Blow off valves

I was wondering how effective a Blow off valve is? Does it just sound good or is there a noticeable difference in performance. Also what companies make them?

Thanks

I dont think it actually effects performance till you get to higher boost levels. Blow off valve just lets out the remaining gases the build up in the turbo and pipes so it doesnt get stuck there for when you hit the accelerator again. The built gas can cause the turbo to lag i guess. Best way i can explain it.

Unless you run higher boost, then all a BOV will do is make better sounds. And maybe even make your stock turbo last longer.

Oh, if you want one, call the people at RPMmotoring.com and they can help you get a good one, thats a cheap price.