The streamlined car moved swiftly, easily parting the air. It used less fuel than other models and gave its passengers a smoother ride.

It was a 1934 Chrysler Airflow, the first mass-produced American car specifically designed to reduce air resistance. It was an engineering marvel in its day--but a flop on the showroom floor.

Now American automakers are at it again--angling and curving sheet metal, flattening wheel covers, and dropping chrome. The idea is to better manage the air flow--the way air moves over, under and around cars in motion.

An aerodynamically efficient car cuts the air cleanly, separating the currents and holding them close to the body until they reach the rear and flow away in orderly patterns. Air resistance is weakened because friction, or drag, is reduced as the air moves about the car.

But power is needed to conquer resistance. "Aero horsepower" is needed to overcome air drag on a car moving at 50 miles per hour on a level highway.

Aerodynamically efficient cars require less aero horsepower at higher speeds, which means they use less fuel. That makes them the most cost-effective way for domestic automakers to try to meet federal minimum mileage standards for new-car fleets.

That is the science behind the hype, the substance behind the numerous ads touting the "slippery" new cars rolling into showrooms across the nation. It is the stuff many domestic and, for that matter, foreign automakers believe they need to boost sales in a still-fragile market.

Perhaps nowhere is that belief more fervently held than at Ford Motor Co., the longtime second sister of the domestic auto industry.

"Aerodynamics will be the distinctive signature on Ford Motor Company products of the 1980s," says a company brochure; and the company has invested $3 billion in a high-risk attempt to back its boast.

So far, the bet seems to be paying off. Ford's radically redesigned Thunderbird is selling well, particularly in California, where Ford and other domestic cars have been outpaced by imports. And sales of the company's Lincoln-Mercury Cougar--a kind of hedged bet because of its more conservative styling--are improving faster than those of the T-Bird. Between March 11 and June 10, Ford dealers sold 33,711 Thunderbirds, a 174 percent increase over 12,285 sales in the same period of 1982. But in the same period, Cougar sales increased 383 percent, to 21,391 from 4,425.

Ford's 1983 production plans now call for 102,000 Thunderbirds, 12,000 special Thunderbird Turbo Coupes, and 100,000 Cougars. The models were introduced Feb. 17. Ford officials, citing still-strong dealer orders, say they expect to sell the full model run by the end of the calendar year.

"If we had to sell more right now, we couldn't do it because we're at capacity," said Jack J. Telnack, Ford's chief of North American automotive design. "We're satisfied, very definitely satisfied with the T-Bird and Cougar sales."

Telnack credits compromise for the apparent success of his company's leading aero models, saying they represent an optimum mixture of engineering and styling.

The Chrysler Airflow models, on the other hand, bombed in the 1930s because they had engineering without styling, "function without flair," Telnack said. There are many "amens" heard on that point.

"The Airflow was not a blend of styling and engineering. It was just engineering," said David R. Holls, General Motors Corp.'s director of advanced design. Holls also said the Airflow offered features, such as improved mileage and power steering, that many consumers did not want at the time.

"We're not doing anything as shocking as the Chrysler Airflow was" at the time of its introduction, Holls remarked. "Our changes are happening more gradually. . . . We try not to shock everybody by going too far, for example, with something like the family car."

GM's most visible investment in aerodynamic styling is in its sports models, notably the Chevrolet Camaro and Corvette and the Pontiac Firebird.

Aero traits also are found in GM's Buick Century and Oldsmobile Cutlass Ciera. But the streamlining of those family cars is broken by formal rooflines.

Industry analysts say that kind of compromise is necessary in something as fickle as the domestic auto market, where buyer taste and need--and the ability to satisfy both--vary widely.

Ford, for example, "still has to hope it hasn't stepped out too far ahead of the pack" with its 1983 Thunderbirds, says J. D. Power, president of J. D. Power & Associates, an auto consumer research firm based in Westlake Village, Calif. "Stepping out too far ahead of the pack turns people off. They simply won't buy what they're not ready to accept."

Ford's aero launch was good, but far from spectacular, Power said. "I don't think it's as successful as the company thought it would be, or would like it to be. But I don't think it's going to be the blunder of the year, either," he said.

Power believes that possible blunder was averted when Ford officials decided to balance "the more radical design of the Thunderbird" with the square-roof treatment of the Cougar. "The Cougar is outselling T-Bird, proportionately, which is a surprise to some people at Ford. The Thunderbird is doing okay in California, but there is some buyer resistance to the car in more conservative areas, such as the Midwest," he said.

Part of the customer reaction to Ford's top aero products stems from the fact that, despite the company's aggressive advertising, "the science of aerodynamics doesn't mean much from the standpoint of the consumer," Power said. "Aero might have had an impact two or three years ago when people were concerned about fuel prices. But those prices have dropped, and fuel efficiency is way down on the list of priorities of car buyers."

"People aren't buying cars because of aero. They're buying them because they like the looks. And if they like the looks, they don't care if the car is aero or boxy," Power said.

Still, Ford officials insist that their commitment to aerodynamics is unshakeable. Similar vows emanate from the executive suites of GM and other automakers who believe that sleek, properly dressed, is better.

A reason: Aerodynamic styling saves corporate money.

Ford, for example, says its corporate average fuel economy (CAFE)--the federal minimum mileage standard for new-car fleets--increased by one mile per gallon since 1977 because of the company's new emphasis on aerodynamics. "Normally, just to get a 0.1 percent increase in CAFE, we'd have to do something to the power train, which could cost anywhere from $100 million to $200 million," said Larry Socha, manager of Ford's aerodynamics department.

That means Ford saved at least $1 billion by relying on aerodynamics to improve fuel economy. Aerodynamic styling basically involves drawing, sculpting, building prototypes and wind-testing the results--a routine, relatively inexpensive process in new-car development, Socha said. He said the $3 billion Ford spent developing its 1983 aero products, including the midsize Tempo and Lincoln-Mercury Topaz, included $7 million for wind-tunnel tests, many of which were conducted at the University of Maryland.

GM has another measurement. The company found that it can get an average increase of 2 city miles per gallon and 15 highway miles per gallon by replacing the current Chevrolet Citation (X-car) body with an experimental, aerodynamically designed "Aero X" shell.

"You would have had to take 800 pounds out that car to get the same improvement in fuel efficiency. And the only way you can take out that much weight is to use exotic metal parts, which really would raise the price of the car," said Holls, GM's advanced design executive.

"Aero is not just a styling gimmick. It's here to stay. It has brought about functional improvements in the cars we build," Holls said.

Foreign automakers agree. Top aero imports vying for a share of the U.S. market include the new Audi 5000S, the Honda Prelude, Datsun 280ZX, Mazda RX7, Toyota Supra, Mitsubishi Starion, Isuzu Impulse, Volkswagen Scirocco (Wolfsburg Limited Edition)--and almost any Porsche.

What about Chrysler Corp., producer of the ill-fated Airflow?

"We're doing aero here, but we're not necessarily convinced that you must have the basic wedge shape to have an aerodynamic car," said Gary Romberg, Chrysler's chief of aerodynamic design.

Chysler, instead, has refined the formal look--a somewhat rectangular design largely dictated by the company's reliance on its basic K-body platform. The Chrysler cars, particularly the New Yorker and LeBaron models, are selling well. But the company plans to sharpen its aerodynamic profile next model year with the introduction of the Chrysler Laser, the Dodge Daytona, and the high performance Dodge Daytona Turbo Z.

"It's a matter of applying aerodynamic technology in what you think is a rational and reasonable amount," Romberg said. "We're doing what we think is called for in our cars; and we think the marketplace will determine if that is rational and reasonable."