Blow Off Valves

A blow-off valve (BOV) releases boost pressure from the charge pipe when the throttle closes. Without it, the pressurised air has nowhere to go and pushes back against the turbo’s compressor. This category covers complete valves, replacement springs and spring kits, O-ring and diaphragm kits, weld flanges and adapters.

The main choice is where the released air goes:

• an atmospheric valve vents to the air and makes the familiar sound;
• a plumb-back, or recirculating, valve returns the air to the intake before the turbo, quietly;
• a dual-port valve splits the flow between the two.

On engines with an air mass sensor, air vented to atmosphere has already been measured, so the engine can run rich for a moment or stumble as the throttle closes. That is why plumb-back or dual-port valves are the usual choice there. Some valves are made to replace a particular factory diverter valve, including electronically managed versions for cars whose original valve is controlled by the ECU. Others are universal and mount on a weld-on flange.

The spring has to keep the valve shut under full boost and let it open when the throttle closes. If the valve leaks under boost or opens early, a firmer spring from a spring kit is the usual fix.

A universal valve needs a weld flange on the charge pipe, usually close to the throttle body, and a vacuum reference from the intake manifold after the throttle. Check the couplers and clamps on the charge side too, as a valve that holds pressure better will show up any joint that was only just holding.