The APC system listens for knocking and retards the boost in increments of 1.5 psi until the detonation goes away and attempts to bring it back as much as possible until detonation is detected then retards the boost again. This prevents the engine for being damaged by knocking and pinging while allowing the maximum boost possible. Neat trick huh?

The control unit receives signals from several transmitters, and in turn processes those signals and uses the data to adjust the charge air pressure. It does this by actuating the waste gate with an electrically operated solenoid valve tapped into the pressure line to the waste gate. The following sensors are used by the control unit to determine charge air pressure: The main sensor is the knock sensor. The knock sensor is a piezo electric sensor that is used a lot like a microphone to listen for detonation. There is a pressure transducer to sense the pressure in the intake manifold upstream of the throttle plate. The APC system also uses a signal from the ignition to determine engine RPM.