Blow Off Valves

When the throttle snaps shut on boost, the compressed air between the turbo and the throttle has nowhere to go. A blow off valve opens on the change in manifold pressure and lets it out, sparing the compressor the surge of air pushing back against it. The decision that matters most is where that air goes.

Atmospheric or recirculating

An atmospheric valve vents to the open air and makes the familiar sound. A recirculating (bypass, or suction return) valve sends the air back into the intake ahead of the turbo, as most factory bypass valves do, and is quieter. Some designs split the charge, venting part and returning the rest.

Why air flow meter cars can dislike venting

Many Japanese turbo engines meter their air with an air flow meter ahead of the turbo. The ECU fuels for every gram of air the meter has counted. Vent some of it to atmosphere and the engine is fuelled for air it never receives, so it runs rich for a moment as the throttle closes; the meter's output can also swing as the air surges. The result can be a stumble, a hesitant idle or a stall when coming to a stop. A recirculating valve keeps the counted air in the system and avoids it. An engine metered on manifold pressure (a MAP sensor) has no such count to upset.

That is why HKS sells a Suction Return kit for its Super SQV: the air returns to the intake as with the factory valve, with less noise from the valve and more from the air cleaner.

Choosing one

• Check how your engine meters air, and whether the valve can be returned to the intake on your setup.
• Vehicle kits from HKS, GReddy and Blitz include the flange and piping for a given car; a universal valve needs its own adapter.
• Match the spring to your boost, so the valve stays shut at idle and under boost and opens on lift-off.

Adapters, return pipes and springs are in BOV option parts, with the rest of forced induction, including boost controllers.