This is What an FCD is:
An FCD is a small device, which essentially acts as a me chanical air pressure switch. A small ball bearing within the valve is held, under pressure, onto a seat by a spring. Air pressure acting on the ball causes it to lift off its seat once a preset pressure has been reached, as determined by the spring pressure on the ball – which can be adjusted of course. Once the ball has lifted off its seat air can flow through the valve. As the air pressure acting on the ball reduces below the spring pressure, the valve closes preventing air flowing through the valve. In essence no air passes through the valve until the spring or ‘cracking’ pressure has been reached.
For this application, this means that the ECU (Engine control Unit or Engine management unit) sees only up to the pressure you set the valve to. This means that if your standard ECU had a factory set “boost cut” at 14.7psi (very common), then adjusting this valve to suit will mean that at any pressure above, say, 14.5psi, the ECU will only ever see 14.5psi as the valve will “leak” air off any air above that pressure.
The advantage with the GBE FCD is that UP TO THE PRESET PRESSURE, the ECU sees the correct reading, so you are unlikely to have detonation in the part throttle or low boost ranges. This can ONLY be a good thing!
Fitting Instructions are HERE