BOV Spring Kits

Spring choice decides when a blow-off valve opens as the throttle closes, and how firmly it stays shut at idle and under boost. BOV spring kits and single springs let you match the valve to the engine’s boost and vacuum, or replace a spring that has lost its strength.

Normally the reference line carries manifold pressure to the top of the piston or diaphragm, so under boost the forces largely balance and the spring holds the valve shut. When the throttle snaps closed, the manifold drops into vacuum while the charge pipe is still under pressure, and that difference lifts the valve. Where a rating is given, it is in inches of mercury (inHg), roughly the vacuum at which the valve starts to open.

A soft spring opens on a small lift-off but can sit open at idle. On an engine with an air mass sensor, a valve venting to atmosphere then lets measured air escape, which upsets idle and fuelling. A firm spring stays shut at idle and part throttle but needs strong vacuum, so on a brief lift-off it may not open fully and the compressor can flutter.

Choose by valve model:

• Single springs: colour-coded yellow, white and blue (soft) springs listed for particular models, and a green spring rated at 15 inHg.
• Spring systems: complete spring systems for one valve family, in versions for the older and the current valve, so check which you have.
• Spring kit: a kit for one diaphragm-type valve.

Start from the valve maker’s suggestion for your boost level, then adjust. Go firmer if the valve hisses at idle or leaks under boost, and softer if it flutters or barely opens on lift-off. Change one step at a time and test under the same conditions. While the valve is apart, check the piston or diaphragm and its O-rings, and make sure the reference comes from the intake manifold after the throttle.