[quote author=Scruff link=topic=3319.msg32923#msg32923 date=1187603386]
You already have both of them fitted to your car. The wastegate is fitted to the turbo and the dump valve is fitted to your intercooler.
Alan
[/quote]
Yep, thats not 100% right Alan but very close
The Waste Gate diverts exhaust gases away from the turbine wheel in a turbocharger when the boost pressure limit is reached to avoid over boost. This causes the turbine to lose speed, which in turn reduces the rotating speed of the compressor. The primary function of the wastegate is to stabilize boost pressure in turbocharger systems, to protect the engine and the turbocharger. The wastegate is controlled by a wastegate actuator in which the actuator is controlled by pressure coming from the intake manifold. So far so good...
The Dump Valve sit between the turbo outlet and the throttle body. When transitioning from a boosted state to a closed throttle state (as in between shifts), due to inertia, the turbo continues to pressurize air, but the closed throttle prevents the compressed air from entering the engine. In this case the pressure exceeds the preset spring pressure in the dump valve and the excess pressure is bled off to atmosphere.
Even with a dump valve the compressed air acts as a brake on the turbo (slowing it down), because the pressure on the backside of the turbo is at a higher pressure than on the front side (and the air actually wants to flow through the turbo backwards).
A blowoff valve (BOV, sometimes hooter valve, dump valve) is a more elegant solution to this problem by allowing the turbo to "freewheel" when the throttle is closed (equalizing the pressure on both sides of the turbo). Unlike a dump valve a blowoff valve can be used at multiple boost settings without reconfiguration.
Blowoff valves are sometimes incorrectly called dump valves because they serve a similar function, but they are very different solutions to the same problem.
I hope I don't bore you with my technical mumbo-jumbo
The Subaru usually has a compressor bypass valve (CBV) also known as a compressor relief valve which is a vacuum-actuated valve designed to release pressure in the intake system when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor. Thats why you hear nothing or just slight whisteling noises when the original CBV is in place
I have a Baileys Blowoff Valve and it sounds great
I would go for it.
Axel