Automatic Performance Control
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Automatic Performance Control (APC) is a system that was introduced on turbo charged Saab H engines in 1982.
2. pressure transducer
3. control unit
4. solenoid valve
APC is a knock-sensing system which allowed a higher compression ratio (8.5:1 as opposed to 7.2:1). This improved 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.
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.
