A blow off valve, BOV for short, is a vacuum or electronically actuated pressure release valve present in engines with turbochargers. Blow off valves release excess boost into the atmosphere when the throttle is closed. The purpose of a blow off valve is to reduce turbo lag and protect the engine from compressor surge.

Compressor surge occurs when the throttle plate of a turbocharged engine closes and the pressurized air is forced back into the turbocharger compressor housing, causing the turbo to slow down. Compressor surge not only decreases the spool of a turbo but can also damage its center cartridge.

When a blow off valve vents compressed air, it emits a distinctive “pfsssst” sound. Some blow off valves are designed to amplify this sound with trumpet shaped vents. HKS produces BOVs with interchangeable “sound inserts” that allow the driver to customize the frequency emitted by the blow off valve.