The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo's compressor from going into surge. When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. Surge can eventually lead to thrust bearing failure due to the high loads associated with it. Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure, helping to eliminate surge. INCLUDE: 1 x WRX Type blow off valve FEATURES: Draw in surrounding air and release a unique discharge sound High quality 1.00" brass piston with teflon seal 2.00" Diameter housing (2.25" Overall width) Holds up to 30psi boost pressure High-quality diaphragm and short stroke significantly increase response NOTES: 100% Brand New Professional Installation is Highly Recommended Installation Instruction is NOT Included