Tuning the C20LET - Cams and their optimisation

It is fortunate that the LET engine is based on a successful n/a version. This means that items like cams are interchangeable. In an ideal world there would be several different evolutionary stages of the XE, each one with slightly different cams. That would give us a good selection of mix'n'match candidates, cheap and reliable. These are OEM items, and their quality is guaranteed (it's easy to check for damage visually). The metals are what they should be, the machining is exact, the lobe separation is spot on, the shape of the lobes is comfortable for the valvetrain. As far as I know, none of these factors is guaranteed with aftermarket cams, whether they originate from reground OEM ones, or from blanks.
I prefer to avoid those if I can.

 

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.

We already know that aggressive timing doesn't always blend with turbos. Depending on the intake/exhaust pressure ratios, there can be considerable reversion/leaks, upsetting the quality of mixture burn. More on this in the exhaust section.

 

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 (click for larger images):

 

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:

1. 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)
2. 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.
3. 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!
4. 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.

 

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