WRX STI will have 2.5T ???

[vrg3]

Our blowoff valves dump the extra pressure back into the intake already.

Some dump it out to the atmosphere, but very few stock configurations are that way.

[ciper]

Dumping to the atmosphere will create a temporary lean condition. The only advantage to an atmospheric dump is the noise. I havnt heard of any turbo vehicle that functions this way stock, only pop off valves usually work this way (pop off and blow off are two different items btw).

[vrg3]

Wouldn't it create a temporary rich condition? The engine would be ingesting less air than the MAF reads, right?

It's possible to have an atmospheric blowoff valve that doesn't result in this transient bad fuel mixture, though; you just need your ECU to know about it and correct for it. The ECU already accounts for other types of transient behavior (for example, when you first open the throttle quickly, it dumps in a little extra fuel to compensate for what would otherwise be a temporary lean condition with bogging).

Most people buy atmospheric blowoff valves for the noise, but there is also the advantage of simpler plumbing.

[ciper]

Yeah you are correct. I was thinking rich and wrote lean

You are right that it can be tuned, but why do it? Personally Id rather not have any noises coming from the car.