blow off valve

i was wonderin what the blow off valve does. i would get it just for the sound and i know it releases the pressure from the turbo. but how does this help?

when you close the throttle plate the air rushing towards the engine can't get in. so it would normally build up in the piping and kinda jam the compressor wheel. this would lead to problems over time. so the blow off valve just sits between the turbo and engine venting the incoming air whenever the intake manifold gets a pressure significantly lower than that of the pipe the b.o.v. sits on.

Sits between the turbo and the exhaust piping out of the turbo

Sits between the turbo and the exhaust piping out of the turbo

Thats a waste gate not a BOV.

Anyways when the throttle plate closes under boost it creates alot of pressure between that and the turbo which can bulid up on the compressure and slowing it quickly which isn't good for bearings and exhaust pressure i'd think... the BOV lets off the excess pressure in order to protect the Turbo. Some say a BOV isn't really nessisary.

Bypass valve (recirculating BOV):
http://hem.bredband.net/b132378/annat/bypass.gif
http://hem.bredband.net/b132378/annat/overview.gif
Note that solenoid valves for bypass valves are quite uncommon but it can help reduce leaks in the bypass valve under boost.

Not to highjack the thread but with my buddies Saab 9000 turbo you drive around and you can heard the B.O.V.. But when when in neutral and you rev it up you can't heard it. Im just wondering why you can't hear it.

something about the turbo not producing boost b/c there is no load on the engine.

Dead on, this was a topic of discussion recently in the turbo forum. Turbo's don't produce boost without load on the engine.