A blowoff valve (bov) is a pressure release system present in turbocharged engines. The purpose of the bov is to prevent compressor surge and reduce wear on the engine. GReddy bov's are easily adjustable to eliminate compressor surge and preventing premature boost leakage, while increasing boost response between shifts. Each bov is made of durable cast and billet aluminum frame and the valve to diaphragm ratio provides performance that cheap piston types cannot offer. The spring stiffness adjustment screw on the bov and the three different sizes give the GReddy user the option to match vehicles with mild upgrades to heavily tuned engines. There are many factors in matching a bov to an application, valve location, turbocharger size and pressure, but as a general rule of thumb, we recommend the Type RS bov for 100-400hp, Type R bov for 260-450hp and the Stiff Spring Type R bov for over 400hp applications. Also available in easy to install kit form for many turbocharged vehicles.
How Blow-Off Valves Work
A compressor bypass valve (CBV), also known as a compressor relief valve or diverter valve, is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged car 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.
A blow-off valve, (BOV, sometimes hooter valve, not to be confused with a dump valve) performs the same task but releases the air into the atmosphere instead of recirculating it. The blowoff action of the bov produces a range of distinctive hissing sounds, depending on the exit design. Some bov are sold with a trumpet shaped exit that intentionally amplifies the sound. The TurboXS model RFL bov is well known among tuners for this kind of design and some turbocharged vehicle owners may purchase a bov solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled bov to the atmosphere. In the United States, Australia and parts of Europe cars featuring unmuffled bov are illegal for street use.
Bov are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor. The compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and imparts severe turbo lag if the driver accellerates immediately after the surge event.
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