Why plastic over metal? ''Weight is the primary thing,'' Mr. Holtzberg said. ''It's also quieter and doesn't rust, but the key element for Detroit would be reduced manufacturing time over the process involved in making metal engines.''

Of course, it is very early to make any judgment about how well the plastic engine matches up against the conventional engine. The auto industry is, for good reason, slow to accept any of the myriad new ideas that pour in every year. And the plastic engine, whatever its virtues, will inevitably be more expensive - and may never be cost-effective in the eyes of Detroit.

Nonetheless, Mr. Holtzberg says the auto companies are interested in plastic engine parts, and have some under testing.

The engine developed by Mr. Holtzberg weighs 168 pounds, as opposed to about 350 pounds for a metal engine. There is still some metal in Mr. Holtzberg's engine - some 68 pounds of the total. Things such as the crankshaft, camshaft, valves, combustion chamber and cylinder liners are made of metal, he said.