well for one thing a BOF is not needed but recomended. and not just for the pretty pshhhh sound it makes.
actually it pretty much is needed if it wasn't there you could really mess things up
Does anyone understand what a blow off valve really does? It is used to vent excess boost from bulding up when the throttle plate is closed (when you shift or you're sitting idling) and potentially blowing your Intake
pipe up (although it would take a TON of boost buildup to actually make something burst). The idea is that the BOV lets the pressurinzed air out of the "hot" Intake
pipe and recirculates it into the "cold" Intake
pipe (before the Turbo). This effectively lets all the boostout of the Intake
tract and prevents any potential "explosions".
Now, venting your BOV to the atmosphere (that "PSSSSSSSH" sound) is an excellent way to a) attract attention to yourself and b) confuse the shit out of your ECU. When the MAF reads how much airflow is entering the engine, it sends a signal to the ECU, which takes this airflow reading and calculates how much fuel the motor will need to effectively and efficiently use the air. Here's the problem:
If the MAF reads the airflow as "10", and then you blow "2" of that airflow out the BOV and into the engine bay, the ECU will send "10" fuel to match with the "10" air that it thinks it's getting, but in reality there is only "8" air.
Therefore: You have "10" fuel and "8" air, when both quanities should be equal. The condition of more fuel than air is known as "rich".
Now, if you are an attention whore, or you just really like the "psssssshhh" sound, it is possible to atmospherically vent successfully, without any ill effects. This is done by either:
a) Placing the MAF downstream from the BOV, so that the MAF is reading exactly the amount of air that engine will recieve.
b) You can compensate for the air that gets vented through tuning, but this only works with a standalone ECU like the PowerFC or the AEM
Here's something else to consider:
At 5psi, your BOV will be quite quiet. The more boost you run, the more air gets blown out of the BOV, and the louder the sound is. You won't even really need a BOV until you're running 10psi, something you'll only be able to do once you have lowered the engine compression and changed all of the stock internals.
Of course, there are also acoustically tuned BOVs that are ridiculously loud and obnoxious. TurboXS makes one, its's called the Type-H RFL (Really Fucking Loud)http://turboxs.com/_images/Video/txsbov.mpg