OK, wastegates 101 coming up.
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Lets deal with a common misconception first:
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Wastegate size has little to do with power figures.
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Unlike almost everything else on an engine, you don't neccessarily need bigger to run more power.
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in fact, you need a bigger wastegate to run less power,
all other things being equal
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(which they generally are not).
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It's all about flowing gas, not about horsepower, as we're about to discuss.
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What is a wastegate?
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A wastegate is a device that diverts exhaust gasses around the turbine wheel.
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You do this to control the amount of boost your are presenting to the engine.
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The more gas that is diverted from the turbine wheel, the less boost the compressor wheel produces for the engine.
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If your wastegate doesn't open, the majority of turbos are capable of pushing far too much boost into an engine.
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This is certainly true of the CT26 and CT20b that are standard on our engines.
Internal or external?
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An internal wastegate
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(such as is stock on our cars) is a flap in the turbine housing that creates a shortcut around the turbine wheel to control boost.
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It's actuator is on the outside of the turbo, but the actual flap is inside.
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The diverted exhaust gasses are put back into the exhaust immediately after the turbine.
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An external wastegate is a valve that bolts on to an exhaust system
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(either designed or modified for this) before the turbo.
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When the valve opens, it diverts some of the exhaust gasses, thus controlling boost.
What's a screamer pipe?
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With an external wastegate, there are two options of what to do with the diverted gases.
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You could run another pipe from the wastegate to create a
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"plumb back" into the exhaust, so that all the gases still go through your mufflers etc.
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Alternatively, you can fit a
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"screamer pipe".
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This just dumps the diverted gas directly into the atmosphere, generally out of a pipe that's aimed at the ground.
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Because there is generally no muffler on the pipe, these make a very loud noise.
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Inside the car it tends to sound like an epic roar, while outside the car it can sound like an obnoxious jet engine sound.
So what's boost creep?
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Boost creep is where you cannot limit the boost generated to your target level.
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Mostly the boost will hold at your desired level until a certain RPM, but above this the boost will increase as your revs increase.
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This basically means that, although your wastegate is fully open, it's not providing an easy enough path to divert enough gas around the turbine and thus too much of your exhaust is still choosing to go through the turbine wheel.
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This creates more boost, hence even more more exhaust gas and thus even more gas going through the turbine.
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Essentially, your wastegate is not up to the job.
Why would a wastegate not be good enough?
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To be effective, a wastegate need to be able to provide an easier route away from the engine than the turbine wheel.
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There are three main, interconnected reasons why this might be so:
1) The wastegate or it's associated pipework is too small and cannot open wide enough.
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Hence to hold boost and thus power at your desired level, you need a bigger wastegate
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(see the very top point)
2) The wastegate connects to the exhaust manifold in such a way that it asks the gas to make a sharp turn.
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The faster the gas flows
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(ie.
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the higher the engine RPM) the less it wants to turn to go down the wastegate.
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3) The turbine is too easy to spin! The turbine provides a certain amount of resistance because it needs to be turned; exhaust back pressure
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(although bad for spool time and volumetric efficiency) essentially multiplies the effort required to spin the turbine wheel.
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So applying the same backpressure to the exit of both the turbine and the wastegate will affect the turbine path more than the wastegate path, making the gas favour the wastegate more and thus making boost control easier.
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Generally, boost creep happens for a combination of all three reasons.
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And this applies equally to internal and external gates.
OK, how does this affect MR2s? Surely Toyota put a good enough wastegate in!
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Toyota designed an overall system that is sufficent for the stock setup.
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The internal wastegate is big enough given the massive restrictions on the stock exhaust system.
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The cat presents backpressure to both the turbine and the wastegate, but that's not the real culprit; the stock turbo elbow presents a massive restriction to the exhaust
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(there's a big constriction right in the path of the exhaust gas) which means the turbine has to work much harder to spin and the small internal wastegate is fine.
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The problem comes when you replace the stock cat with a decat downpipe; particularly a 3" straight through system.
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You've massively reduced the backpressure so that the turbine is much easier to spin up, reducing lag and freeing up some power.
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BUT, you've disproportionately made the turbine an easier path to take compared to the wastegate.
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Suddenly a wastegate that was fine is now too small and at an angle that's difficult for the gas to turn into.
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Hence creep!
What can I do?
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When faced with creep, you can either:
1) Restrict your exhaust again, which is kind-of counter productive.
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2) Improve your wastegate, which is often difficult and expensive
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(I can discuss all the MR2 specific options if you really wish
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- I've tried pretty much all of them!)
Internal or external?
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part two!
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So why choose one other the other?
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OEMs like internal gates because they're easy to build and package.
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Given they require no extra pipework they also work out cheaper and easier to fit.
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However they tend to be smaller than even the smallest external gate, so on a free flowing system you may well run into problems.
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External gates are very often favoured for turbo swaps and engine builds because they just work better.
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provided the exhaust system is designed well.
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there's more pipework to think about, more space needed in the engine bay and maybe even coolant lines required.
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External gates are also easier to work on
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/ replace
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/ change spring pressures on etc.
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and you have the option of a screamer if you wish.
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If money
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/ labour
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/ time were no object, I would always say go external.
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But in the real world comes down to your skills and your project's goals.
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