Saab's APC is a restraining device used to ensure that the engine is always operating at peak boost for the current conditions of the engine.
It is comprised of 5 components:
- Knock sensor
- APC control unit
- Pressure transducer
- Solenoid valve
- RPM signal from ignition distributor
The basic purpose of the whole device is to control the Solenoid valve, which sits between the turbo output to the engine and the wastegate actuator. When 'open' the valve will direct pressure back into the air intake of the turbo system. This allows the wastegate actuator's sensitivity is to be set to an extremely low value so that the wastegate will open up very quickly.
Normally this would result in the turbocharger being effectively emasculated and becoming mostly useless, however this is where the APC comes in. During normal driving conditions the valve is about 95% open, directing most of the boost back into the air intake and away from the wastegate actuactor. During boost the valve is moved to some default duty (discussed later), this reduces the pressure bled off the wastegate actuator, allowing the wastegate to open up and slow down how quickly boost rises.
Upon reaching maximum boost pressure the Solenoid valve closes completely, directing all pressure to the wastegate actuator. This results in killing the boost provided by the turbo and keeps your engine from exploding.
If one were to crack open their APC unit there are three potentiometers which are used to control it's behavior. They are, from left to right "P", "F" and "K". They stand for pulskvot, fördröjning, and knackning in Swedish. In English you can think of them as controlling the boost curve, delay of boost, and engine knock sensitivity respectively.
Turning the "P" pot controls the default duty of the solenoid valve discussed earlier. The "F" pot controls the maximum boost at which the solenoid valve shuts off, and the "K" pot controls the engine knock sensitivity. One can tweak the "P" and "F" potentiometers to control the boost behavior of their turbocharger fairly precisely, moreso than most aftermarket turbochargers allow. The "K" pot should never be turned down as it is used to set the threshhold at which engine knock forces the solenoid valve closed. This is important to the keeping your engine from exploding part.