The engines show the (r)evolution in auto technology

Nothing reflects the changing times here more than the engines that power American cars. Before 1985, 4-cylinder engines will be dominant in this country and the rest of the world; and by 1990, the once-mighty V-8 may be a museum-piece relic alongside the running board and the rumble seat.

The big power plant is a victim of the times. First, a federal law requiring an automaker to achieve 27.5 miles per gallon on average by 1985 was enacted. And second, the fuel-pump price shock is accelerating small-car demand.

Take the case of General Motors Corporation, the world's largest manufacturer of motor vehicles. The company intends to attain an overall fleet fuel-economy average of more than 30 m.p.g. by 1985, actually hitting 30 m.p.g.-plus in 1984 and 31 in 1985.

By 1985, it is projected, 4-cylinder engines will power about 60 percent of GM's cars, virtually changing place with the V-8, which accounted for 60 percent as late as 1979.

Six-cylinder engines will be in 20 percent of GM cars by 1985, while diesels will grow to almost 20 percent.

More specifically, look at what is happening at one of GM's major car divisions, Pontiac. A few years ago, an engine with four cyclinders was as uncommon as a cow with eight legs.

The cow is still a novelty, but the 4-cylinder engine is not. In 1978, Pontiac was turning out about 750 L-4 engines a day.Last Jahuary it climbed to 2 ,500. That will nearly double to 4,600 daily during the 1982 model year, at which time Pontiac will be churning out 4s at a million-year clip.

With GM leading the way, Detroit automakers are undergoing the fastest, most massive power-plant shift in history. By 1984, domestic manufacturers will have the capacity to produce more than 8 million 4-cylinder engines annually in North American plants. Besides GM, the producers will be Ford, Chrysler, American Motors, and Volkswagen.

It is possible that even Cadillac, pride of the GM luxury stable, will have a 4-cylinder engine by the mid-1980's. Cadillac in the 1981-model year adopted what it calls a "modulated displacement" power plant.

The unusual product, standard equipment in new Cadillacs, operates on 8, 6, or 4 of the V-8's cylinders. It has an electrochemical system, controlled by a microprocessor, that automatically actuates only the number of cylinders needed to satisfy the demand.

It is designed to improve fuel economy some 10 percent over an unmodulated V- 8, say Cadillac engineers. Eight-, 6-, or 4-cylinder operation is entirely automatic and operation is based on driving demands.

At a recent auto industry-management seminar sponsored by the University of Michigan, the future of engines was put into perspective by F. James McDonald, who becomes president of GM in January when E. M. Estes retires. Between now and 1985, he said, "we expect most engines will operate on petroleum-based fuel, with the battery coming along in the latter part of this time frame. The light-duty diesel and stratified-charge engines are both technically feasible."

From 1985 to the year 2000, what he calls the "intermediate term," the GM president-to-be says, "it appears the gas- turbine engine, electric-battery cars in sizable numbers, and the methanol-fueled, spark-ignition engines are most likely."

The third time period is beyond 2000. "The hydrogen- fueled engine and fuel cell look like strong possibilities at this point," Mr. McDonald suggests.

But it is the present period with which both he and GM are most concerned. "We're literally tearing up our old plants and either rebuilding them or in some cases starting all over."

There obviously is little room in GM's plans for big V-8 power plants. In fact, down the road the next GM president sees "a neat little two-passenger car that runs on a 3-cylinder engine getting 86 m.p.g. at a steady 25 miles an hour."

Dust off a spot in the auto museum for the V-8 passenger car. Its days are numbered.