaust. Some of this may be beneficial actually, cooling the valves a bit.

Turbocharged engines are totally different beasts. Exhaust backpressure rises rapidly right after the max torque revs, while boost pressure doesn't. The result: reversion. But before max torque revs, intake manifold pressure is higher than exhaust backpressure - boost leak territory.

Change the exhaust and ditch the cat, and the whole balance may change - raising the rev point where boost leak stops and reversion rears it's ugly head.

That's why generalisations and sweeping 'expert' statements can be embarrassing later on.

3. OK now, how on earth can a free-flow exhaust fail to help the turbo spin faster? Surely there's no downside to this one. Take a stock turbo car, fit a bleed valve, fit a 4" downpipe with no backbox, and you're King of the Hill.

...Boost spikes anyone? The main way to control boost is through the 'integrated' wastegate.



Yes, they are proudly advertised as a bonus, when in reality they're a miserable compromise of low-cost and low-flow. The wastegate typically sits next to the turbine and as it opens up exhaust gases are diverted from the turbine's way. For the same opening of the wastegate valve, flow is controlled by the backpressure after the turbine. The 'freer' the turbine spins, the less of an incentive for the gases to go around the wastegate. A 4" straight-through pipe will seriously diminish the wastegate's effectiveness. If the car is running high boost as well, then the stock crappy wastegate is under even more pressure.

The result is boost spikes, that can allow the turbine shaft to spin momentarily far faster than designed. Doesn't help reliability.

But I've got a boost gauge, I hear you cry. I don't see no spikes.

I've got news for you: Your boost gauge is heavily dampened. If it were not, it would be unreadable, the needle jumping up and down continuously. The spikes are evened out in the gauge's damping fluid, what you see is an average value. Oops.



Can a free-flow exhaust reduce efficiency?

Actually, it can. The efficiency of a turbocharged engine relies heavily on the cylinder head operating at the right temperatures - more precisely the gases between the exhaust valves and the silencer (or cat). The speed and temperature of these gases dictate the force that will drive the turbine. If the gases are too slow or too cold then the turbine isn't driven as hard as it could be, resulting in increased backpressure and a slower compressor (less boost).

An exhaust that is *too* free-flowing can result in the engine feeling 'gutless' at the bottom of the rev range. This is not always placebo, the gases leaving too early result in lower exhaust gas temps, therefore lower torque produced at those engine revs. This is only the case at low revs, because the very same exhaust design also results in higher flow at high revs, and lower EGTs there too. The only difference is that the lower EGTs are now welcome, because they are pushed below the maximum (safe point), while at low revs they were below the minimum (efficiency point).

Such an engine will produce improved max bhp figures, but looking closely it will be apparent that it's at the expense of low-down power. Once it's recognised however, it can be fixed - exhaust wrapping could help bring EGTs back up again, while the free-flowing exhaust can retain the max flow potential. Best of both worlds.



Surely after the turbine the ideal exhaust is free as a bird. I mean - there are no exhaust pulses in turbos, are there?

Here is a handy little calculator for gasflow in a pipe



Wastegates

Besides noise, resonances etc, it's the (in)effectiveness of the integrated wastegate that dictates how 'free' the exhaust can be. Cracks on the wastegate 'face' don't help, but they do appear due to the constant expansion/contraction, and the penny-valve banging on it all the time as the actuator pulses it (via the amal valve).

Tiny hair-cracks are not very disruptive, but once they grow to these proportions then the whole core has to be junked

The wastegate won't shut anymore, so the turbo will take ages to spool


There are mods where the wastegate is slightly enlarged to allow it to flow a bit more. I don't think it's worth the aggro. For serious performance an external wastegate is a must.

Another big advantage of an external wastegate is that it can be set to dump the excess gases out in the atmosphere, without them interfering with the 'normal' exhaust flow. This interference creates even more backpressure when you least want it: at full boost. If it has to merge with the rest of the exhaust, it can be set to merge at a very shallow angle and a few feet away from the turbine wheels. That would minimise the interference with the turbo's efficiency, that always robs power in 'integrated' setups.

How stiff does the wastegate have to be? Surely there are no vast forces pushing it open!

The exhaust gases are quite forceful at that stage. Just sticking your hand at the back of the exhaust tailpipe doesn't give you the right idea. Don't forget it's these gases that spin the turbine to 100K+ rpm!

Under low boost the exh/inlet pressure ratio is just over 1, quickly climbing to 2:1
At 1 bar boost it's around 2.5:1 and from then onwards it grows disproportionately.

Of course it depends on the turbine diameter, the exhaust A/R ratio and the backpressure of the rest of the exhaust. The pressure ratio will be lower if the turbine is bigger, the A/R ratio larger and the exhaust see-through.

Very roughly, for a stock(ish) turbo and a cat-less exhaust, the exh backpressure is about 2.5 times the boost pressure. So if we're running 1 bar boost, we've got 2.5bar pushing at the wastegate (against the actuator spring)
Then we calculate the area of the flap that the gases can see. Say it's 1 inch sq.
We've got (2.5)*14.7= 35.8psi (that's pounds per square inch!)
...so the gas force pushing the flap is 35.8 pounds. Simple.

That's why actuators that can hold high boost pressures need to have much stronger springs.




Uprated Actuators - there may be a catch!

An uprated actuator should be one with a stiffer spring. But beware, any actuator may appear as 'uprated' just by looking at it, and it can even appear to perform as an 'uprated' item if it's wound up long enough. But that's not the way to do it. Here's why:

Suppose that the rod has to extend 20mm in order for the wastegate to open fully. If you have an actuator with a weak spring, then you'd have to wind it up by (say) 15mm, so that it can hold 20psi (or whatever). But what you're effectively doing is simply pre-loading the spring. That's NOT the same as having a heavier (stiffer) spring.
You have a soft spring, but preloaded. Yes, it will start pushing the rod at 20psi, but it will only extend by another 5mm (the other 15mm you've already used up while preloading it!)
5mm travel are nowhere near enough to open the wastegate flap properly, and you'll end up with runaway boost.

Your 'tuner' may blame the ECU, the turbo, the weather, the 'small integrated wastegate', whatever, but it really is their fault in preloading the actuator spring too much.




Elbow

A halfway house would be to change the 'elbow' with one of a better design. Due to space limitations the stock one is a very restrictive 90 degree bend, so any improvements there are welcome.

This is how an alternative flange would look.

It would allow the separation of 'normal' exhaust gases from the wastegate gases - quite important for performance at higher boost pressures.

Thanks to Chris Reay for the pic


Note the gentler turns - essential to reduce backpressure, as it's so close to the turbine. This is a KKK by the way:



Note how the wastegate gases are separated during those critical first inches, so that they can't wreck havoc with the 'normal' gases. This is felt as better boost control during gearchanges under boost.

Below is the racing version of the LET. It clearly follows the same design principles:





Here is the truth behind the evil and sneaky Wastegate Creep



Actuator-controlled exhausts

Another interesting idea is to have a stock (or at least not offensively wide) exhaust, Teed off at an early stage, with the sneaky extra branch coming to play only under high boost.

Smart and simple.




Silencers

The silencer is too far away from the turbo to have a big impact on back pressure - but this doesn't mean the engine has to be strangulated or the car has to be noisy. For maximum flow straight-through silencers are the best, but for noise reduction AND good flow a design is needed similar to this one (see Ray Hall in the 'links' page)



Here's why we need to keep an eye on Exhaust Gas Temperatures This is meant for turbo diesels, but the principles are the same (local copy here)

EGT monitoring tips from Hahn the turbo guru (local copy here)

Here is more on what monitoring EGTs can and cannot do (local copy here)

We recently mentioned superchargers: why not have both a turbo and a supercharger on the same engine? Why not indeed...Lancia tried it and found the complexity was not worth the aggro.



There are LET enthusiasts all around the globe. From Australia and South Africa, to Romania and Sweden. Hot and cold climates --- but they all agree on one thing: these cars do tend to overheat.

The cooling system is only just adequate for the donor engine, the C20XE, but the turbo produces 54 horses more, and the super-hot turbocharger in front of the bay doesn't help at all. To make matters worse, the radiator size took a bit of a hit, so that the stock intercooler could fit up front (kinda). As the icing on the cake, the super-hot turbo bearing dumps most of its heatload on the coolant, which is then promptly emptied into the (plastic!) expansion tank.

Cut-out of a turbo waterjacket:

If the car also has aircon, then another massive radiator sits up front (the condenser).

We're not finished yet: Any turbo worth its salt needs a proper intercooler, and that will be front mounted. Yet another heat exchanger preheating the air BEFORE it enters the engine bay.

There's an oil cooler as well, integral part of the lubrication circuit

A tuned turbo will produce 50-100% more power under load, and all the heat that goes with it.

No wonder people get desperate in the summer, with coolant temps hovering over 100C. Switch the engine off and it can easily exceed 110C for a short while. Nothing good can happen at these temps.



Is there an ideal coolant temperature?

It depends on how the car is used.
On a track day, you'd like the coolant temp to be around 90C all long (whether it stays there is another matter!)
On normal start/stop driving where part-throttle response is more important, mid-nineties is better.
Emissions are lowest around 100C, that's why manufacturers try to keep temps around there.

But the Octane requirements of the engine increase with coolant temperature. For an 11C increase of the water jacket (say from 80C to 91C) there needs to be a raise of 2 AKI points, if we are to keep the safety margins intact. That could be 2 RON and 2 MON points, or 4 RON points. Quite a lot, eh? Look at the Ignition and Fuelling sections for more.

On the other hand, a cold engine (below 70C) should never be pushed hard. Metals need to expand, fluids need to warm up. Let them assume battle stations before applying full throttle!

These two thermostats are on the two sides of the spectrum.

On the left is a pretty 'cool' one, 72/85 (Meaning that it cracks open at 72C and is fully open at 85C). In the winter the engine will struggle to reach 80C and will not exceed it on motorway journeys. Too cold for the engine's own good.

On the right is a standard one for the aircon LET, 92/102. Much better.

Remember that the engine's efficient operating range is 90-100C

To the left is a pattern thermostat. The build quality is nothing like the German on the right. The bleeding screw has been omitted to save costs (not that it's needed anyway...)

If you find yourself 'needing' a cooler thermostat, then you need to clean up the whole cooling system. A thermostat cooler than 92/102 should only be needed in tropical climates. Otherwise it is a botch, masking the inadequacy of the radiator, expired coolant, neglected cooling passages (full of scale) or even water pump. Don't fall for the "easy fix" of a cooler thermostat, far from being a cure --- it's not even an aspirin.



Coolant

Manufacturers claim (and punters believe) that antifreeze is good for ya. 50/50 mix all year long they say, keeps the engine happy. Hey, it's got anti-boil additives, so a strong antifreeze mixture will resist boiling - isn't that a good thing?

Well, no. While the anti-boil additives might raise the boiling temp by a few degrees, all that antifreeze (instead of water) will be far less effective in absorbing the heat from the engine and dumping it on the radiator.

Eh? why do they recommend it then? Surely they don't want people's engines to fail prematurely? Exactly that is the reason they recommend it - they know that the average punter isn't too bothered to check anyway (unless it breaks down) so the easy-to-remember 50/50 ratio is convenient for all-round the year. They don't want people running weak coolant mixtures and claiming on warranties for their cracked engine blocks in the winter.

But for optimum performance you need a different mixture, depending on the ambient temperature. Antifreeze has one major role: to stop the coolant from freezing. It also contains anti-corrosion and lubrication agents. The truth is that water is the best cooling medium around. That's distilled water, or at least de-ionised. The heat-transfer properties of water are superb. Antifreeze can't match it, so running a 50/50 mix when you can avoid it is plain silly. There is a minimum mixture strength and that is 25%. It's not wise to go lower, because the anticorrosion and lubricant additives will be diluted too much. Even so, running 25%antifreeze 75%distilled water makes a big difference in the summer. At the end of the summer, just drain off 2 litres of coolant (2.5 if you're in the highlands) and then topup with pure antifreeze. That would bring it back to a safe 50/50.

It must be stressed that all coolant should be drained and replaced every year or two, because it eventually turns acidic. There are chemical reactions happening there, it's not like a glass of water left covered in a dark room. The coolant reacts with the metals around it and produces contaminants that appear as 'furring' on the inside of the coolant passages. A 0.3mm such layer can cut down heat transfer by a third. Hard to clean it, too - best to avoid it in the first place!

A warning for those with pets: antifreeze is toxic but sweet. It smells and tastes sweet (apparently) so pets go there and lick it. Then they die.

Methanol-based antifreeze was nasty even to smell (it's banned now via an EU directive)



Checking the coolant with a Voltmeter

Eh? you're kidding right? Coolant testers use boyancy, the small eye-drop things with the floating balls.

Actually a digital multimeter can be very useful in checking the condition of the coolant. Water is not a very good insulator, that's why regulations don't allow installations of electric sockets in bathrooms.

Coolant is a mixture of (hopefully) clean and deionised water, along with all sorts of additives that help make it quite a conductor. The older the coolant, the more acidic and conductive it becomes - that's not good.

Set the DMM to measure low voltages (below 1 DC hopefully!) Stick one of the probes of the DMM in the coolant, the other somewhere on an exposed metal part of the block or the inlet manifold, whichever is closer. Even with the engine switched off you may be surprised to see a voltage.

If it's below 0.1V then it's fine.
Between 0.1 and 0.3V it's not perfect, but you should be prepared to change the coolant at some point.
0.3V - 0.5V is not good.
Over 0.5V then you have a problem and should flush the lot immediately!



Coolant flow

Liquids flow in strange ways, sometimes counterintuitive.


You think it may be better if you run the water pump a bit faster, and sometimes this is indeed a good idea, as it circulates the coolant quicker. Other times this is counterbalanced by eddies in the wrong places, *reducing* flow and losing pistons (one reason turbocharging the Vauxhall V6 is tricky)


Ah, but then you think it may be better to run the pump a bit slower, giving more time for the coolant to do its stuff, and sometimes this is indeed a smart move. In other cases you get localised boiling and hot spots - not good at all.


You then have a stroke of genius and decide to increase the diameter of the waterways to promote flow -- and you end up actually decreasing pressure, depriving remote spots of their cooling. This is a common issue with aftermarket 'performance' cylinder head gaskets, where the coolant holes have been enlarged to promote flow. It's like the manufacturer was negligent or absent-minded and made the gasket passages smaller than the block/head waterways. The aftermarket 'guru' fixes this oversight, pressure in the head goes down, and remote chambers in the head lose their coolant circulation altogether. The head goes west, and something else takes the blame (usually the turbo!)

The motto is THINK before forcing an 'improvement' in the coolant circuit - it may have far-reaching repercussions you never thought of.




WaterWetter

Does it make the water any wetter? Yes. Well, at least it appears so, it reduces surface tension.

Does that make a massive difference? No. If you just add it to your 'recommended' 50/50 mixture you'd be hard pressed to feel any difference.

Where it really excels is on the track, because it contains all the needed additives (apart from antifreeze!). So during the track day you can run just 100% distilled water and WaterWetter. This does make a difference, but it's not the wetting effect at play, but the lack of antifreeze. Nice though.

Warning: WaterWetter might react with some additives in the antifreeze and create a 'curry froth' in the expansion tank. It looks like the head has gone porous, but it's a false alarm. Best to change the coolant though, and avoid mixing WW with that particular brand of antifreeze.



Water pump

This is a weak point of these engines, as it tends to suddenly deteriorate, leak and eventually die - sometimes taking the engine with it. It's therefore good practice to have it changed every time the cambelt is up for change (30-35Kmiles), since most of that work has to be done anyway.



Both of the above pumps are on sale, the one to the right is better (it will not cause cavitation).



Radiator

When a FMIC has been fitted, the old intercooler should be disposed of. There is no point in it being there restricting airflow just because the coolant rad leans on it. The rad mounts can be extended by small brackets, allowing the rad to be positioned a bit more centrally and leaving some space for extra cooling air to get into the bay.

These are close-up shots of the new rad mount extensions:





An even better move would be to replace the radiator with a bigger one, that would use up this extra space for a good cause. The coolant hoses might need to be extended in this case.

An intermediate solution is to use an aftermarket rad, direct replacement of the existing one, but with a thicker core. It wouldn't cost much, and it would bring the following benefits:

new radiator is totally clean outside, the fins are straight and the paint is intact
new radiator is totally clean inside, all the muck, scale and electrolytic garbage are thrown away. No rad cleaning can ever achieve that.
new radiator core can hold another 0.5 litre of coolant. More coolant in the system means less chances for it to hit critical temperatures.


Can I run the turbocharger without the coolant?

Funnily enough, yes.

It's not recommended of course, and the extra heat load will be dumped on the oil. People have ran the LET turbocharger on engine blocks without coolant pipes. The turbo obviously doesn't live as long (around half the lifespan, which is not too bad really) and the oil changes have to be frequent and only use fully synthetic (which is a good idea anyway!)



Engine-bay Airflow

This is a very important issue, which some people neglect, or take for granted.

Fresh air enters through the front intakes, moves through radiator fins, pipes, hot components etc, then exits via the vents at the back of the bonnet, as well as underneath the engine. At speed these 'exit routes' are well defined and part of the original design.

Vents like these help the hot air from the engine bay exit quicker, increasing the efficiency of the cooling system and the intercooler core as well.
In contrast, these are scoops: in this Mustang they seal against the V8's air intakes underneath, creating a ram-air effect.

If such scoops are introduced on a conventional engine bay, the airflow will be disturbed and probably made worse.

Scoops create positive pressure, while behind the radiators we want negative pressure.

For extra air intakes on the bonnet, the best way (aerodynamically) is to fit a NACA duct:

Ill-placed vents and scoops can disrupt the airflow towards the alternator or the ignition coil, and create eddies that reduce the efficiency of all rads under the bonnet.

Rule of Thumb: if in doubt, leave it alone.



Battery

Everything starts from the battery. If it produces less than 12V (or 13.5V with the engine running) the ignition will never give it's best shot. Most later turbos have a voltmeter, if not it's a good idea to add one. Low voltage is the root of many 'weird' electrical or fuelling problems. Bad earth connections also account for many X-files type of faults.

If the battery is more than 3 years old and it can't hold charge properly, it's better to ditch it for a new one. If it's in a good condition though and just needs a good, slow charging, then an 'intelligent' charger like this "OptimateII" is a good investment:





Coil

It should never be touched with the engine running, the shock could be lethal! The stock coil comes with an 'ignition module' on the side, which is just an overpriced transistor regulating the voltage. Although the official parts guides have a different (i.e. rare, expensive!) part number for it, rumour has it that the XE coil is practically the same. Similar case to the Oxygen sensor.

The battery terminals tend to corrode and need cleaning thoroughly. Perhaps the connector washers and nuts need replacing with new ones, too. It's stupid to let dirt and oxidisation compromise the voltage it receives from the battery.

If the coil is suspected of not delivering the goods (misfires under boost) then a couple of drops of water on the top of the coil will show if there are hairline cracks. With the engine running, look for tiny bubbles. Easy on the water, we don't want to short it out, just see the bubbles. If there are cracks, it's got to go.

Left: stock coil
Right: alternative coil that NOS people use routinely. Unfortunately I didn't find it to be compatible with the engine. Maybe it's the primary resistance that's 1.6Ohms instead of 0.9OOhms, who knows...

At some point I'll come across a coil that fits the LET nicely and will produce a stronger spark.

Beware though, a stronger spark is not necessarily a good thing, as it will demand a pristine path and will try harder to earth itself through the chassis (causing misfires)

Note the lead with optimised shape and length (aircon engine bay). Also note the different plug.

Too many coils, too little time...

Here is how to check your coil as well as the poor man's coil tester (local copy here)





Aftermarket Super-Duper Coils

There is a lot of hype regarding 'spark amplifiers', 'hyper 50KV coils' and the like. I've fallen for it myself too in the past - it just sounds reasonable and techno. You know, loads of KiloVolts igniting your pressurised mixture - hey, it can only help, right?

The sad truth is that most are snake oil, much like Slick50, Splitfire plugs and the like. Those that do produce a stronger spark (many don't even achieve that!) have no measurable difference whatsoever, simply because the stock system produced enough spark in the first place. The 'ectstatic' reviews (where they were real) would be from people who had replaced a half-dead ignition system with a brand new aftermarket one - of course it felt better! Mind you, it's not always easy to diagnose a coil that doesn't perform. It may measure right, look right and perform right at most engine speeds, but micro-cracks in the insulation might lead to misfires at full boost. In bad cases just placing a few drops of water on the top of the suspect coil would show this - if they appear to 'boil' then there are cracks hidden.

Modern ignition systems (like the LET for example) are capable of producing more than enough spark energy than that needed for combustion (even at 20psi). If there are misfires, they're probably because of dead leads/plugs/rotor/dizzy/lead routing, or the wrong AFR.
Yes, a wrong AFR can make ignition a nightmare. Either too lean or too rich - it just won't ignite, no matter what the coil output is. It's just NOT COMBUSTIBLE.

Changing bits around, like the chip or the oxygen sensor, would probably also change the AFR at those conditions, making it more (or less) combustion-friendly. But because you've also changed the coil (or the leads or whatever) you think it was the coil (or the 'trick' leads) that solved your problem.

This doesn't mean that they are ALL junk: some might indeed be a genuine improvement.
What I would question is the *need* for a (so-called) stronger spark when we're talking about an LET doing 20psi overboost. We're not in 30 or 40psi territory here - now that would definitely ask for some serious ignition improvements!



Distributor

It consists of the main body on the cylinder head, the dizzy cap and the rotor. There are three O-rings in there, change them for new ones if it happens to be disassembled.



These engines eat dizzy caps and rotors for breakfast. They should be treated as consumables, along with spark plugs and the various filters.

These points are less than 5K miles old: (click for larger image)



The dizzy cap is the same as the one from the early 2.0XEs, so it can be found everywhere. The rotor can be a genuine part for peace of mind, but it's cheap anyway.

Be careful, as many motorfactors supply the wrong dizzy cap for the turbo - this is the correct one:

(click for larger image)




Just because a rotor arm fits in the shaft, it doesn't mean it's native.

Both of these arms are sold for the LET, but the thinner one also sits 2mm lower


The (vertical) distance between the centre of a dizzy point and the mating surface is around 21mm



Ideally the distance between the rotor arm tip and the mating surface should also be 21mm.

With some rotor arms this distance can be a few millimetres off. Not good for the quality of the spark, and could potentially lead to misfires. Since everything will be pushed in firmly, there will be no easy way to troubleshoot what's wrong.




Rotor Arm Mod

Also known as the "NOSWizard mod", because it was first described by the NOSWizard. It aims to increase the strength of the spark by 10-20%. It involves scrapping the inline resistor that lives in the rotor arm. (Click for larger images)

Below is a new rotor arm intact. The resistor is within the black bit.



Scratching the resistor reveals a lot of the stuff:



I used a Dremel with an engraving tip, but other tools might do the job:



Below are two rotors - one is stock with the whole resistor, to the right is the modified one. It's filled with solder, effectively short-circuiting the two copper ends.



The stock rotor measures 1KOhm between the two ends. The modified one is practically zero Ohms.

This has to be done with attention to detail because it rotates at half the engine's speed, which is a few thousand rpm. If it's not secure it might disintegrate, leaving you with a weak (or none at all) spark. Also try to use some epoxy resin to cover the lot, in the manner the OEM item does it. Good quality solder has a resin in the core, which floats to the surface once molten - but add more if it doesn't look enough. This will prevent the air within the cap from becoming ionised (used to be a source of inexplicable intermittent faults in older automotive designs). Better safe than sorry.

Be careful to measure the resistance afterwards, and verify that it's zero (or thereabouts). If the copper ends are not electrically connected, then the resistance will be infinite, of course. The car may still run though, as the spark can jump a few millimetres - but it will be weak, and under boost it will misfire.

This is a no-nonsense mod. The car feels stronger and smoother at full boost - no question about it. It also appears to pull cleaner through the whole rev range, an effect similar to advancing the ignition timing by a few degrees. I can only think of two reasons that this could happen:

Without the resistor the coil now discharges slightly quicker.
Without the resistor the spark duration is now slightly longer, increasing the chances of finding an ignitable charge specimen within the plug tips.
An inspection of the spark via an oscilloscope would indicate which of the above two is actually happening. Either way, the mod works a treat. Stock turbos may not feel the full benefit, but high-boosted engines will definitely feel happier (apart from the higher cylinder pressures, they also tend to run 'colder' plugs that are less efficient off-boost).

Then why did the manufacturer fit the resistor in the first place? Good question:

One reason is to contain the 'backfire' that occurs after the coil has discharged (a high-voltage spike is sent back into the coil). Having zero-resistance wires is regarded by most people as a 'good thing'. In fact it just shows that they don't understand how these ignition systems work. The manufacturer has added resistors on the path for reliability reasons, these cars are not meant to be rebuilt every 1000 miles!

Another reason is to reduce interference, which is why the recommended spark plugs also have inline resistors ("R" in the name). The stock ignition leads have carbon resistors in there, too (that's why they age and weaken the spark). Getting rid of all these resistors should theoretically make the car fizzle like a mobile phone mast - interfering with the ECU, the radio, the ABS - even the aeroplanes and satellites. In reality it runs just fine - all the people in the NOS forum run their cars like this and they've reported no dental fillings tuning to radio stations.

Please note that the long-term effects of such a modification are not known. There is now extra load on the coil and it might be forced to fail prematurely. Try it at your own risk - just like every other attempt to deviate from the manufacturer's design.

NASA knew of the beneficial effects (in terms of reliability of ignition points) of resistive leads back in 1945!



Leads

There is nothing wrong with the OEM leads, quality-wise. But after a few years they will have deteriorated, although this is not always visible. When I comes to replacement leads, my preference is for Magnecores, the 8.5mm red ones. They fit nicely, click everywhere, seal the plugholes, and insulate well.

Leads need to be physically clean, with no carbon deposits or marks at all. Neither should they touch each other or anything metallic. The reason is that sparks find the easiest path to earth themselves, and if they find an alternative route, they'll take it. Misfire or crossfire might result (a lead sparking another cylinder). The stock (plastic) spacers are there for a reason. Use them.

There is a lot of claptrap 'hyperconductivity' theory surrounding trick leads. Read here about Magencore's version of the truth (local copy here)

Tip #1: make sure the leads are securely pushed in, especially on the distributor side. The engine will run fine even if one or two have not fully clicked, as the spark can still jump the small gap (but it weakens). The car however will feel sluggish and that is a fault hard to pinpoint.

Tip #2: if you have an aircon engine, the coil is probably very close to the distributor. This means that the king lead might be far too long. If you cannot source a shorter lead, it's better to coil and secure it with cable-ties (nothing metallic!) so that it doesn't touch anything metallic in the bay, even as the engine rocks wildly under load. The spark can (and will) leak through intercooler pipes, aircon pipes, steel clamps, gearbox cable, whatever. Beware of anything metallic within an inch distance from the leads.

Ideally you'd want a shorter king lead though. Various 'experts' might say that you need a special crimp tool for the Magnecors. Rubbish.

Here is the king lead in pieces
The famous core: wound up spring, nothing exotic
Just cut half of the lead length, and put it back together. The rubber boot will be a bitch to slip back into the outer silicon body.

Don't worry - use some liquid soap, and it will just slot in. After the soap dries, it's good as new

Special crimp tool?

Yeah, right...

Short versus stock lead.

Much better.


Another by-product of the shorter lead is lower electrical resistance. The stock one is 2KOhms, the shortened one a bit less than 1.5KOhms. Every little helps.

Here is more about leads (or 'wires' as americans call them). If it's down, try a local copy



Plugs



The OEM plugs are fine for a stock car. But when the power is raised by 50%, then they are no longer optimal. As a very rough rule-of-thumb, every 50-60 bhp increase should be accompanied by plugs one grade colder. Therefore modified LETs should go for plugs one or two grades colder. Be careful, because not all manufacturers use the same naming conventions for their heat ranges. The 'width' of their ranges can be different, too.

As boost increases, typically the plug gap has to shrink, if misfires are to be avoided. Go too small, and misfires is all you'll get. Ideally the gap should be as wide as possible, just enough to avoid misfires. The stock gap is 0.7mm. An engine running 1 bar would be better around 0.65mm. A high-boost LET at 20psi will probably need 0.6mm.

Iridium and platinum plugs are the best, because they last longer under high-boost conditions. Cheaper ones will also work (if the gap and heat range are correct) but they will need replacing more often. In theory at least.

For a stockish turbo running high boost the NGK BCP-7EV is a good start. It has no extra resistor to weaken the spark (no "R" in the name), it is gold/palladium-tipped for long(er) life (that's the "V") and it's one grade colder than the stock LET items (heat range 7 instead of 6).

The nearest equivalent Iridium is BCPR7 EIX, and can be bought for £36/set from the internet.

Tip: if the car has trouble passing the MoT because of too high HC, and everything else appears fine, then try again with a set of plugs one grade hotter - like BCP - 6EV, or better still, some OEM-replacements.



The instructions recommend that you do not try to adjust the gaps on these, simply verify that they are correct (0.7mm). Banging the ground electrode gently (ish!) on a wooden bench should close the gap if you have to, but pulling it back again can be more tricky as you could exert pressure on the thin (and fragile) tip.

Iridium plugs are claimed to operate more efficiently under high boost and last longer too. Their heat range is claimed to be broader as well. The secret is that the tips are very thin indeed.

My experience - and those of others- however tends to differ a bit.

Robustness is an issue apparently, as Denso iridium plugs have been reported to lose their tips during high-boost operation.

The Denso iridium to the right was lucky and only lost some of the insulator in the centre. Others lose the whole tip.


One wonders where the lost tip ends up eventually. Probably the exhaust valves won't appreciate the encounter, no more than the turbine blades...

The claims about longer life have not materialised in my experience, either. What's worse, I've had them foul (dry) under rich conditions, and they could not 'burn' themselves clean afterwards, resulting in misfires. A new set solved the problem immediately, but left a few question marks on the manufacturers' wild claims about 60K miles service life...

This is an article on iridium plugs from a specialist, trade magazine.

Nothing is substantiated, looks like an infomercial really...

Another similar 'trade' article, both kindly scanned by Chris Hall



Another important aspect is spark plug tip protrusion



The image above shows how I measured protrusion (the gasket thickness is excluded obviously, as it can vary)

ZFR6FIX: quite clearly it protrudes in the combustion chamber 2mm more than the other ones. Hmmm...
BCP7EV
BCPR7EIX



If you feel that this stuff is totally elementary, here is a slightly deeper view of some ignition aspects, explaining detonation, preignition and more



More on NGK's plug naming conventions

Here is a typical page on spark plug diagnosis (local copy here)

Here's another one (local copy here)

Here is yet another one - quite interesting, too (local copy here)

Pseudo-science meets spark plugs with the Fuel Blaster. (local copy here) More ridiculous than Splitfire.



Importance of oil

Oil plays an extremely important role in an engine, even more so if it's turbocharged.

It keeps the metal parts from touching each other and becoming one large metal blob
It cools many hard-to reach places, as the coolant passages are not everywhere
It keeps the engine clean by storing all solid contaminants in the oil filter and keeping the rest in suspension until they are drained out along with the oil.
Lubrication is not equally critical in all parts of the engine. Some parts may go on running with no damage even if they are starved from oil flow for a few seconds, but others can't. The turbo bearing is one such example.

It's called a bearing, but it's not like the camshaft or the crank bearings at all. It's a floating bearing, meaning that it's a ring within a ring and these two are a bit 'loose'. The bearing surface really is pure oil under pressure between them.



As long as there's oil pressure this works well and leads to low frictional losses and negligible wear. But we have to remember that during full boost the turbo shaft revs at 100Krpm, maybe more. To complicate matters, there are strong forces trying to 'rock' the shaft about, and shift it along it's axis as well. It gets worse: air pressure differences may develop between the intake and the exhaust side of the shaft, and these will lead to the bearing oil being sucked in the exhaust or the intake. That doesn't help the bearing's effectiveness.



The oil is meant to be delivered to the turbo under pressure, (the banjo bolt at the top) then follow the path of gravity and go through a maze of tiny oil galleys, through the bearings and finally drain into the sump. Hence the sump oil level should always be lower than the turbo drain pipe. Sometimes this doesn't happen, either because there is too much oil (and it's kicked around by the crank) or because the sump is abnormally pressurised (faulty ventilation, piston blowby). In this case the turbo oil return cannot flow - it's stuck. New oil is shoved in the turbo bearings under pressure, and the old one has nowhere to go (or drains at a lower rate). This excess of oil will inevitably be pushed either through the intake or the exhaust side of the shaft, depending on the engine revs and load (the air pressure ratio at the time). Won't do the turbo any good, that's for sure...

During the short stay at the turbo bearing, the oil has a very hard time. It gets a beating that could break it apart. It looks like whipped cream, so much is the froth and the thermal stress. There is no place in the engine where the oil is tested so hard: this is important when deciding on oil quality.



Viscosity

This is the most well-known, and probably most important of oil properties. Oil of SAE (Society of Motor Engineers) 10 is thinner than SAE50. But that stands for single-viscosity oils. Manufacturers tend to recommend multi-viscosity oils, like 10W40. This means that as you start the engine in the winter (when oil wants to be 'thicker') this product will perform like a straight SAE 10 and at operating temperatures (when oil wants to be thin as Kate Moss) it will behave as if it were a straight SAE 40.

How do they do that? In the case above, they would typically use a base of SAE 10 and then add Viscosity Improvers. Simple (for them) and it works most of the time.

...But there's a snag: under extreme stress this '****tail' may momentarily break apart and revert back to the base (in this case SAE 10). Cams and turbo bearings may not be happy at that point (that's why many cam manufacturers recommend 'breaking in' their products with straight SAE 30 or thereabouts). Turbo bearings can get extremely hot, and the whipping action of the shaft brings out the worst in this sort of oils. If not changed very regularly they completely break apart and the VIs evaporate - you're left with a thin crispy base: this may be fine for a pizza, but the turbo (and the rest of the engine) will disagree.

VIs are a real pain for turbos, because they are the same substances that tend to carbonise on the ultrahot surfaces inside a turbo. Getting rid of them would be nice, but then we'd have to change oil every season (manufacturers' marketing departments would never agree to this). So we're stuck with them, and try to minimise their effect by using only fully synthetic oils (they are far more stable) and regular oil changes.

There is an exception, and that is Mobil1: This uses no VIs, but a patented sequence of distillations. As a result it doesn't coke and keeps it's viscosity for way longer than anything else. There was an independent test publicised on the internet a few years ago, where enthusiasts used their bikes to compare different brands of oil. A viscosity tester was used before and after each test. Synthetics came out ahead of mineral oils (in general) and Mobil1 was a long way ahead of the best synthetic: It managed to hold 90% of it's original viscosity after 2500miles, while the others had lost it after around 1000 miles. Even their competitors agree - if we read between the lines! (local copy here)

When I was into turbo bikes, most of them were air-cooled, so oil had a very hard time indeed. I had tried several oils on my 750turbo, and after stripping it down, Mobil1 was ahead by a long shot. It was the only oil that didn't leave a black sludge in the sump. Here is another 'independent test' that has done the rounds of the internet in several guises. Look at the bottom and in 'viscosity retention' Mobil1 is highest (it's based on motorbikes that also use engine oil for the transmission). Pity they don't have figures for 5K miles though. (local copy here)



Other oil properties

Viscosity is not the only game in town. Oils do oxidise, even if they are not used at all. Antioxidants (of various shapes) have to be present. Also detergents and dispersants: contaminants and debris have to be moved away via the oil, and get trapped within the oil filter. If the engine is dirty inside, then a good oil will get dirty very quickly. That is a sign that it manages to keep all the rubbish in suspension, protecting the engine. It's also a sign that it's time to change it, or else the suspended garbage will end up everywhere!



Oil changes

There is a lot of debate on how often should engine oil be changed.

The truth is that this varies, depending on the:

Engine design (very old engines might have different clearances and oil needs, but then oil spec is just one of the incompatibilities there)
Car use - motorway miles are far easier on the oil than start/stop city traffic, or a track day
Ambient temperature - cold winters degrade the oil very quickly
Number of warm-up periods - each time the engine cools down, there is condensation everywhere inside, especially in the winter. For the first 10 minutes or so, all this condensation is picked up by the oil and you're effectively running on an oil/water mixture. When the engine reaches operating temperature (coolant and oil > 90C) then it starts getting rid of this water (evaporation) but some of it remains diluted in the oil, making it thinner and less effective. Repeated warm/cooldown cycles with the engine never really maintaining operating temp for a while, lead to oil degradation far sooner than one might think. These short runs in the winter can kill the oil within a month or two even if the mileage in that period is minimal (because it was minimal in reality!)
Quality of oil itself - mineral oils tend to break up easier than semi-synthetics and these don't last as long as fully synthetics.
State of the engine - a dirty engine will overwhelm the oil with rubbish. Even the best oil cannot cope with this and only an oil flush can ease the situation. Also poor-sealing piston rings will allow fuel to find it's way down to the sump. This is even worse than water dilution, because oil and fuel mix really well, and the result is dead oil. Apart from optimising crank ventilation (or fixing the rings!) frequent oil changes are the only remedy.
Car manufacturers like to quote long oil-change intervals, because it makes their products look like low-maintenance appliances (the marketing department really loves that!). They claim that this is down to improved designs, superior metals, availability of good oils etc. etc... All these are valid arguments, but after 10K (sometimes more) miles the state of the oil can be horrific. Even the best oil in the world will deteriorate if it's mixed with liberal quantities of water and fuel, then is heated up at 100C and stirred at 8000rpm.

Perversely, oil manufacturers don't try to dispel this 'low-maintenance' myth, maybe because it would make their own product look inferior to the competition (If Mobil were to recommend that their ultra-expensive top synthetic needs changing every 3K miles, and Shell recommended 10K miles for their base £10/barrel stuff, which one would YOU buy?) <local copy here>

My experience is that turbocharged petrol engines should only be fed fully-synthetic, changed every 5k miles (at least!). If a lot of start/stop occurs in short winter trips then this should be brought down to 2K miles.



Coking

A lot is expected from the oil during start-up, as it has all drained down in the sump and there's only a very thin film covering the parts that are about to start rubbing themselves. Turbos also create expectations for lubrication right after shutdown. There is acute need for lubrication around the turbo bearing immediately after the engine is stopped, but the oil pump is on lunch break and there is oil pressure no more! There is the myth that the turbo keeps spinning for a long time after the engine is shut - complete rubbish, if you look at a turbo idling and then shut, it stops almost instantaneously. If it were doing full boost and the ignition were suddenly killed (very stupid indeed!) then it might take a couple of seconds for the turbo to come to a complete stop. You get the idea.

It's not that the turbo stays spinning forever after, but that the temperatures around the turbo bearing suddenly shoot upwards (not that they were low before!). The flow of pressurised oil takes with it a lot of heat, and this stops in its tracks when the engine is turned off. Therefore the remaining tiny quantities of oil are expected to keep the shaft from melting within everything it touches (one side of the shaft has the turbine, which can be white-hot at times! How do oils cope with this? With difficulty!

What happens is that the oil is trapped between an extremely hot shaft and a superheated bearing. Lubrication is the last thing in the oil's mind - survival is more like it. It is baked and all the additives evaporate. The VIs turn into a carbon form, hard as a diamond. What's left is some black ash and not much more. Repeated cycles see this build-up altering the shape of the oil galleys. Not pretty, and it's practically impossible to clean this stuff without dismantling the turbo. No oil flush or 'magic in a bottle' will get rid of it. It just adds up.



Oil filters

Same as with oils, no heavy-weight independent tests can be found on this subject. The commercial interests are so large, that the cost of lawsuits afterwards will cripple whatever organisation tries to clear up the muddy waters.

Enthusiasts have cut down filters though, and put them to the test, posting the results and the photos on the internet. (local copy here)

The general picture is the following:

OEM oil filters tend to be the best available
Many expensive 'big name' performance filters are rubbish
Some cheap generic ones are almost as good as the OEM ones
Some aftermarket filters (irrespective of price) can be dangerous - either they let large particles through, or they are not robust enough and collapse under high oil pressure, or their bypass malfunctions.
Race oil filters have a lower 'micron' rating, meaning that they stop even the tiniest particles that would otherwise flow freely through OEM-spec filters. This is a dangerous thing for someone who might forget to change this filter after a few hundred miles. It will clog up quite fast (as it's meant to) and then the relief valve will override it, letting all sorts of garbage circulate. Very dangerous!

The oil filter should be always changed along with the oil. The new filter should also be filled with oil before installation, or else during the first few seconds that it takes the pump to fill it with oil, the turbo will be running on oil fumes! After a few such oil/filter changes people complain that the 'turbo burns oil', but it's their carelessness that killed it.



Oil level

This is an important issue, as there are warring factions once again, the full mark, the low mark and the middle earth people.

First of all the oil level should be checked (ideally) with the engine warm, but after it's been left to cool down for 10 minutes. The difference between the LOW and the FULL setting on the indicator is usually 1lt. Running higher than FULL is stupid, since the crankshaft will splash on a lot more oil, eating up power and reducing the remaining oil into froth. At high revs a lot more oil will be stuck at the wrong places (due to centrifugal forces of the crank) and the sump pressure will be higher. Even worse, the turbo oil return will struggle to flow properly, and we know that's not a good thing.

Don't think that you're immune from something silly as that, because nobody is. I've lost count of the times garages have overfilled my sumps, I think it's the norm rather than the exception. I don't know the exact reasons behind this conspiracy, but probably it's a combination of 'can't be bothered' and the fresh oil being transparent and not easily visible on the stick (especially under bright light). Beware.

Maximum power will be available when the oil level is at MIN. However this is not safe, as during sharp turns the oil pickup may suck in air or bubbles, and they are not a good lubricant for the turbo. My favourite is slightly below the middle mark, but if you're going on a long trip better play it safe and keep it near MAX.



Oil Flush

The following procedure should be credited to 'Mycroft', a colourful character from the Skyline forum

The best flush is as follows:

Drain the existing oil/filter while it's still warm. Let it drain completely.
Pour in a good quality *thin* oil, like a straight SAE10 or SAE20
Drive the car around for 20-30 miles without revving it too hard. Not motorway miles, just around the neighbourhood, it needs varying speeds and loads.
Drain this oil/change the filter (again). Let it drain for as long as possible
Pour in the new oil.
This is arguably much better than any flush can.



Importance of regular oil changes

This issue cannot be overstated. If there is a single maintenance operation that you intend to perform on the engine, that should be regular oil changes.

These are pictures of a supra engine, freshly imported from Japan, where the owners had missed an oil change or two. Maybe three...





They were posted in SupraForums some time ago and present a good example on the horrors that can be hidden inside a neglected engine





Fuelling





Correct fuelling is extremely important on n/a engines, but on turbos it's critical. Get it wrong and you could lose the engine before you know it.

With a n/a engine we're talking accelerated wear like losing the bores early if it's running too rich, and general overheating consequences when it's running too lean. A few clicks and pops in between.

In contrast, a turbo running lean under boost lives on borrowed time. Keep it at full boost while lean, and there's overheating and detonation within seconds.
If you're lucky the head gasket will give way and you'll have to slow down either you like it or not. If your luck's out then a piston or two will have melted, like they've been attacked by a blowtorch. The melted aluminium may land anywhere - on the chambers, the valves, the exhaust turbine...you get the picture.



Flashback --- let's start from the beginning:

The engine burns a mixture of air and fuel, but not any mixture. If it's one gram of air and one gram of fuel it won't burn, because it's too 'rich', i.e. it's got too much fuel, it's rich on fuel. If it's got 1Kg of air and 1 gram of fuel it won't burn either, because it's too 'lean', i.e. it's lean on fuel, this one is practically all air!

It has been found experimentally that the ideal mixture, the one where these two elements are perfectly balanced is 14.7:1 by weight, i.e. 14.7grams of air and 1gram of fuel. Or 14.7lbs of air and 1lb of fuel. This is called 'stoichiometric', meaning in Greek measure of the elements.

.

Knowing that it's ideal is very nice, but ideal for what exactly? Ideal for max power, ideal for max economy, ideal for minimum emissions, what?

...You've guessed it, it's emissions. Save the whales, hug a tree etc...

A stoichiometric mixture will burn and produce the least amount of toxic goodies, say hydrocarbons and CO. As it gets leaner it produces less CO and more hydrocarbons, as it gets richer more CO and less of the others. Stoich is a fine balance, where even more nasty stuff of a different nature gets produced, but the catalyst takes care of that. It's the point where all oxygen has been used up, and there is no excess fuel lying about.

Mixtures leaner that stoich (15:1 ~ 18:1) provide better fuel economy (unsurprisingly, since they contain less fuel!) and sometimes we do want our fuelling to be this way, especially during deceleration, or light throttle openings (cruising). But put your foot down at the strip, and if you're running 18:1 you ain't going anywhere. Acceleration needs richer than stoic, 14:1 ~ 13:1, sometimes even 12.5:1. A turbo under low boost needs to be around 13:1 minimum, and under full boost richer than that. HOW MUCH richer depends on chargecooling, ignition mapping and other factors.

Two extreme examples: a light-pressure turbo with a kick-arse chargecooler that brings the chargetemps down to ambient all the time, with perfect ignition mapping and well-designed combustion chamber, running 100-octane fuel would not need to run richer than 13.5:1 under full boost. In fact, it would best if it didn't, because it wouldn't make any more power and the extra fuel would be a waste. The same engine running TWICE the stock boost, on 95-octane fuel, crappy intercooling, fudged ignition mapping and overtaxed cooling system, would run best at around 11:1, or even 10:1. This is 20-30% more fuel, mainly to patch up the cooling issues. In the trade it's called a 'fuel shower' as the extra fuel droplets absorb all that heat from the red-hot valves, pistons and compressed air. It also affects the speed of burn, but heat absorption is the main reason. It wastes fuel, and it's a botch (because the specific heat of fuel is not that high), but it's simple and it works.



AirFuelRatio gauges

There are two kinds of oxygen sensors: narrow-band and wide-band. The first ones typically have 2 or 3 wires and they are found as OEM equipment on catalyst-equipped cars. The other ones are more costly, have more wires, and are usually found in Rolling Roads, Tuning shops (serious ones) etc. You can buy WB sensors from the internet nowadays from several sources, but beware of the displays. They might need a 'translation' stage to display within the 0-1v range, and even then the linearity has to be verified. They also need to be recalibrated every so often, as they can get contaminated and lose their accuracy. Most cars paying for RR time are tuned and tend to run rich and even sometimes use leaded fuel. Both of these poison the sensors pretty quickly. Ask a RR operator when was the last time he had the sensor recalibrated, if you want a cheap laugh!

Lambda (or Oxygen) sensors really are air batteries, creating voltage depending on the oxygen content of their (filthy) environment. That's why they slowly deteriorate with age and use, and after a few years they are slow and inaccurate. It's also why you should never hook the sensor to a multimeter and try to measure its resistance. The tiny current from the multimeter might kill the sensor dead. Here is more info on lambda sensors (local copy here)

AFR gauges simply display the output from the sensor, so they can't really be blamed for erroneous results. An exception would be if the gauge's impedance would be low enough to affect the signal going to the ECU - high quality gauges from reputable companies wouldn't have this problem.

We have to remember that the purpose of the OEM lambda sensor is not to produce a lightshow for the driver, but to provide feedback to the ECU about the exhaust gases being slightly leaner or richer than stoich. This is vital, if the cat is to operate properly and not self-destruct. Rich mixtures can coat it with carbon deposits and kill it, and lean mixtures will cause it to overheat and die.

Therefore, the OEM sensor is only accurate around stoich (14.7:1) and wildly inaccurate beyond that point. For example, a 9:1 mixture will register as 'full rich', as will a 10:1, along with a 12:1. Similarly, an 16:1 will appear as 'full lean', same as 20:1. Not only is the useful band narrow, but accuracy drops sharply the further away the reading is from stoich. It's also very temperature-sensitive. The ECU refuses to listen to the sensor until the engine has reached operating temp, and there's a reason for that. While the sensor is cold, rich appears richer, and lean leaner (depicting our society). As it heats up, the useful range 'shrinks', and the same voltage now relates to a different AFR!



Note how CO and NOx are at their highest at stoich (but the cat takes care of them). Also note the almost linear relationship of CO in the 'rich' section. That's why CO emissions at full boost are a good indicator of how rich the engine runs.

Two caveats then - the readings are not linear, and only worth something when the sensor is hot. Even so, the sensor has to be in good operating order (hard to tell just be looking/measuring it). Even so, the most accuracy we would expect would be in the range between 13.5:1 ~ 15.5:1 with not much linearity either.

This lack of linearity can be easier seen in this graph from the Autometer site:





This gauge has 6 green LEDs, to indicate 'rich'. A 750mV output from the sensor will register as one green LED, and for a fully warmed-up sensor that means AFR = 14.5. Two green LEDs would result from 800mv, or AFR= 13.3 and three green LEDs AFR = 12. Due to the lack of linearity it's highly unlikely that such a gauge will ever show 4 green LEDs (900mV), certainly not at operating temps. I have verified these measurements with a Wide-Band sensor operating in parallel. However after 20K miles that accuracy had gone out of the window. The sensor was not even accurate around 14.7:1, it was showing leaner than reality, fooling the ECU into dumping more fuel than necessary. Not good for a clean engine without carbon deposits.

The red LEDs standing for 'lean' are even more tightly packed, more like a lean indicator (and not gauge of how much)

For tuning and ECU-mapping purposes, a narrow-band sensor is nowhere good enough - but for troubleshooting after the car has been setup, it's brilliant. A narrow band sensor cannot tell you if the engine runs rich enough, but it can tell you if it runs too lean. Tuned turbos shouldn't be without one, as running lean under boost can cost dearly.

The graph below displays this particular shortcoming of narrow-band sensors, compared to a wideband. See how evenly the wideband spreads from very rich (10:1) to very lean (20:1). In comparison the narrowband struggles between 11.5 and 16.5, with usable resolution only between 13.5:1 and 15.5:1



Finally some more technical take on narrowband sensors (taken directly from an old, long and extremely interesting thread of the GTR forum: (if the link is dead click here)

The oxygen sensor utilises the "Nernst effect." Nernst's law measures the voltage between two materials in close contact, one of which is a known constant. When the sensor reaches a temp. 640degrees F ±25F, it produces a voltage which is interpreted by the ECU and adjusts the fuelling to suit.

The outer shell of the sensor is Zirconium Oxide, this material is very sensitive to contamination, think of a nice piece of polished alloy and how easily it will absorb the oil excreted by you skin, well ZircOx will take up all those additives in petrol, but for the most part they don't effect it too badly, but Optimax and all those Octane Boosters do contain contaminents and if over applied or applied constantly (as in using Optimax continually) the voltage engendered is not as it should be, Citric acid cleans ZircOx perfectly, the acid actually cleans down into this 'rough' surface, we are talking very small scale here, so to the hand it is quite smooth.

Lead from fuel screws them, Phosphorous (Optimax, and most OBs') does the same only slowly. If your turbo has ever blown the seals and blown smoke out the back, the phosphorous in that burn out will kill it fairly quickly, you should always replace the sensor if this has ever happened.



Wide Band sensors for DIY use

Wide Band sensors used to be a bit rough (for non-professional use) and expensive.
Now they're coming down on price, and getting more reliable too (self-calibrating) mainly because the new generation of engines will have to stay closed-loop even at full throttle. The new Golfs have a WB as standard fitment, for example.

Late 2004 a new breed of WB has come out based on a variant of this sensor.

Replacement sensors are cheap, too (what you'd pay for a pattern narrowband!)

Standard dimensions, fits nicely in standard pods. Nothing like the previous generation of WideBands, that needed the exhaust to come out, drill and weld a bung, then use a laptop for the display.

Narrowband on the left, Wideband on the right
This plug has to pass through the firewall. Driving cables through a slot in the firewall is hard enough, but this beast is a challenge.

John Holmes had an easier time perforating young and innocent actresses.

The idea is to cut the smallest possible slot otherwise toxic fumes from the engine bay will be entering the cabin. Tight passage.

Detail shows where to look for the output selector. It's not easy to spot, you need a bright torch and good eyesight. Use a micro-flat screwdriver to turn it fully clockwise for the narrowband emulation (to keep the ECU happy!)

Every time the ignition is ON, the controller inside the gauge will restart and should display briefly P04 or position 4 to verify this.



Currently they are sold at competitive prices (considering rip-off Britain). They could be had for less from the States, but adding postage and (perhaps) duty would bring it close to this UK price.

The one shown also has a narrowband simulation, so if the fittings have the same threads, you simply swap the existing one and keep the ECU happy.

 

[Jaspa]

o god wata read lol sticky FTW!

 

[wayne1]

jesus, if i read that lot i,l be here all night , paul where did you get all that from ,,, i,ll have to read that when i,m not drinking

 

[Space Cadet]

If you just sat there and typed that then you really need to get out more but defo sticky!

 

[pdc78502]

no *** typing all that copy and paste of a site found it a good read havent read it all yet just bits of it lmao

 

[Alex]

That's from www.max-boost.co.uk iirc

 

[fennyLET]

That's from www.max-boost.co.uk iirc

Thats what i was thinking.