Flat-four engine is the heart of Subaru, Toyota sports coupes

Masuda said the boxer engine's flat profile allowed the partners to place the engine's center of gravity "this high," gesturing at his knees.

That placement is almost five inches lower than where a conventional I4 would have been, and lower in the car than any car this side of supercars such as Ferrari, he said. The low center of gravity produces better handling, he said.

Other than the engine, Subaru is mum about most of the details of its version of the car. Subaru showed off a see-through mockup of the "Boxer Sports Car Architecture" at the Geneva show. Look for more details on the Subaru version at the Tokyo motor show this fall.

On the other hand, a second generation of Toyota's version made its debut at the Geneva show as the Toyota FT-86 II concept. Wassim Kanoun, Toyota Motor Europe product communications manager, said the two cars share 95 percent in common.

Since Subaru offers all-wheel-drive models exclusively in the United States, we're not used to equating the Subaru boxer engine with better fuel economy. However, Subaru says weight, and therefore fuel savings, were additional reasons the partners used the boxer four.

Masuda said the horizontally opposed movement of the cylinders in the Subaru boxer eliminates the need for balance shafts, which would have been needed in other engines. In addition, the block is aluminum. Because of those factors, the boxer weighs close to 50 pounds less than a comparable inline four-cylinder, he said. That's a big savings in an industry where saving a single kilo (2.2 pounds) is a big deal.

The rear-drive Subaru sports car will be a departure for the brand in the United States when it goes on sale here next year. Subaru has stuck with all-wheel drive exclusively in the U.S. market since 1994, according to Subaru of America. In other markets, Subaru continued to offer front-drive cars.

Subaru of America spokesman Michael McHale said Subaru figures U.S. enthusiasts won't mind the departure from Subaru's usual practice. "If you could forgive us for anything," he said, "it would be this car."