Blow off valve is a pressure release system present in turbocharged engines. Its purpose is to prevent compressor surge and reduce wear on the engine, and releases the air into the atmosphere instead of recirculating it. The blow off action produces a range of distinctive hissing sounds, depending on the exit design. Some blowoff valves are sold with a trumpet shaped exit that intentionally amplifies the sound. The TurboXS model RFL blow off valve is well known among tuners for this kind of design and some turbocharged vehicle owners may purchase a blowoff valve solely for the auditory effect even when the function is not required by normal engine operation.
Blow off valve is 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.
Blow-off-valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, manifold vacuum without pressure develops in the intake manifold after the throttle plate and "sucks" the blow off valve open. The excess pressure from the turbocharger is vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.
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