What is a blowoff valve and what does it do?

My roommate is all into the blowoff valve that he wants to put into his 1.8t jetta. he keeps making this sound.. it somewhat resembles the sound a turbo makes.. that hissing sound.. but anyway, what is it?

 

A blow off valve releases residual boost pressure between shifts or when the engine sees vacuum. It eliminates what is called compressor surge (can't really define it more than that... it has something to do with a shockwave going through the compressor blades), and also minimzes turbo lag so you can reach full boost after a shift faster.

There are ways of minimzing that "swish" sound though. If you hear the loud swish then the boost is most likely venting to the engine bay. You can route a hose from the vent tube off of the blowoff valve back into the intake... this way you will get all of the benefits of the blowoff valve but be more stealthy.

 

[QUOTE]Originally posted by RWD_RCKT
[B]A blow off valve releases residual boost pressure between shifts or when the engine sees vacuum. It eliminates what is called compressor surge (can't really define it more than that... it has something to do with a shockwave going through the compressor blades), and also minimzes turbo lag so you can reach full boost after a shift faster.

 

Does it increase performance any? or is it more the sound?

 

It increases performance also...but as stated above you have to have a Turbo. (I think)

 

In response to performance or sound, the blow-off valve was designed for a reason, and it wasn't because it sounded cool. That may be why some (many) people throw them on their turbo cars (and even on SC cars ), though. I think the reason why it helps with performance is that backpressure built up while shifting, or letting off the gas is removed so that the air can freely flow out of the engine towards the turbo when you get back on the gas. It doesn't have to force all of this backpressure air out of the way (through the turbo) first. Either someone explained it to me this way or I'm just making it up. Either is possible.

 

Wes, we just think you're making it up...

 

[QUOTE]Originally posted by Red Blur
[B]Wes, we just think you're making it up...

 

ok.. i just want to make you all away, that i wasn't planning on putting it in my car... my roommate has the 1.8t and he is all talking about it and making the sound.. i didn't want to ask him what it is all about.. and sound stupid.. so instead.. i will voice my question here.. to sound stupid... but better be stupid here... than my roommate.. nice huh...

 

Blow off Valve, Bypass valve... different terms for the same equipment many times... I use an HKS blow off valve, which I guess would be referred to as a Bypass valve, since it has to keep the mass air flow sensor happy...