Saab Turbo

Big-Engine Feel -- Without Big-Engine Thirst

It's no big surprise that as cars get smaller and more economical, the engines have to get smaller and more economical right along with them. Trouble is, a small, economical engine in a car feels like, well, a car with a small, economical engine.

There are ways, however, to get a big-engine feel from a small engine, without getting the big-engine thirst for fuel along with it. The trick is to have the desired power only on demand. The method that was pioneered by the Swedish Saab company for regular street cars, and which now many others are beginning to use, is turbocharging.

All internal-combustion engines run on a combination of fuel and air. The more fuel and air, the more power. The amount of fuel ingested is directly related to the amount of air ingested. The amount of fuel is carefully regu- lated by a carburetor or fuel injection system. It's the amount of air that's the big variable.

The typical normally aspirated engine relies on the vacuum created by the motion of its pistons to suck in the needed air. If all you want to do is toodle along, that's usually just fine and dandy. But if you want to get around that tractor-trailer or get on a crowded freeway before next Christmas, you probably will find the small, economical engine a bit short-winded.

A turbocharger uses your car's exhaust gases to force feed your engine more air when you need it. As the exhaust leaves the engine, it spins a small turbine in the exhaust's stream. The turbine spins a compressor wheel in the intake system to force more air into the engine. Turbine and compressor are mounted on a common shaft, sealed in their own halves of a small, relatively lightweight housing. It's a nice compact and effective system.

Trouble is, as long a$ the exhaust spins the turbine, it spins the com- pressor, and the intake system gets charged with more and more pressure -- called boost. If left to its own devices, the turbo would eventually destroy the engine. So a safety valve, called a wastegate, allows the exhaust to by- pass the turbine when the boost reaches a specified level. That boost level has a lot to do with the turbo's effectiveness. Too much and the car is good for about two runs to the A&P; too little and you wonder why you paid the extra money for the turbo.

But a properly designed turbocharged engine can last as well as a non- turbo...and still give all the desired traits. For example, Saab is able to get 135 horsepower from its turbocharged 2-liter (that's about 120 cubic inches) four cylinder. Not only is that well over 1 hp per cubic inch, it's 20 horses more than the normally aspirated version of the engine. All without a penalty in fuel economy when the driver doesn't have his foot in it.

Of course, that leads to one problem of turbocharging the small, economical car - It makes driving so much fun again that the driver likes to put his foot into it.

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