What is that sound?

[TcEnthusiast]

Some cars in my area have this sound 'shrrrsh' when they shift. If you don't know what I'm talking about then, visit this page and click on 'WRX' video.

HKS USA: Hi-Power Exhaust

That video has it on there wrx. I was just curious what it was because I hear it alot in my area when people shift gears.

[Redneck4csa]

Ok. Well, I can't watch the video but I'm pretty sure I know what you're talking about. I am a big Subaru fan and plan on purchasing an 06 STI some day. Anyway, I'm sure the sound you are referring to is a blow-off valve. That is because the cars you hear making that sound are turbocharged. A blow-off valve is a vacuum actuated valve that releases the pressure in the intake to ensure you don't get compressor surge. Compressor surge will force the pressure to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The loud ones that you really hear are the vent-to-atmosphere. The blow-off valve just releases the excess boost under the hood in that case. However, what alot of people don't realize is that a blow-off valve with the stock turbo on a WRX or an STI will cause it run lean.

Now, this is not to be confused with a bypass-valve. A bypass-valve will recirculate the the unused boost back into the intake before the turbocharger, but after the MAF.

Hope this helps.