PHOENIX, AZ - General Motors Corp. will soon launch a new aftermarket spark plug it claims dramatically outperforms high-profile competitors, including the Bosch platinum tip and Splitfire.

The plug, the AC Rapidfire, pays for itself in lower fuel consumption, GM says. The plan is to have plugs available for 90% of the cars sold in the U.S. by the end of the year. It's also charted for OE applications.

The plug not only promises significant improvements in fuel economy and engine smoothness, it should build enthusiasm and commitment among GM's components engineers and salespeople, says Richard N. Carr, general product manager for AC Rochester at GM Service Parts Operations.

"This plug is more than a new product for us," Mr. Carr says. "It is a core, something for people to get excited about. You need that to motivate people. I hope it's the first of a string of products like that."

The U.S. replacement spark plug market amounts to 550 million units annually, says James F. Obremski, GM SPO marketing manager for AC Rochester products. Premium plugs like the Rapidfire, Splitfire and Bosch platinum tip account for about 10% of that. He says the total replacement market is shrinking because of longer plug life and fewer cylinders in engines. However, the premium replacement segment, which did not exist until five years ago, is growing.

The Rapidfire combines several technologies AC Delco Systems had already developed. GM began consumer clinics to find out what people wanted from a plug early in 1994. Focus groups and a 1,200-person survey showed the top concerns were faster starts, quicker throttle response, smoother idle and improved mileage.

"Our tradition is to take OE parts and put them on the aftermarket," Mr. Carr says. "This is a breakthrough product because it began as consumer research for the aftermarket.

"A bunch of engineers went into a room with the consumer research posted to the walls and said, `How can we give them this feature?' By the time they were done, we had something patentable, although all the individual technologies already existed."

The plug combines new materials and an advanced design to improve performance. GM claims 2% higher fuel efficiency and 27% smoother idle than its premium competitors. The improvements were immediately apparent in back-to-back tests of used cars running new Rapidfire and OE plugs. In addition, GM plans to beat the competition on price. Mr. Obremski says the Splitfire retails for $5.92 each. The Rapidfire carries a $3.99 price tag.

The technological advances include a redesigned electrode to reduce heat loss and improve flame propagation. GM achieved that with a "clipped gap" tapered sidewire at the bottom of the plug. The shape reduces the amount of cold metal impeding the flame front. In conjunction with a serrated and tapered electrode, the plug also exposes more of the sparking area to the fuel mix for better ignition. It requires less energy to spark and transmits more energy to the fuel mixture than standard plugs.

The plug also uses a nickel-silver alloy developed by AC. The alloy is more conductive than materials in other plugs, so the sidewire is smaller, further reducing heat loss and interference with the flame front. The plug also extends 1 mm deeper into the combustion chamber to advance the spark.

The cooler sidewire also reduces corrosion of the plug, making it more visually attractive over the long run and easier to remove, says Dale L. Hostetler, AC Rochester ignition and filtration chief engineer.

GM says all of these innovations will make it into its original equipment plugs, beginning with the 1996 1/2 Quad 4 engine.