Automatic Performance Control (APC) is a system that was introduced on turbo charged Saab H engines in 1982.
APC is a knock-sensing system which allowed a much higher compression ratio (9:1 as opposed to 7.2:1) and boosted performance from 145 hp (108 kW) to 155 hp (116 kW). This system also produced 20% better fuel economy and allowed the use of low-octane petrol without causing engine damage.
A knock sensor (basically a microphone) detects engine knocking and sends its signal to a control unit. The control unit combines this signal with the engine RPM and inlet manifold pressure. In case of knocking, the control unit drives a solenoid valve that opens a wastegate to bypass exhaust gases from the turbocharger, lowering turbo boost pressure to a safe level.
