Hey folks! I'm about to purchase an Audi A3 in the next few days and of course I'm spending countless hours researching modifications I can do to it once I get the keys. A little auto history of mine, I've only owned V8's. I currently have a 1966 Mustang Fastback and a 2001 Camaro SS. I plan on selling the SS and getting this A3. The V8 bug has been scratched and ever since my German friends got an A3, I fell in love. Anyway.... about this blow off valve thing... I've read that an aftermarket one can cause you to lose some power. I've also heard that it won't. Basically, I love the turbo sound and would like a loud blow off valve but I'm curious if it does in fact cause a loss of power. Go easy on me... After all, I've been a V8 guy all my life.

 

Audi uses a diverter valve which reroutes excess boost back into the intake system, not to the atmosphere like a BOV. This air has already been measured by the MAF, so leaking it for "that cool turbo sound" is actually messing with air/ fuel ratios, even if for a split second until the computer compensates for the sudden (unanticipated) drop in the amount of air in the intake.

A BOV can be use with the correct set up and tune to run it, but if you just want it for the sound, get a Neon SRT-4.

 

Read about diverter valves.

 

I installed the latest Forge BOV (~ $250) for my wife's car per her request and had no check engine lights or performance loss so far. Car has been running fine for a good year since that "upgrade". I did not notice any power loss or gain.
The only other upgrade I recommend is a cold air intake. The standard intake sucks in too little air and is not "cold". The BOV sounded much better with the extra air from the CAI and I did notice a slight performance increase from that.

In regards to the previous comments:
There are (it seems like) millions of threads and articles about the BOV and the closed system Audi is using (and other manufacturers). I could never find enough evidence that it's bad or harmful to the engine - but if not properly installed or an aftermarket manufacturer didn't do its job right it will throw a code.
Anyhow I took the risk and so far had no issues. I also installed an oil catch can (again might be a gimmick not worth paying for) for a peace of mind.