Automatic Performance Control

The sensor generates a small voltage that is sent to the electronic control unit, which processes the signal to determine if, in fact, knock is occurring.

If it is, then the control unit activates a solenoid valve that directs boost pressure to the turbocharger's pneumatically controlled wastegate, that opens to bypass exhaust gases from the turbocharger directly to the exhaust pipe, lowering turbo boost pressure until the knock subsides.

Knock events that are managed by the APC can be discerned when the in-dash boost needle "twitches" slightly.

Because the knock sensor becomes less accurate at high revolutions, the APC tapers maximum boost pressure after approximately 4,500 RPM.

Although knock sensors are common even on non-turbocharged and turbocharged engines today, Saab has continued to use the APC name prominently as a differentiating feature.