During Saab-Scania's salad days of the 1970s and 1980s, its interests were varied and far-ranging. Aerospace, automotive, defense, and even computing were all areas of research within the realm of the quirky Swedish company. The benefits of turbocharging - originally a means to compensate for altitude on piston-driven aircraft - were well understood to Saab. The company became known as a bit of a pioneer in the relatively under-developed field of turbocharging cars in 1978 with the Saab 99 Turbo. The 99 saw to it that the company with fighter-jet heritage had cars with performance to match. The old Triumph-based "B-Engine" was suddenly a force to be reckoned with thanks to the addition of the little exhaust-driven turbine.
The Basics of APC Operating Principals:
Knock occurs when the temperature inside of a cylinder's combustion chamber becomes high enough to detonate the fuel/air mixture before intended by the spark plug - near top-dead-center of the compression stroke. Since forced induction naturally raises pressures, the compression ratio of a turbocharged engine must be low enough to ensure knock does not ever occur. This does not mean that a turbocharged engine cannot happily run at higher boost pressures or compression ratios, it simply means that the conditions must be right.
Basic turbocharging principals dictate fitment of a wastegate to allow excess exhaust gasses to bypass the turbocharger, thus preventing too much boost from entering the intake of the engine. It is operated by a pressure line from the compressor side which forces a diaphragm to open the wastegate once boost levels become too high.
1) Knock Sensor
2) APC "Brain Box"
3) Pressure Transducer
4) Solenoid Valve
5) RPM Sensor
The APC box is connected to a knock sensor (#1) which can detect if pre-detonation is occurring within the engine, and also to an RPM sensor (#5) to factor in engine rotational speed versus intake manifold pressure, as determined by the pressure transducer (#3). If the APC system detects knock, it will allow the wastegate to open sooner and reduce turbo boost to alleviate the problem. Because of this ability, this is why Saab was able to maintain a higher compression ratio in their APC-fitted turbocharged cars. Before APC, the ratios for turbo "B201" engines was 7.2 to 1. This increased to 8.5 to 1 with APC and, on the 16-valve "B202" engine, 9 to 1. APC can also control fuel delivery to an extent. When boost becomes excessively high, due to a stuck wastegate or misguided modding for example, it has the ability to cut fuel to the engine.
As the name suggests, Automatic Performance Control also controls performance - with the emphasis being on performance. It is not simply a means to prevent the engine from destroying itself, but also a way to vary tuning for the sake of economy, emissions, simple marketing strategy, etc. Saab's own SPG model 900 used a tweaked APC box (known in the Saab world as the "Red Box" to allow for a more powerful engine.) This is where things get rather complicated.
Exactly how the APC's "brain" works has been a part of Saab lore for decades now. To this day there are conflicting and diverging schools of thought on how a would-be Saab tuner can go about tweaking the APC for more power. They more-or-less all involve three circuit board potentiometers ambiguously labeled P, F, and K as well as some playing around with transistors and a soldering iron.
Though this author admits to having spent some time boldly tinkering with the APC box of his own 900 Turbo, this is not something for the weary or faint-of-heart. For those interested, I will direct you here but beware - more than once I've gleefully watched my Saab's boost gauge eagerly run into the red, only to have all the boost suddenly dump and the fuel delivery cut out and leave me coasting at 75 mph trying to restart the car. After a while, you may just find yourself tearing the box apart again to turn the screws back to their original settings, desperately hoping to regain the driveability of a normally functioning APC.
The APC system lived on in Saab turbos relatively unaltered until 1990. For the 9000 models only, which were fitted with direct ignition, DI/APC integrated the engine's ignition control into the mix. Therefore, the system was able to control both turbo boost as well as ignition timing to keep everything running smoothly.
The success of the APC system certainly didn't go unnoticed. Today, nearly all turbocharged cars have a similar system in place. Personally, this has allowed my cheap self to be running low-octane gas in not only my APC equipped Saab, but my Volkswagen with a 1.8T as well. Many Volvo enthusiasts have adopted and adapted APC functionality to their old 240 and 740 turbos.
Unfortunately, and no doubt thanks to years of General Motors ownership, it's been quite some time since we've seen truly great and unique innovations like this from Saab, but lest we forget, we must give a nod to Trollhattan.