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BOV

Car culture term

BOV
Photo: Alison Cassidy / Wikimedia Commons · CC BY-SA 4.0

What is BOV?

Blow-off valve. A valve that releases boost pressure when the throttle closes, preventing compressor surge in turbocharged engines. The famous "psh" sound.

BOV (blow-off valve) is a valve fitted to turbocharged engines that releases excess intake manifold pressure when the throttle plate closes. Without a BOV (or its closed-system equivalent, the bypass valve), the closed throttle creates back-pressure against the spinning turbocharger, which causes compressor surge (a chattering sound and potential turbo damage). The BOV vents this pressure to atmosphere.

The famous "psh" sound is a hallmark of aftermarket BOVs that vent to atmosphere. Factory BOVs typically recirculate pressure back into the intake (no audible sound) because emissions and intake metering require the air to stay in the system. Aftermarket atmospheric-venting BOVs are popular in tuner culture for the sound but cause check engine lights and fuel mixture issues on cars with mass air flow sensors.

BOV culture is associated with the broader Japanese tuner aesthetic. The HKS BOV was the iconic aftermarket BOV in the 2000s, with its distinctive Tiny Trumpet sound. Other popular brands include GReddy, Turbosmart, Tial, and Forge. The choice of BOV (sound, response, build quality) became part of the tuner identity even when the modification had no performance benefit.

Related terms

TurboWastegateJDM

Frequently asked questions

Why does my BOV make that "psh" sound?
Atmospheric-venting BOVs release the boost pressure to outside air, which produces the distinctive "psh" or "chu" sound on throttle lift. Factory BOVs typically recirculate the pressure quietly. The sound is purely a side effect of venting to atmosphere; some aftermarket BOVs are tuned for specific sound characteristics.
Is venting to atmosphere bad?
On modern cars with mass air flow sensors, yes. The MAF measures air entering the engine. If air enters and is then vented out, the ECU is fed incorrect information about how much fuel to add, causing rich running and check engine lights. Cars with manifold absolute pressure sensors (no MAF) are not affected.
What is the difference between a BOV and a wastegate?
A BOV controls intake pressure (downstream of the turbo). A wastegate controls exhaust gas flow (upstream of the turbo) by venting exhaust away from the turbine to limit boost. They are different components with different functions but both manage turbocharged engine pressure.
Do all turbocharged cars have a BOV?
All turbocharged cars have either a BOV or a recirculation valve. Without one, the turbo would suffer compressor surge every time the throttle closes. Factory cars typically use recirculating valves; aftermarket modifications may add atmospheric BOVs.
Will a louder BOV add power?
No. The BOV is purely a pressure relief valve. A different BOV provides different sound and possibly different response speed but does not increase engine power. Power-adding modifications are turbo upgrades, intake/exhaust improvements, and tuning, not BOV swaps.

BOV on the blog

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