Tuning the C20LET - Wastegate Creep

Behind this creepy term lies another unpublicised sad story:

Say your wastegate is set to open at 10psi (your car is stock, you're young and naive. Keep an eye on your wastegate now...)

Is the wastegate shut when you're idling? ... yep - what sort of question is this?

Is it shut when you're doing 3psi? ... well, yeah

At 5psi? ... not so sure, it looks like it's cracking open. But why? (Has my HiFlo performance cone filter done me turbo?)

At 7psi? ... it's definitely cracked open, whoa, it's almost half bloody open. No wonder the turbo can't build boost quickly, all those gases are escaping too soon! (Is it my 'performance' backbox sucking out the air as it says on the box?)

At 9psi? ... It's almost fully open now mate! it will take ages to go up to 10psi will all this leakage.(Could it be the Slick50 I sprayed earlier? Friction eliminator?)

At 10psi - hell yeah, it's been open for ages now, the boost rise rate has been steadily dropping since 6 psi. (It's a miracle we managed to hit 10psi, before the weekend).

What's happening? Is this unavoidable? Are the wastegates so small (and the springs so loose) that we need to start opening the door while the boost is halfway up?

The definitive answer is perhaps 'yes and no'.

Wastegates are not digital devices that can snap open/close in a picosecond. They have mass, inertia, tolerances etc. So if the gate is supposed to be fully open at 10psi, it will have to start opening bit before that. But 5psi? gimme a break.

That's because manufacturers want the power delivery to be smooth and civilised. Just like a normally aspirated car! But if I wanted a n/a car I would have bought one - gimme back me power. Sorry, we're civilised now - go away...and close that wastegate of yours...

If you want to know how fast your car really boosts, you have to disable the wastegate. It does feel quicker. It's not recommended though, because boost will rise so fast that you won't believe how quickly your stock engine registers a few bar. Fuelling and intercooling are more geared to half a bar though, so something will give.

But the fact remains that a lot of what many people see as 'lag' is intentionally built in the design. It saves warranty claims on shafted transmissions, too.

Aftermarket boost chips rely on this to give you a feeling of kick, that the stock car cannot match. Well it can, and it does. It's just that the chip resalers claim it's the clever code in the computer that does it's magic, not the hand that physically tightened the nut on the actuator. THAT's the source of the kick, mainly, not the chip. A wastegate setup for 22psi, will crack open at 11~12psi, so the intake will shoot up to 11psi (the stock boost pressure) as if the wastegate were on/off. The rest of the difference (naturally) is the fact that higher pressure is ultimately achieved, hence the exaggerated pre and post-boost state.

But with most chips the real power increase is unimpressive (compared to the boost increase they allow). A stock LET 'chipped' for 100% more boost will only produce 20% more power. It's the new actuator setting that does the trick. We've seen that it's intercooling that makes the big difference at high boost pressures, and chips can only minimally help in that respect (a modest fuel shower, if you're lucky).

Finally let's clear up one thing - the wastegates in ECU-controlled cars don't really open and close, they pulse. That's why inline pressure valves like the Dawes device don't always work well with them (but they do eliminate boost creep if connected properly). It's also why the wastegate bush wears off so easily, even in stock cars that don't hit the rev limiter too often. The ECU controls the pulses so that everything is smooth and civil. Just like a n/a car...

 

Boost creep and the LET

There are two sources of boost creep on the LET:

1. ECU pulsing of the wastegate (via the amal valve)

2. The spring of the actuator giving way before maximum boost is reached

Which one is more pronounced can only be found experimentally, and it depends on the make of ECU and actuator make/settings/age. It's easy to check it out, all you need is a boost gauge and some common sense.

Disconnect the pipe from the back of the actuator and plug it with a bolt. Keep it from swinging about with a cable tie. Beware: Now the ECU has no way of pulsing the wastegate, so the 1st gear boost restrictor doesn't apply. If it's a hot day and the temps go over the hill the ECU will be unable to cut down boost, so try this on a cool day, and only if you have a proper intercooler fitted. You'll see that max boost is reached quicker and it will be more brutal too! If you feel a big difference in driveability, that means that the ECU is working overtime pulsing your wastegate, so that might indicate something's wrong.

Next step is to disconnect the wastegate altogether - wind up the nuts several turns, so that the wastegate is kept shut irrespective of boost pressure. Now we're in danger territory, because it's very easy for boost to get out of hand and damage the engine with a boost spike. Avoid full throttle and keep your eye on the boost gauge. If the engine feels a lot more keen to rev, it means that the actuator is tired and it's half-open even when it's supposed to be shut. The diaphragms in the actuators don't live forever, and the springs do get tired after a few years. Get a new one and adjust it properly.

Here's a bit more on mechanical (pneumatic) boost control for the LET

Here's another attempt to tackle boost creep with a bigger actuator

 

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