Automatic Performance Control (APC) is a system that was introduced on turbo charged Saab H engines in 1982.
1. knock sensor
2. pressure transducer
3. control unit
4. solenoid valve
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 an electronic 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.
turbo/APC boost gauge in a Saab 900
Saab Full Pressure Turbo (FPT) models with this unit include the APC name displayed on a non-numeric boost pressure gauge in the instrument panel. Although knock sensors are common even on non-turbocharged engines today, Saab has continued to use the APC name prominently as a differentiating feature.
Automatic Performance Control