900 loving – the Saab Turbo System

This article continues the series lifted from a Saab Engineering brochure from the early 1980s, as scanned by 1985 Gripen.

This one is an overview of the turbocharger setup used in the 8-valve 900 Turbo. Enjoy.


….ranks among the best and most exciting automobiles we have ever tested. We were impressed with the Saab Turbo to the point that it will probably become the standard by which all other cars are judged. Need we say more? (AUTOSPORT CANADA)

Turbocharging has long been used for increasing the output of truck and aircraft engines. In high-performance competition cars, turbochargers have been employed to achieve very high top speeds. Saab- Scania had already accumulated a wealth of experience in turbocharging of truck and bus engines when the turbocharged Saab 99 was launched in 1977.

Saab was the first among the world’s car manufacturers to adapt the turbocharging technique to everyday motoring. In a Saab Turbo, the boost in performance starts at low engine speeds. The engine delivers maximum torque at a speed of 3000 rpm. This provides massive power resources for overtaking – without the need for changing down. So the Saab Turbo has been designed for normal, everyday motoring instead of the extreme top speed demands of the competition driver.

With the turbocharger, the performance of the 4-cylinder Saab engine is on a par with 6-cylinder or 8-cylinder engines. But experience has shown that the high performance of an engine is not used during 80 – 85 per cent of the motoring time, at which time the turbocharger runs without adding to or detracting from the engine output. So during this time, the Turbo runs with the good fuel economy of the normally-aspirated fuel injection engine. In a Saab Turbo, one “pays” for high performance only if it is actually used.

Saab turbo brochureThe turbocharger consists of a turbine section and a compressor section. The exhaust gases flow through the turbine, while the compressor handles 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 starts to boost the pressure of the intake air. Due to the pressure increase, more air is forced into the cylinders. As a result, more fuel can be injected and burned. The engine delivers more energy per piston stroke and the torque and output of the engine thus increase.

Click to enlarge.

Saab turbo

The turbine is combined with a charging pressure control valve – also known as a wastegate which automatically controls the charging pressure throughout the engine speed range. If the charging pressure – irrespective of the warning pressure control valve – sometimes should exceed a predetermined value, a pressure switch will temporarily interrupt the fuel supply. The gauge on the dashboard also shows the charging pressure. In addition, the system is provided with engine overspeed protection. When the engine speed has risen to 6000 rpm, a switch will open the ignition circuit until the engine speed has fallen to an acceptable level. So the engine is monitored by two switches designed to prevent overloading.

Not tuning

The Saab turbocharging system is not the same as tuning in the usual sense. The engine actually delivers peak output at a lower speed than the standard engine and the compression ratio is also lower. The wear of an engine can generally be classified into peed-dependent and temperature-dependent wear. The speed-dependent wear of the Turbo engine is lower than that of the standard engine. Partially due the fact that the power resources available enable the car to be driven in a higher gear than a car powered by the standard engine. In situations such as on uphill gradients and in city traffic. Another reason is that Turbo cars incorporate overspeed protection. The temperature of the exhaust gases from the turbocharged engine is no higher than that of the exhaust gases leaving the conventional fuel injection engine – but the gas flow is higher. Components exposed to heat are therefore protected. Modified or uprated to withstand the higher output with a comfortable margin, the turbocharged engine is also equipped with a radiator with a larger number of cooling passages and a radiator fan with a higher output than that of the conventional fuel injection engine. Whenever necessary, the fan runs on, even after the ignition has been switched off.

Saab Turbo with APC

The automotive world has become accustomed to major items of news from the Saab Development Department. The “everyday turbocar” is merely one of a succession of innovations. Whereas many competitors are still in the initial stages of turbocharging technology. Saab has already announced the second generation of turbocharged engines: the Saab Turbo with APC (Automatic Performance Control). This engine adjusts itself automatically to the fuel used. The higher the octane number, the higher the engine output. But the most important benefit is that the higher ratio made possible by the APC system makes the turbo engine even more fuel efficient. Saab has again been in the lead in launching an innovation. But Saab has also manufactured and delivered more than 70,000 turbocharged cars to all corners of the world, and must today be regarded as the leading European manufacturer of gasoline fuelled turbo engines

Saab turbo

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