Diverter valves often leak, it’s common knowledge. Regardless of the amount of air that escapes a valve, if any air escapes it is correct to say there is a leak.

Common tests used to detect a diverter valve leak include smoke testing the intake tract (read more on this here), removing the valve and pressurising the ports (usually
in conjunction with a bucket of water or some soap to find a leak), a vacuum pump on the top port or any combination of these.

When such tests are applied and indicate a leak coming from a GFB TMS valve however, what is often not understood is the scale of a leak, where it originates from, and whether or not it is detrimental to engine performance.

GFB TMS valve design criteria

GFB TMS valves are designed with maximum throttle response in mind first, followed by the ability to vent to atmosphere for noise (if desired) on cars that typically don’t allow atmosphere venting.

The piston-type valve design used by GFB satisfies the broadest range of design requirements and applications, which is why it continues to be used. For example, the dual outlet Hybrid design pioneered by GFB back in 1999 is only possible with a piston-type valve – dual outlets and 50/50 venting simply cannot be done with a factory poppet type valve.

Sealing a piston in a bore is simple, right? Use an O-ring and no air will be able to leak past – this is how hydraulic and pneumatic pistons and rams work. GFB TMS valves however, do not use an O-ring on the piston as is sometimes found on other brands, for two important reasons:

1. Minimum sliding friction is a key requirement for both optimum performance and successful atmosphere-venting operation. Friction in the piston travel can cause undesirable side-effects such as poor throttle response, compressor surge, idling problems, stalling and backfiring.

For optimum throttle response, the piston must be able to move freely in reaction to very small changes in pressure (the forces involved can actually be very small). O-rings create significant friction, no matter how loosely compressed, and worse still, the amount of friction varies greatly depending on the amount of lubrication, temperature, and how long the O-ring has been stationary (they tend to take a “set” after a while of not moving, and require significantly more force to get moving again).