Why do people replace the stock bov(recirculation valve)? Is there an improvement from doing this modification? Does it increase performance? What's the point of replacing it :confused:

The stock CBV (it's not technically a BOV) leaks at higher boost pressure. Going with something more aggressive generally gets you less leakdown, too. Then there are the people who are going for sound; they're the ones that get BOVs. :rolleyes:

When you say "at higher boost pressure", do you mean above 10 psi?

Yes. Sometimes sooner. :( Depends on the shape of the CBV you're starting with, but I (personally) think that Bosch 0280142108 (http://www.socalsupras.com/tuning/bypassvalve.html) is a good investment for somebody that's not going to be running lots of boost (over 1 bar). That link even has a .WAV sound file to listen to it operate. ;)