By John Lamm

Mazda often has created race cars with a twist, dating back to when Car and Driver campaigned a Wankel rotary–powered RX-2 in 1973. Come 1991, the last year before the rotary was banned from the 24 Hours of Le Mans, and the Japanese automaker took its 787B Wankel-powered racer to a win in that round-the-clock classic. Old song, new tune for Mazda in 2013 as it puts its new Mazda 6 on the track and again steps away from the crowd, racing the sedan with the same turbocharged 2.2-liter Skyactiv-D turbo-diesel that will arrive in the 6 later this year. John Doonan, who heads Mazda’s racing efforts in the U.S., smiles as he explains how they take pride in “being different.”

Diesel-powered race cars aren’t new; Cummins began racing them in the Indy 500 in the early 1930s and put one on the pole in 1952. Audi’s R10, R15, and R18 have proved the diesel’s worth at Le Mans, but there aren’t many oil-burners powering non-prototype racers. Mazda wants to make the point that its Skyactiv-D production engine is as quick as it is reliable by racing it in Grand-Am’s new GX class.

Doonan quickly points out the teams using disel-powered 6s in Daytona are “able to use so much of the production engine in the race engine. We’re up to over 250 part numbers straight out of the road-car engine that are in the race engine. It’s about 63 percent of the motor. The crankshaft, rods, pistons, and valves are all racing-based for material changes. Those pieces that have been changed have had that done only for durability and materials. If you set a stock valve and a race valve next to each other, they would look identical.

“We have to run a stock Bosch ECU because that’s spec for the series. We’ve had the Bosch guys on because they run the Audi program. Our stock ECU is Nippon Denso, so making that change has been a long process and probably the hardest part of transferring over from stock to race car.”

Also stock is the 14:1 compression ratio, and with Garrett bi-turbos pumping to a max of about 70 psi, the diesel is rated at 400 horsepower and 445 lb-ft of torque. Other than the manner in which it is mounted in the car, this engine is almost identical to the 450-hp, 500-lb-ft diesel that Mazda will race at Le Mans in the LMP2 class.

Doonan adds, “The other big piece is the fuel story. We’re running a renewable synthetic-diesel fuel from a company called Dynamic Fuels. They’re using non-feedstock or non-food-chain remnants of fat, oils, and other meat processing refuse to make this fuel. Dynamic makes 75 millions gallons of it each year and the Navy is currently flying fighter jets off the aircraft carrier Nimitz using this fuel. We’re not using any particulate filters and no after-treatment on the engine. You can sit in the paddock, see the driver push the starter button and there’s no smoke whatsoever.”

Doonan explains that, “When we poured the Dynamic fuel in we didn’t change any of the mapping and the exhaust gas temperatures dropped 50–100 degrees. The horsepower and torque numbers showed a smidge of an increase by 2–4 horsepower.” Given diesel’s reputation for better mileage, Mazda figures the 6 and its mandated smaller fuel tank—14-and-a-half gallons—will go about the same distance as its gas-fired competitors.

While the engine in the 6 race car is new, the chassis, Doonan points out, “is converted from the RX-8. We lengthened it six inches behind the driver’s bulkhead, but it’s essentially the same underpinnings.” Those would include the Alcon racing brakes and the EMCO six-speed sequential transaxle, which has one interesting addition, a strain gauge that automatically blips the engine to keep the drivers from overrevving the diesel, which is sensitive to high revs. While the redline in the race engine is 5200 rpm, they limited it to 4750 for Daytona.

Mazda brought three diesel 6s to Daytona. The “factory” car had drivers Jonathan Bomarito, Marino Franchitti, Tom Long, Sylvain Tremblay, and IndyCar driver James Hinchcliffe. The second car was entered with young drivers who have risen through Mazda’s driver-development program: Joel Miller, Tristan Nunez, Spencer Pigot, Tristan Vautier; but also 65-year-old Yojiro Terada, who has run Le Mans 29 times and is a big icon at Mazda. Car three had drivers Andrew Carbonell, Tom Long, Rhett O’Doski, and Derek Whitis.

After practice at Daytona we talked with Franchitti and Hinchcliffe about their experience in the diesel-powered Mazda 6. Not surprisingly, they liked the chassis, Franchitti saying, “You can feel the RX-8 DNA in the car. Even though it’s a longer-wheelbase, heavier car, they’ve got it feeling very similar, which is nice because the RX-8 was a really nice-handling car. I think the thing that limits these Grand-Am cars are the tires. And there is more downforce than with the RX-8.”

Hinchcliffe agreed, adding, “The brakes are phenomenal, the transmission is excellent. The big difference, of course, is the engine, which revs so low and yet is still so torquey. It’s a big adjustment. And we’ve come from driving what is essentially the loudest engine ever produced by man (the rotary) to one of the quietest ones.” A point we can confirm as the cars (relatively) whispered past at speed.

Franchitti thought that, “Sound isn’t a big deal. Maybe it’s quieter outside, but you still have a good amount of sound inside the car so it’s not like you don’t have any reference. You’re aware of the engine noise and can still drive by ear.” Hinchcliffe added that, for him, “You have to rely a lot more on the shift lights on the dash because you cannot hear the motor as much, especially out on the banking with all the wind noise.”

As for the engine, you have to “get used to the turbo and how it spools up,” according to Hinchcliffe. “It requires a bit of a different driving style. You have to keep the turbos spooled up to stay in the power band and you have to work more with the throttle to keep the rpm up, so you get into the throttle a bit earlier.” The shifter came in for specific praise from Franchitti, saying, “It’s beautiful. It blips the throttle because the engine is sensitive to over revving on the downshifts, so you just brake, push the lever, it blips and the gear just slides in. It’s like butter; so nice and natural.”

• Comparison Test: 2010 Mazdaspeed 3 vs. 2010 Volkswagen GTI
• Instrumented Test: 2012 Volkswagen GTD Diesel
• First Drive: 2013 BMW M550d xDrive

The Skyactiv-D Mazda 6s are the first diesel-powered race cars to compete in the Grand-Am series or at the 24 Hours of Daytona. While the race cars were beautifully finished, their lack of long-distance testing showed too soon. The team had been able to fix such initial problems as a troublesome belt tensioner, but a recurring vibration harmonic problem was bothersome. Before sunset, the 6s weren’t part of the parade at Daytona. Reality had hit the fan.

Two of the cars lost a cylinder because of problems with their fuel rails, while the third had a main-seal failure. Mazda wasn’t willing to assign fault to the rails or the seal, and will be chasing down that harmonic problem to see if that was the source. Doonan summed up Daytona this way: “Today will be noted as a learning experience, a data point, if you will. The Mazda and SpeedSource engineers had over 400 hours on the dyno, but only a few on the track.” Now that process will continue.

Even before the problems, Franchitti had said, “This is just the starting point for this car.”

Jim O’Sullivan, who heads Mazda in the U.S. and is quite passionate about the diesel race cars, looked to the next race, which is at the Circuit of the Americas, and said, “Just wait until Austin.”

Source: FULL ARTICLE at Car & Driver