I've think it's because of how the ECU is tuned, I believe the car has an Air Mass Meter like mine. I'm not exactly sure about the S4 about the performance problems it may have or not have at all with a blow off valve. I guess it's possible since I've seen one company carry them and there are people with them who say that it hasn't affected drivability. Others say that the BOV letting air into the atmosphere causes the engine to run rich. One guy I read got a whole new air mass meter system and allows BOVs to work on the S4.

It's not good to put a BOV on MY car because then the AMM will give a weird reading of the amount of air coming in therefore giving a check engine light and when the CEL comes on my cars only goes half boost and slows down drastically. Basically the air instead of going back into the intake rather goes to the atmosphere therefore messing up the AMMs original reading of the air that was in the intake pipe. It measured air which doesn't exist anymore therefore the ECU goes crazy and gives a CEL.