OK, it looks pretty much like next year’s top-of-the-line Mazda sedan. But there are obvious signs that it’s lots more than that. This 400hp metallic-maroon-red car, spotted with logos and numbers,  is hunched way down, like a tomcat about to pounce on a field mouse, its front bumper so low you could barely stick your foot under it. There’s a distended air scoop and a low-projecting lip spoiler. There are low-slung silhouette side skirts, aerodynamic side-view mirrors and an adjustable rear wing and rear under-spoiler to push that high-slung tail down at race-track speeds.

But it’s the Mazda 6 Skyactive GX’s mechanical innards that make it one of the most radical “stock” racing cars on the track today. If not yet the most successful.

The car didn’t finish in its first big race this year—possibly, according to Road & Track magazine, because it had just been delivered the week before. It finished second in its next two races, so you could at least say it’s a very promising design. But this mixed showing also reflects its singular, developing technology—it’s probably the only diesel race car on the American auto racing scene today. 

At its Mazda Racing press rollout in Irvine on Tuesday, Florida racing entrepreneur Sylvain Tremblay explained how, despite an incredibly detailed computer-generated design process that went into its development, the GX engine remains a work in progress as its racing career advances.

Mazda likes to stress that the car’s little 2-liter Four contains more than half stock Mazda engine components, but the engineering of the crankshaft, connecting rods and other moving parts remained a huge challenge because there just haven’t been very many diesel powered racing cars. So Mazda’s engineers had to pioneer an entirely new highly-stressed, high-speed diesel technology.
It’s not completely new, though. Giant U.S. truck-engine manufacturer Cummins fielded diesel-powered Indianapolis cars in the early 1950s. Six years ago, Audi won the Sebring 12-hour race with its 12-cylinder diesel R10, a custom super-racer that looked like a Batmobile designed by Klingons and cost a reported $15 million just to develop.

Mazda’s taken a far different approach, which is to be innovative, but also to stay closer technologically to its production cars that the consumer can relate to. Theirs, for instance, is the first diesel race car to have a double compound turbocharger, Tremblay notes, but it also runs with a stock-production engine block. Diesel’s power pulses are much stronger than a conventional engine’s, though, so the top half of the engine was subjected to a lot of redesign—much of which is still going on. Says Tremblay, “It’s a continuous development, we’ve changed to rigid valve lifters and have had to redevelop the fuel (injection) system.”

But, he notes, the problems the cars have experienced since their first run at the Rolex 24 in Daytona have generally not involved the engines. It’s hoped that in future competition, starting with the Road Atlanta this weekend, the Mazda GX racing diesel can show more of its strengths over the conventional gasoline engine, such as  higher torque for faster acceleration, and much lower fuel consumption, meaning fewer fueling stops.

“It would have taken far longer to find the faults if we hadn’t raced these cars first,” Tremblay said.

(Listen to Marc Haefele and John Rabe's conversation about the new Mazda diesel race car with one of its drivers.)