Blow Off Valves

Under boost, the charge pipes and intercooler are full of pressurized air. When the throttle closes, that air reverses toward the compressor wheel, causing compressor surge, a fluttering sound that stresses the turbo. A blow off valve senses the change to manifold vacuum and opens, releasing the trapped air so the turbo can keep spinning freely and boost returns faster on the next throttle input.

Blow off valve vs bypass valve

A blow off valve vents air to the atmosphere. A bypass valve recirculates it back into the intake before the turbo. Both protect the turbo from surge. On MAF-equipped engines, the MAF has already measured that air, so venting it commonly causes a brief rich condition. Bypass and hybrid valves avoid or reduce this. See our turbo bypass valves for recirculating designs.

Hybrid blow off valves

Hybrid or dual-port valves can vent part of the air and recirculate the rest, often with an adjustable split. They're a common choice for owners who want some BOV sound on MAF-equipped engines while limiting drivability issues.

• Choose direct-fit or universal. Direct-fit valves replace a factory valve or bolt to a vehicle-specific pipe or adapter. Universal valves mount to a weld-on flange on the charge pipe.
• Check your engine management. Speed density engines typically handle vent-to-atmosphere valves without issue. MAF-equipped engines commonly need a hybrid or recirculating valve, or tuning adjustments.
• Size the valve. Higher boost and larger turbos move more air. A valve too small to vent quickly can allow surge. Some high-airflow setups use two valves.
• Choose piston or diaphragm. Piston valves are common for durability and quick response. Diaphragm valves are commonly sensitive and smooth-acting.
• Set the spring correctly. The spring must keep the valve closed at idle vacuum and under boost. Adjustable valves let you fine-tune this for your boost level.
• Plan mounting and reference. Mount the valve close to the throttle body for fastest response, and connect the reference line to a manifold source after the throttle.

Common blow off valve problems

Leaking at idle is common when the spring is too soft for your engine's idle vacuum. On MAF engines, this lets unmetered air in, which can cause idle hunting and lean codes.

Stalling or stumbling on shifts commonly comes from venting metered air on a MAF-equipped engine.

Flutter usually means the valve is too small, too stiff, or not opening fast enough.

Opening under boost points to a spring that's too soft for the boost level.