<P> A blowoff valve, (sometimes "hooter valve" or BOV) performs the same task but releases the air into the atmosphere instead of recirculating it . This type of valve is typically an aftermarket modification . The blowoff 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 . 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 . Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere . </P> <P> Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine . The sound produced is called turbo flutter (the slang term' choo - choo' is sometimes used). 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 resulting collision of pressure waves creates an effect similar to cavitation producing the unique noise . </P> <P> A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate . When the throttle is closed, the relative manifold pressure drops below atmospheric pressure and the resulting pressure differential operates the blowoff valve's piston . The excess pressure from the turbocharger is then vented into the atmosphere or recirculated into the intake upstream of the compressor inlet . </P> <P> In the case where a mass airflow sensor (MAF) is used and is located upstream from the blowoff valve, the engine control unit (ECU) will inject excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements . The engine then briefly operates with a fuel - rich mixture after each valve actuation . </P>

Where does the blow off valve connected to