Saab remained a relative 'small' car manufacturer, having developed a fine reputation for cars of quality, starting production of the front-drive in-line four-cylinder 99 series in 1969
, from which the longer, more luxurious 900 range was derived.
Whereas the originally announced 900 three or five-door saloons (and Combi Coupe) were virtually hatchbacks, 1980
saw the addition of 900 saloons with four-door 'three-box' bodywork incorporating a vast and separate boot of some 14.61 cu. ft (414 liters) capacity.
The big Saab novelty for 1980 however was APC, or Automatic Performance Control, an intriguing futuristic device. SAAB had correctly predicted that motor fuel octane ratings would vary dramatically in the forseable future, being inclined to fall rather than to rise.
Octane numbers denote the anti-knock (or anti-ping) qualities of various fuels, normal petrol around the 1980's being from 89-92, with Super at 97-99 octane. The lower the figure the greater the risk of knocking, (Iow-compression engines react better to low octane numbers than high-compression units) due to poor combustion of the mixture - a situation that cannot be remedied by automatic ignition.
The Per Gillbrand Solution
Saab's Per Gillbrand developed his idea of a system that would adapt the engine automatically to the petrol's octane rating, enabling it to function in the most favourable thermodynamic mode. Starting point was to regulate the engine's function to the limit of knocking, but not by the traditional method of advancing or retarding the ignition.
Saab's Automatic Performance Control System was applied to turbocharged engines, and acted upon the blower pressure via the wastegate. An engine block sensor followed the working conditions of the motor, recording every knock caused by high loadings. The sensor sent signals to an electronic unit which passed instructions to a solenoid valve located adjacent to the valve controlling charging pressure delivered by the turbocharger.
The electronic unit was also in communication with the pressure sensor on the inlet manifold and could thus balance the charging pressure to the engine, so that it will always be exact. There were nine pulses per second in the Saab APC system, enabling the engine to adapt to petrols from 91 to 99 octane ratings, and amazingly, it permitted the compression ratio of the turbocharged engine to be raised to 8.5: 1 (from 7.2: 1). The high compression ratio, of course, gave greater efficiency, and at the same time a fuel consumption improvement of some 8 per cent over then current model Saabs. Yet another advantage was that, without increasing the rated power of the turbocharged engine, APC boosted, just briefly, the supercharge pressure which resulted in enhanced acceleration. The momentary power increase was claimed by Saab to be in the region of 20 per cent, but we can find little evidence to support this figure.