Well, it seems like I've answered the same question thirty times in the past week, so I figured I'd make a sticky post and leave it there for everyone to see....
Q: What does a boost controller do?
A: A boost controller allows the user to specify a certain boost level for their turbo.
Q: Are they okay to put on a WRX/STi?
Q: Why not?
A: All engines run best at a certain Air/Fuel ratio. Different cars run better at different AFR's, but forced induction engines (turbo and supercharged) usually require a richer AFR than naturally aspirated engines.
Your computer is programmed from the factory to control the boost that the turbocharger makes based on RPM and load (how hard the engine is working). It takes those two factors into account and determines how much fuel to put into the engine to reach a desired AFR. It also adjusts your timing to get the best performance while still avoiding knock (detonation).
When you raise your boost, you throw all those readings out of whack. Your ECU reads throttle position and load, and determines how much boost and fuel should be going into your engine to reach the desired AFR. If you use a boost controller (either manual or electronic) you are overriding the ECU's control of boost. The ECU thinks the boost has not changed, but in reality it has. So, it puts the same amount of fuel into the engine that it would put in if it were still in control of boost. However, you are putting more boost (more air) into the engine, which creates a leaner AFR. Lean AFR's are the #1 cause of detonation, and detonation is the number one cause of engine failure.
Q. What about a downpipe? Doesn't that raise boost?
A. Yes, it does. But it only raises peak boost slightly, and only under full load. A boost controller allows full boost at any load and RPM. Your ECU runs richer at full load and high rpm to take extra precautions against detonation.
Q. What is Partial Throttle Full Boost?
A. Partial throttle full boost is the biggest danger associated with boost controllers. We have already discussed how the ECU regulates boost based on rpm and load. A boost controller does not factor load or rpm into the equation. It works on pressure only. It will allow your turbo to reach full boost at 3000 rpm or 7000 rpm. And, it will allow it at 10% load or 100% load, or anywhere in between. At full throttle it may be okay, because the computer is still adding the fuel it thinks the car needs. But, at partial throttle it is adding very little fuel, because it thinks the car is only making a small amount of boost, which leads to extremely lean conditions. I have seen AFR's in the 19:1 range pretty commonly with a boost controller at partial throttle.
So, just say no to boost controllers.
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