Automatic Performance Control

Automatic Performance Control(APC) was the first engine knock and boost control system. The APC was invented byPer Gillbrandat the Swedish car maker SAAB.U.S. patent 4,372,119

SAAB introduced it on theturbo chargedSaab H enginesin 1982,and the APC was fitted to all subsequent 900 Turbos through 1993 (and 1994 convertibles), as well as 9000 Turbos through 1989. The APC was sold to Maserati to equip the carburetedMaserati Biturbo,with different settings for the Biturbo, and was known as the Maserati Automatic Boost Controller (MABC).

1. knock sensor
2. pressure transducer
3. control unit
4. solenoid valve

The APC allowed a highercompression ratio(initially, 8.5:1 as opposed to 7.2:1, and, on 16-valve variants introduced in 1985, 9.0:1). This improvedfuel economyand allowed the use oflow-octane petrolwithout causing engine damage caused byknock.

The APC controls boost pressure and the overall performance, specifically the rate of rise and maximum boost level - and it detects and manages harmful knock events.

To control the turbocharger, the APC monitors the engine'sRPMandinlet manifoldpressurevia a pressuretransducer,and uses these inputs to control a solenoid valve that trims the rate of rise of pressure as well as the maximum pressure by directing boost pressure to the turbocharger's pneumaticwastegateactuator.

To detect knock, apiezoelectricknocksensor(basically amicrophone) bolted to the engine block responds to unique frequencies caused by engine knock. The sensor generates a small voltage that is sent to theelectronic 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 turboboostpressure until the knock subsides. Knock events that are managed by the APC can be discerned when the in-dash boost needle "twitches" slightly. The APC unit has a 'knock' output where anLEDmay be connected. This LED will then light up if knock is detected. Because the knock sensor becomes less accurate at high revolutions, the APC tapers maximum boost pressure after approximately 4,500 RPM.

APC boost gauge

turbo/APCboost gaugein aSaab 900

SaabFull Pressure Turbo(FPT) models with this unit include the APC name displayed on a non-numeric boostpressure gaugein the instrument panel. 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.

The white area on the left side of the scale showsmanifold vacuumunder normal driving conditions, the short white dash is atmospheric pressure (engine off), the orange scale is where there is safe turboboost,the red scale is boost above 0.5 - 0.7barwhere the wastegate may be opened or afuel cutdue to overboost may occur.

Saab integrated the APC's boost control functionality with ignition control in 1990 with the introduction of theDI/APC system, available in 9000 models only. The DI/APC system managed knock not only by decreasing boost via a solenoid but by retardingignition timingas well; DI/APC also managed the engine's basic ignition timing.

See also

• Saab Information Display
• Trionic