Molecule by molecule toward the plastic car

Matty Holtzberg, a New Jersey inventor-engineer, has an idea for fuel-stingy drivers: a plastic engine. The 80 percent polymer motor weighs less than half as much as a conventional engine. It has stood up under the equivalent of more than 50,000 miles of testing.

Next year his company, Polimotor Research, will put the engine to the acid test in a Formula II racing car.

The plastic engine - too expensive and unproven to be put in anything but industrial engines and specialty vehicles for years to come - shows how far thinking has gone in the use of plastics in cars. Yet little of that thinking is seeping into Detroit these days.

In fact, in the mid-1970s automakers saw plastic as a way to blunt the twin thrusts of foreign competition and rising fuel costs. It was one of several lightweight materials - ranging from aluminum to exotic ceramics - that would hatch a generation of lighter, fuel-efficient cars.

Today, molecule by molecule, plastics are showing up in cars - but not as fast as Detroit orginally envisioned. A few carmakers have even cut down on use of high-priced structural plastics. Stabilized gasoline prices have made weight reduction less of a cause celebre in Detroit board rooms.

''The issue now is very much one of price,'' says John Hammond, director of the automotive unit at Data Resources Inc., a Lexington, Mass., consulting firm. ''Only to the extent plastics prove to be less expensive than some other materials will they be used.''

Seven years ago some automakers were forecasting that the average car would carry 500 pounds of plastic by 1995. Today that number sits at about 200. By the mid-1990s, estimates Predicasts Inc., a Cleveland market-research firm, it will only edge up to 275.

This is not to say Detroit has completely lost its penchant for plastic. As a material, it still holds several trump cards: lightweight, corrosion-resistant, undentable, and sometimes easier to fabricate. Plastics used in cars range from the polyurethane in seat covers to high-performance engineering polymers and composites, some of which are tougher than steel. (Torlon, a ''superplastic'' Holtzberg is testing, can withstand temperatures of up to 500 degrees for long periods.)

Up to now, plastics have been mainly used in three areas: the interior (seats and dashboards), exterior trim (grills and wipers), and under the hood (for instance, in wiring). But slowly they are showing up in body panels and structural parts, too.

Japan's Honda Motor Company recently unveiled a sports car with a body that's 40 percent polymer alloys. Next year Volkswagen of America plans to put a plastic fuel tank in its diesel Rabbit. Ford is putting all-plastic bumpers on at least three models.

Probably the most ambitious step into plastics, however, will be General Motors' Fiero, a two-seater sports car that will hit showrooms next month. It will have an all-glass-reinforced plastic body. Unlike the Corvette, another plastic car, the body of the Fiero will be bolted instead of glued onto a steel frame. Result: panels that can be easily replaced. Some analysts see the Fiero's precision-fit, bolted-on plastic panels as the forerunner of what cars will be sheathed in in the late 1980s. ''If it catches on it could revolutionize the whole industry,'' says Lawrence T. Harbeck of the University of Michigan's Office for the Study of Automotive Transportation.

Edmund Burke, director of GM's composite program, is more cautious. He sees the panels being used on a few lines over the next few years. But expensive retooling will prevent any massive switch to plastic panels on most cars for years to come.

Where else will plastics soon show up? According to Mr. Burke, in flooring, trunk liners, and more parts under the hood. He doesn't see them used much in gas tanks and other ''high safety'' areas.

In a new wave of exotic engines being developed for the 1990s, high-performance ceramic looks like a better bet than plastic, Mr. Holtzberg's efforts notwithstanding. Ceramic can withstand higher temperatures.

But in the end, Holtzberg is the first to admit that not everything should take on a plastic tint. ''No one material is going to revolutionize any industry ,'' he says.