ok i don't know much about turbos but i know the blow off valve is the thing that makes the Pshhh noise when you let off the gas right? If it is then when i finally get my STi can i put on of those on easily to get that sound? If so what brand is good and is it something easy to install and will it hurt anything?

 

To the best of my knowledge, the STI comes stock with a blow off valve or BOV. Most modern turbo cars have them. The purpose of the BOV is to relieve the pressure that is built up in the plumbing between the turbo and the throttle intake. When you take your foot off the gas, the throttle plate shuts off the intake and with the turbo still spooling, the pressure has to go somewhere which is out the BOV. If their wasn't a BOV, the back pressure would stall the turbo. Not only is this rough on the turbo but now it has to spool up all over again.

With that said, there is some merit to upgrading a stock BOV. Sometimes the stock BOV can't hold the pressure that the turbo is generating and it 'leaks'. This can rob you of some psi generated horsepower. Others, like yourself, like that BOV sound. Manufacturers make some BOVs that amplify the sound with a horn like shape on the BOV outlet. Stock BOV tend to muffle this sound in order to keep customer concerns to a minimum. I'm sure there will be quite a few options for aftermarket BOVs. I'd guess that Greddy and HKS will have them available for the STI as well and many others.

 

i thought it only had a wastegate..

 

The wastegate is a different component than the blow-off valve (or pressure relief valve or whatever the "stock" unit is that returns it into the intake). The wastegate is what regulates how much exhaust passes through the turbine housing and how much bypasses it. This is what determines your maximum boost pressure. (Well there are other factors too but this is the primary controlling mechanism.) Once maximum boost is achieved, the wastegate opens and bleeds some of the exhaust through the bypass. This prevents overboost.

 

yah.. isn't that a less sophisticated form of BOV?

 

No. They perform two separate functions and most turbo'd cars have both.

Let's review. When the car is accelerating under load, the turbo is rushing compressed air into the intake manifold. Then you lift off the accelorator to, say, shift gears. Just because the throttle plate slams shut doesn't mean the turbo suddenly stops spinning. It is still compressing air, and that compressed air now has to go somewhere else. That's where the BOV steps in; it relieves the pressure built up after the throttle plate closes. Otherwise the compressor blades would take the brunt of that pressure and take a hell of a beating. The stock unit in the WRX routes it back into the intake line before the turbo. Some aftermarket units simply vent it into the atmosphere. Some do a half-and-half sort of thing.

The waste gate is an exhaust bypass that routes exhaust around the turbine housing rather than through it, as a way of regulating boost pressure. This is achieved by a pressure-actuated valve or solenoid. Say you want a maximum boost of 15 PSI. Once you hit 15 PSI, the wastegate will then open by way of aformentioned valve/solenoid to allow some exhaust to bypass the turbine housing. Otherwise, if all the exhaust was still routed through the turbine housing even after maximum boost was achieved, the turbo would continue to build boost beyond the desired maximum.

Hope that clears things up.

 

got ya.

:]