PeteW

What does a BPV do, and changing it to an aftermaket one, is that good preventative maintenance?

Lucid

No one is going to read this or answer because it has been covered many times before. Search the archive. I'll give you the summary to save you time. A bypass valve allows the air to recycle when the throttle plate closes (e.g. when you let off the gas). Vacuum pressure inside the engine opens the valve and diverts the compressed air back into the air intake, forming a loop. This keeps the turbos spooled longer and protects the turbo from backflow which can stress the turbo.

The question of aftermarket valves is a good one. Many people like the piece of mind of the aluminum, maintainable parts. Are they worth $300? Maybe for your piece of mind and knowing that you can't do any better, the answer is yes. However, unless you are running more boost, your valves are probably adequate. There are cases where stock Bosch valves tear, but I think that might be in older model S4's. You can buy better quality Porsche or TT valves for about $50 that are also made by Bosch, and will serve your purpose. You might like the aluminum valves though if you like to meticulously maintain your car. You can open them up and lube the spring or put in a stiffer one. I have Forge valves and have noticed a difference with my chipped S4. I am rewarded by sustained power while shifting up, and it feels like I my turbo lag has been reduced. I could just be imagining things though.