If the throttle is closed while the engine is overrunning, back pressure will develop in the compressor housing due to the constant pressurization. This back pressure significantly reduces the speed of the compressor impeller, with the ultimate result of reduced boost pressure and increased turbine lag. To avoid this, the turbocharger shunt air valve is opened by an electric actuator. The valve opens a bypass pipe that allows compressed air to rotate through the compressor blades back to the intake side of the compressor loop. In this way, the required turbine speed is maintained. When the throttle opens, the turbocharger shunt air valve closes and the booster pressure is immediately available. This Part Fits the Following Vehicles Make Model Submodel Engine Year Chasis Porsche All GT2 All All 996t Porsche All Turbo All 9442 Porsche All Turbo All 964 Porsche All Turbo All All 993 Porsche All Turbo All 993T Porsche All Turbo 3.6 All All 964 Porsche All Turbo S All All 996T Make Model Submodel Engine Audi All All 1.8T Audi B5 S4 ALL 2.7T Audi C5 A6 Quattro 2.7T Audi C5 Allroad Quattro 2.7T Audi TT MKI FWD 180HP Audi TT MKI Quattro 225HP Volkswagen 337/20AE ALL 1.8T Volkswagen Golf IV All 1.8T Volkswagen Jetta IV All 1.8T Volkswagen Jetta IV GLI All 1.8T Volkswagen New Beetle All 1.8T Volkswagen Passat B5 4 Motion 1.8T Volkswagen Passat B5 FWD 1.8T