The engine started: varoom. Not with a big and powerful VAROOM, but with a little varoom befitting a small car.

But what came rolling off the Pontiac Fiero assembly line here didn't look like a car, small or otherwise. It had no door panels, no roof, no trunk lid. It had no carpeting, no ceiling--nothing, except a black steel frame with engine and wheels, a center-placed fuel tank and electrical wires running here and there, and a driver who removed his portable seat after parking the thing in a row of identical motorized frames.

In any other domestic auto plant, the car would have had nearly all of its exterior parts and interior trim at this point of production. In fact, traditional auto manufacturing would have required the wedding of frame to body panels, largely because the outer bodies of most cars have a support function.

But that's not the case here where the people say they "build excitement," and where General Motors Corp., Pontiac's parent, is betting $300 million that buyers will accept the first American-made, two-seat, mid-engine, four-cylinder, rear-wheel-drive sports car--which can be driven with an all-plastic, bolted-on body, or no body at all.

However, there is more riding on the bet than money. At stake is a whole new concept of auto manufacturing and materials usage--a gamble that, if it goes as planned, could give plastics the prominence of steel in an industry born and bred of metal. Even GM's competitors are rooting for the success of the Fiero ("very proud" in Italian), hoping that it will open a market that they later can help exploit.

"We do things backwards here," boasted Leo VanHouten, pointing to the row of skeletons on wheels. VanHouten, a 22-year GM veteran, had a job repairing glitches in new cars before they rolled out of the plant to dealer showrooms. But he works in plant labor relations now. And he says that's just as well, "because there's not much work for a repairman to do on the Fiero."

"Before, we had to take the car apart, remove a piece of the body or something" to get at a defect, VanHouten said. "But because the panels don't go on this car until after all of the mechanical and electrical things have been checked, that takes a lot of the work out of the repairman's job. It makes a hell of a lot more sense, too."

The development of the Fiero, GM's P car ("P" for "Pontiac, product, people, and philosophy," said Ronald Rogers, the P-car project manager) began in 1978. The idea was to create a two-seat sports car attractive enough to boost Pontiac's sagging image and priced low enough to gain wide customer acceptance.

GM's Chevrolet Division already has the two-seat Corvette. But at $25,000 to $28,000 a copy, the market for that car is limited. The company normally sells 30,000 Corvettes a year. But the pricey 1984 model has been greeted with kudos in the automotive world--despite some early recalls--and GM expects to sell 37,000 in the new model year.

Pontiac, on the other hand, is planning a 1984 production run of 100,000 Fieros, which are being introduced at prices ranging from $8,000 to $11,000. Pontiac already has orders for 40,000 Fieros, and the division is struggling to meet new orders, which Pontiac officials say will take two to four months to fill.

"Two-seaters for most people are either too expensive and unattainable, or they come with a low price and poor performance," said Fred H. Simmonds, a marketing specialist in GM's Pontiac Division. "We wanted to create a two-seater with good performance that would come out somewhere in the middle of the price picture."

The problem was coming up with a design that would justify production costs. Two-seaters generally are low-volume cars, which means production costs have to be recovered in a narrow market.

Manufacturing costs of metal-body cars tend to be high because of the initial investment in tools and dies used to shape and form the metal. A tool-and-die system to make one metal piece--one shape on an entire car--can cost as much as $1 million, according to auto industry analysts.

The high cost of metal tooling is recouped through high-volume production, or through very high sticker prices. (The Corvette always had a plastic/fiber glass body. But, unlike the Fiero, the Corvette's body is integrated with the frame, as is done in the more traditional and more expensive auto manufacturing process. Also, much of the new Corvette's price is in its drive train--the engine and transmission arrangement--and in its mystique.)

Plastic tooling, on the other hand, is substantially cheaper than tools used to work metal. But the use of plastic also involves a greater use of time--about three minutes to turn out a plastic piece, versus 30 seconds to stamp out a piece of metal, according to design engineers at Chrysler Corp.

The reason is that plastic involves a complex mix of polymers and chemicals, all of which must be brought together properly and heated and treated properly to produce the right kind of plastic for the right part. But plastic has three major advantages over steel: It doesn't rust, its low tooling costs mean that designs can be changed with relatively little financial pain, and its lighter weight affords greater fuel efficiency.

"Plastic is the right material for a low-volume car," Rogers said. "The decision to go with a plastic body was a breakthrough in the design of the P car."

There were other breakthroughs, chief among them the idea to use a driveable chassis, which P-car engineers call the "space frame."

The space frame is the steel bone structure of the car, similar to a race car's roll cage. It consists of six modular units, all made of high-strength steel, welded to form a framework that surrounds and protects the passenger compartment.

The frame has 39 mounting pads to which the exterior plastic panels are attached after the frame has been rustproofed and fitted with all of the mechanical and electrical equipment needed to operate the car.

The Fiero basically uses three kinds of plastic: reaction-injection-molded (RIM) polyurethane, a "soft" plastic designed to take knocks on the front and rear bumpers; reinforced-reaction-injection-molded (RRIM) polyurethane, a harder, but still supple plastic used on the sides and doors of the Fiero; and a harder sheet molding compound (SMC), used on the roof, the hood and trunk lid.

Rogers--a man of usual middle-age girth--said one of Pontiac's toughest sales problem is to persuade customers that "plastic doesn't mean cheap or flimsy." To make his point, he picked up a painted Fiero door panel, slammed it on a concrete floor, and then jumped up and down on the panel with both feet. The only damage was a paint scratch. The panels won't melt on an especially hot day or become brittle and break in severe winter cold, Rogers said.

The panels, of course, could be ruined in a car crash. But the ease of replacing damaged panels should help lower Fiero owner insurance costs, "or at least keep them in line" with other sports cars in Fiero's class, Rogers said. There is no indication, yet, of how the insurance industry will view that claim.

A 35-minute drive away, at a meeting in Detroit last week, delegates to the annual convention of the Society of Plastics Engineers were discussing the same question. But the discussion was more upbeat than worried, more filled with anticipation of profits to come.

The Fiero is a very dramatic example of the use of plastic in cars, but it represents only one of many such applications, the engineers said. Honda's 1984 Civic CR-X uses a largely plastic body, and all of the domestic manufacturers are using plastic in bumpers and in other exterior treatments. Ford Motor Co. is experimenting with a car, the Flair, that uses plastic in the frame as well as in outer-body construction.

Plastic now accounts for about 8 percent of the weight in most cars, a figure that "is bound to grow if cars like the Fiero do well in sales," said John W. Cottone, executive vice president of Hughes Plastics Inc., of St. Joseph, Mich.

"The Fiero will act as a catalyst for renewed interest in plastic materials," Cottone said. "It's going to make it clear that plastics can beused in a wide variety of applications, that plastics are here to stay."