Turbo vs. super: the race for extra oomph

One of the many technological battles going on behind the scenes at automobile companies all over the world is whether cars should have ''power induction systems,'' either a turbocharger or a supercharger.

The turbos are winning so far, but the fray is far from over.

The function of a power-induction system is to provide compressed air to burn with fuel in the engine to increase power. Normal air going into an engine has a barometric pressure of 14.7 pounds per square inch (p.s.i.) at sea level. When a turbocharger or supercharger boosts this pressure to 21 or even 30 p.s.i., power and performance rise 50 to 100 percent.

This technological option is proving extremely valuable for several auto companies because of recent trends in oil supplies. The oil crisis in 1973-74, and again in 1979, convinced some companies that the world was entering a period of long-term oil shortages and sharply higher gasoline prices. As a result, they discontinued or downsized their large V-8 engines.

But an oil glut in the last few years has turned this situation around, producing lower-than-expected gasoline prices. This has led the car-buying public to order more-powerful cars. Using forced-air technology, the auto industry has been able to meet this demand without retooling the big V-8s or causing auto companies to fall below the federal fuel requirements.

While the supercharger can't be counted out yet, the turbocharger is winning the competition by a wide margin in the United States as well as elsewhere around the world. The current score in the US is 50 to 0. In other words, 50 cars now have turbos, but none of them are using superchargers at this time.

Turbochargers are being installed on one or more models sold by Buick, Pontiac, Ford, Lincoln-Mercury, Plymouth, Dodge, American Motors, Mercedes, Toyota, Nissan, Peugeot, Volvo, as well as other car manufacturers.

How these two power-induction systems produce their compressed air may explain their relative popularity so far. The turbo more or less gets its power free by directing hot exhaust gas past a turbine rotor, or ''wheel,'' attached to a compressor rotor that scoops up the air, forcing it to become compressed before it is directed into the engine's combustion chambers.

The movement of the exhaust gas (which is subsequently dumped into the exhaust system again) and the expansion of this gas cause the turbine and compressor rotors to revolve at speeds of up to 170,000 revolutions a minute.

This extreme speed produces several problems. The mechanism and the surrounding area get extremely hot and sometimes require special cooling. At high road speeds, the exhaust produces so much heat that it must be bypassed around the turbocharger with a so-called ''waste gate.''

In addition, the high speeds of the rotors could cause the whole turbocharger to explode, although there is no record of this actually happening, because the mechanism is contained in a heavy iron housing.

A major weakness of the turbo is that it ''lags,'' meaning that it usually doesn't produce the extra power until the car or truck is traveling at 25 or 30 miles an hour.

In contrast, a supercharger is usually directly driven by a belt that runs off the crankshaft. There's little or no lag, with the compressed air becoming available at low speeds and producing a corresponding boost in engine performance through the entire range of the engine's operation.

Because the supercharger is driven directly by the crankshaft, however, it takes more fuel than the turbo, and it will normally require more redesign of the engine.

The Garrett Corporation, which produces 65 to 75 percent of all the world's turbochargers, is still working on superchargers because some engineers feel a breakthrough or two could make the supercharger superior. Garrett declares that ''if anybody's going to make the first automobile supercharger that really works , we're trying to make sure it's us.''

Eaton and Holley are also working on superchargers, while Ford Motor Company has tentative plans to develop one.

Garrett will also introduce a number of significant improvements in its turbo-chargers in the next couple of years. Among these will be a variable-geometry system which pretty well eliminates the low-speed lag of most current turbos. In addition, the company is coming out with a new model of turbo that is more suitable and less costly for certain engines, as well as some revolutionary compressor rotors made of ceramic, which will be lighter, more efficient, and ultimately less costly.

Is a turbocharger for everyone? Carmakers, which only install them on about 5 percent of total production, say the answer is no. Engines with turbos usually require more frequent oil changes because of the high temperatures. Also, the turbos often complicate the servicing of engines.

But for the motorist who'd like a little extra performance after the poor power delivered by some past models, and who has additional money to pay for his car, the turbo can be an excellent choice.