The wastegate is a valve inside the exhaust housing of the turbo. It is controlled by pressure from the intercooler piping. When the pressure in the intercooler piping gets high enough, it pushes a rod that is hooked to an arm. The arm connects to a valve inside the turbine housing of the turbo. When the valve opens, it allows the exhaust pressure to bypass the turbine wheel. This means less exhaust pressure to spin the turbo. The turbo speed and therefore the boost pressure is regulated by the wastegate and the rest of the boost control system.
Internal wastegate valve open:
Internal wastegate closed:
An aftermarket external wastegate is used with large turbos. You will usually find it hanging off the exhaust manifold. Larger wastegates don't make more power. They allow you to keep control of the turbo speed of a big turbo better:
A Blow Off Valve, Compressor Bypass Valve for your purposes are essentially the same thing. When you are mashing on the throttle, the turbo is pumping air at full force. The throttle is open, the engine is taking all the air in that it can. Everything is good. When you let off the throttle, everything stops dead. The turbo gets bogged down trying to pump the air into a closed throttle plate. The air has nowhere to go and tries to go back out the turbo as the turbo is trying to pump more. The turbo gets slowed down in addition to all kind of bad forces acting on the compressor wheel.
So starting in the early 80s manufacturers started putting a valve that would open when there is a certain level of vacuum in the intake manifold. The valve opens when you let off the gas allowing the turbo to kind of freewheel. This maintains the turbo speed better so that you have more boost when you get back on the throttle.
This is going to come up so I might as well answer it here:
The Holy Grail of anyone under 25 years old with a turbo car is to have this excess pressure release heard by other kids under 25 years old. High school boys are particularly impressed by this PH-Shhhhh sound. Word is that their high school girlfriends will leave them and come and jump into your car. Cools stuff indeed.
You can spend time and money trying to achieve this sound by taking off the factory BOV and replacing it with an aftermarket one. The aftermarket one will need to blow the air out so that it can be heard by the high school boys. You just cap off the hose where it was blowing back into the turbo inlet pipe.
On Mitsubishis this creates several issues. The ECU has seen all the air enter the system since it was measured by the air flow meter. The ECU gives you the right amount of fuel for all this air. If you blow a grip of air out for the high schools boys to hear, the ECU has no idea you blew it all out. It still gives you fuel for the air you let out. The car goes rich and almost stalls or bucks. The high school girls do indeed get wet though. It is a trade off like all things in life.
The answer is to tighten the aftermarket BOV so it does not blow off as much air so that the motor stalls. Now you hear this turkey gobble noise like “cha-chooo-chooo-chooo”. Think any HKS BOV you have heard. That is the sound of compressor surge destroying the turbo. Congratulations! ;-)
But sp4rkl3bunn3y on the forums says he runs his car blowing out and it does not stall. Well sp4rkl3bunn3y has a stock turbo running low boost and the little compressor surge he gets isn’t going to hurt much for now. He thinks the compressor surge sound he is hearing is a cool BOV noise. Higher boost and a real man sized turbo will make it undriveable.
Also on your RalliArts the way that the tranny is always up shifting and the ECU has more control over the throttle than you do I doubt you would hear much.
With the Mitsubishi drive by wire system there are a bunch more checks and balances with what air flow the air flow meter should be seeing at any given RPM and throttle position. I have seen so many EVO X guys fight SES lights and limp modes with various aftermarket BOVs. It is just another big mess to deal with. Even if you are recirculating the air back in, if the BOV is looser or tighter and allows different than expected air flow past the air flow meter, it is just misery.
Stock Blow Off Valve/Compressor Bypass Valve
I was looking at the stock BOV and saw that it was plastic. I thought that it might leak under higher pressure. I noticed that a stock EVO X BOV would bolt right on! SWEET!
But I looked inside the two BOVs and saw that they both had an o-ring seal on the valve seat. I tested the spring pressure and they are both the same. Changing this stock plastic BOV on the stock turbo will not make any power gain. If you will be running over 30 psi someday, then think about changing the BOV. Otherwise a different BOV is for nothing other than vanity or headaches.
Mike W