"This brochure presents the philosophy and technology behind the various models of the Saab 900. The contents of the brochure are based on the specifications for the 1988 models valid at the time of final editing. The model range, technical specifications and equipment vary from one market to the next."

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Saab-Scania had accumulated a wealth of experience in the turbocharging of truck and bus engines when the turbocharged car engine for everyday motoring was launched in 1977.

Turbocharging

Saab was one of the world's first car manufacturers to adapt the turbo-charging technique to everyday motoring. In a Saab Turbo, the boost in performance starts at low engine speeds. The engine delivers its peak torque at a mere 3000 rpm. This provides massive power resources for overtaking, without the need for changing down. So the engine is designed for normal, everyday motoring, not for the extremely high top speeds demanded by a competition driver.

The performance of the turbo-charged four cylinder Saab engine can match that of six cylinder or eight cylinder engines. But experience has shown that the full power resources of an engine remain unused for 80-85 per cent of the time it is in use, and the turbocharger is then "idling", without affecting engine output. So during this time, the turbo engine offers the same excellent fuel economy as a normally-aspirated fuel injection engine.

The turbocharger consists of a turbine and a compressor. The exhaust gases flow through the turbine, while the compressor supplies the engine intake air. In brief, the system operates as follows: The engine exhaust gases flow through the turbine casing, in which a bladed wheel is induced to rotate at very high speed-up to 120,000 revolutions per minute. The turbine wheel drives the compressor impeller through a shaft.

When the turbocharger has reached a certain speed, the compressor will start to boost the intake air pressure. Due to this pressure increase, more air will be forced into the cylinders, and more fuel can thus be injected and burned. The engine delivers more energy per piston stroke, and the torque and power of the engine will therefore increase. The turbine is combined with a waste gate - a pressure control valve which automatically controls the boost pressure throughout the engine speed range.

The Turbocharger consists of two wheels mounted on a common rotating shaft. One of the wheels-the turbine-is driven by the exhaust gases. The other wheel- the compressor- is located in the intake system of the engine and its function is to compress the intake air. When the driver presses down the accelerator pedal, the flow of exhaust gases and their velocity will increase, and the wheels will therefore rotate at increasing speed. At a certain speed, the compressor will start boosting the pressure in the intake system, so that the engine will be supplied with more air than it were naturally aspirated. A regulator ensures that the correct air pressure is maintained.

The system also includes an engine overspeed protection switch which will open the ignition circuit when the engine speed has risen to 6000 rpm, and will close it again when the engine speed has fallen to an acceptable level. So the engine is monitored by two switches that prevent overloading

The exhaust gases from the turbocharged engine are at roughly the same temperature as those leaving a conventional fuel injection engine, but the gas flow is higher. The heat-affected components of the turbo engine are therefore protected, modified or uprated to withstand the higher engine power by a comfortable margin. Moreover, the engine is equipped with an intercooler which cools the engine intake air by about 60°C. The engine oil cooler and water-cooled bearing housings on the turbocharger ensure that the engine oil temperature maintains a steady, low temperature even after hard driving. The turbocharged engine is also equipped with a radiator with a larger number of cooling passages and a radiator fan with a higher rating than that of the conventional fuel injection engine. Whenever necessary, the fan runs on after the ignition has been switched off.