What do you do if your assembly lines are turning out small, fuel-efficient cars at a time when motorists once again are demanding power and speed?

You turbocharge the engines.

American and foreign car companies are putting millions of dollars into advertising campaigns to herald this "new" development. "Turbo" is the latest chrome-plated word on car name tags, such as the Chrysler Laser Turbo, the Peugeot 505 Turbo, Volvo 760 Turbo and Mitsubishi Mirage Turbo.

An Italian entry, Maserati, has gone the competition one step better, importing a Biturbo -- a lightning-fast machine (160 miles per hour, top speed) with one turbocharger located on each side of a 2.5 liter, V-6 engine.

But there is a price. Turbocharged engines are more costly to own and operate. For example, Volvo's new non-turbo 760 model with automatic transmission has a base list price of $21,485. The turbo version carries a base sticker price of $23,060.

And some critics warn that turbocharged engines may wear out faster or break down sooner because at high speeds, they run at a significantly higher temperature than conventional engines.

Nonetheless, turbochargers are finding their way into cars because of what auto makers say is a kind of consumer schizophrenia.

"People wanted fuel-efficient cars. So we began downsizing our fleet, getting rid of the big V-8 engines and the V-6's, and going to four-cylinder models," said Robert Sinclair, Chrysler Corp. vice president of engineering.

"But after we got the fuel efficiency, people said: 'Well, that's nice. But whatever happened to performance? We want to save fuel, but we also want to have some fun.' "

"Every time you make gains in fuel efficiency, you lose horsepower," said John Saia, technical field supervisor for Goodyear Service Stores, a subsidiary of Goodyear Tire & Rubber Co. "But an engine, basically, is just a big air pump. The better you move the air, the better the combustion. The better the combustion, the more power out of the engine," Saia said.

Turbochargers are super air pumps. They operate off exhaust system gases that turn a turbine. The turbine drives a compressor, which pumps clean air into engine cylinders at high pressure -- often increasing that pressure, measured in pounds per square inch, five to 10 times higher than the pressure in non-turbo engines, Saia said.

He said that the resulting increased density of the air in the engine's combustion chambers allows a greater mixture of fuel with air. Burning more fuel creates more power, Saia said.

"It sounds complicated, but it's really an extremely simple system," Saia said. "You're not actually putting more fuel into the engine with a turbocharger. You're putting in more air, which means you're burning the fuel that's there more efficiently."

For example, a four-cylinder, non-turbocharged engine that cranks out 90 horsepower and gets 25 miles per gallon might get about 110 horsepower and 22 mpg with a turbocharger, Saia said. "You still have fuel efficiency; but now you've also got a little zippiness," he said.

Chrysler, which relies mostly on small cars with four-cylinder engines for its sales, is the turbocharging leader among the nation's Big Three auto makers. Ten percent, or 100,000, of the 1 million cars Chrysler built for the 1984-model year were equipped with turbocharged engines.

Chrysler's Sinclair said the company will produce 160,000 turbocharged cars in the 1985-model year.

By comparison, turbocharged models make up less than 1 percent of the new-car fleets of General Motors Corp. and Ford Motor Co.

"Basically, the experience that we've had over the years says that if you have a larger engine available and a neat package to put it in, you will come out with a better engine, overall, than you would with a turbocharged smaller engine," said Max Freeman, an advanced design engineer at GM.

"Turbocharged engines have durability problems," mostly because forcing air into engines at high pressure creates lots of extra heat, Freeman said. That extra heat causes low-octane gasoline to detonate -- burn unevenly -- faster. Detonation creates greater engine stress and louder engine noises, Freeman said.

The lower the octane rating, the faster gasoline will burn. The higher the octane rating, the slower it will burn. That means turbocharged engines tend to run better on high-octane gasoline -- which usually is more expensive than low-octane fuel, said Freeman. "Stuffing air into an engine" and buying more expensive fuel to make that engine work efficiently would seem to increase motorists' operating costs, Freeman said.

"We've made more turbos than we've sold," Freeman said.

But in its otherwise exuberant promotion of turbos, the industry is once again making a virtue of necessity, just as it did in promoting front-wheel-drive and aerodynamic styling as great advances in auto design. In fact, both features were pulled off the shelf to help meet fuel efficiency requirements.

Turbochargers are not new. They are found in all sorts of aircraft, and the technology "has been under development almost as long as there have been internal combustion engines," according to a report on the future of the automobile industry published by the Massachusetts Institute of Technology.

"Auto technologists are nearly a decade into the second-generation applications of the turbocharger," said the MIT report, titled "The Future of the Automobile."

The technologists "have still not mastered all of the problems of these applications, problems that demand solutions as turbochargers are incorporated in a much wider range of vehicles in quest of adequate performance with the smaller-displacement engines now needed to improve fuel economy," the MIT report said.

The report added: "Mass-market consumers will be less impressed with the "turbo" label on the side of the car than with durability and driveability, so one may expect the technologists to redouble their efforts in this area."

Sinclair said he is aware of those complaints.

"Some guys have shied away from turbocharging because there have been some reliability problems in the past," Sinclair said. "But we've worked liked hell to solve those problems, and we've done a good job of it. Why else would we offer a five-year/50,000-mile warranty on our cars? We'd be fools to do that if we did not have confidence in the product," Sinclair said.