Cadillac continues to dominate the new IMSA DPi (Daytona Prototype international) class with its 6.2-liter LT-based V8, winning the season’s first seven races and scoring 13 of the 21 podium finishes. EngineLabs first profiled the engine that was designed and built at ECR Engines shortly after Wayne Taylor Racing driving the Konica Minolta Cadillac DPi-V.R car won the Rolex 24 Hours of Daytona. Since then, more information and additional photos have been released as the team won the Sebring, Long Beach, COTA and Belle Isle events.
Recently, the Mustang Sampling team captured the IMSA race at Watkins Glen, then the Whelen Engineering car won in Canada. The most startling revelation has been that all three Cadillac factory teams used just two engines each to compete in the season’s first five races.
“This season we exchanged engines only once for each car, each receiving a fresh engine after the Rolex 24 At Daytona. We disassembled the first engines and found no issues, problems or anomalies after the Rolex 24,” reports Richard Brekus, Cadillac’s global director of product strategy. “The three cars have each run the engine we installed after the 24-hour race, taking the green flag on the first practice at Sebring, running and winning at Sebring, Long Beach, the Circuit of the Americas and Detroit with no major issues.”
From Sebring to Belle Isle, the total race distance was about 2,822 miles, and each engine experienced approximately 36,000 upshifts over more than 13 hours of wide-open throttle—according to the data loggers. During that 4-race stretch, none of the engines for the three teams had an oil or filter change.
As noted in the first story, the engine was developed at ECR on a fast-track schedule that coincided with the chassis and body styling. Durability was the primary focus of the engine program, since IMSA set a target of 600 horsepower—a performance level easily achieved with 6.2 liters of displacement and cylinder heads patterned after the LS7 ports. Mass was somewhat of a concern since the Cadillacs would be racing against smaller purpose-built race engines that didn’t have any allegiance to production heritage. But ECR solidified the bottom end with ARP 10 mm main studs and a fully counterweighted Bryant Racing billet crankshaft that was not treated to any weight-reducing measures. When compared to a NASCAR engine, the main bearing diameter is larger but the connecting rod pins run the standard 1.850-inch diameter. We’ve also learned the Carrillo H-beam rods are not scalloped to reduce weight, and Mahle pistons are secured with 20mm wrist pins.
The block is a special aluminum casting but still retains familiar LS/LT dimensions, such as the 4.40-inch bore spacing and 9.240-inch deck height. The Precision Racing Components lubrication system features a split dry-sump design where the 4-stage scavenge pump and air-oil separator are located on the right side of the engine. The pressure side of the system is belt-driven on the left side of the engine and includes the filter and heat exchanger. Between the two side of the dry-sump system is a 2-gallon oil reservoir mounted in the front plate. Also in the front plate is the water expansion tank, so the engine can be pulled from the car as a unit without disconnecting any coolant or lubrication hoses. As noted earlier, the engine is a semi-stressed member of the chassis, basically acting as a spine for the carbon fiber monocoque.
We’ve also learned a little more about the valvetrain, but the durability strategy doesn’t waver. The block is machined for 60 mm roller cam bearings that support a Comp Cams billet camshaft and Jesel .937-inch-diameter lifters. Smith Bros. pushrods and T&D Machine 1.7:1 rocker arms carry the lobe instructions to Del West titanium valves.
“Nothing really in that valve train is all that far off from production, to be honest. We took some knowledge from our NASCAR side, as far as valve sizes, which are very close to what production runs,” says Brian Goble, sports car engine program manager at ECR, which is located in Welcome, North Carolina. “But the mass of the components, and the geometry of the rockers, are much less aggressive compared to what NASCAR runs. It’s a standard steel rocker arm assembly.”
One deviation from the production LT engine found in Cadillac performance vehicles is the absence of direct injection. Goble says the development team found the necessary powerband and driveability using port injection, so DI remains an option for marketing purposes or even as a test-bed for a future production part.
A closer look at the unique 2-piece Kinsler intake manifold reveals the bottom half is machined from billet aluminum. It attaches to the cylinder heads and locates the GM fuel injectors. The top half supports the eight butterfly throttles that have different-length trumpets to even air distribution. The electronic throttle control and throttle shafts are all contained under the carbon-fiber airbox mounted above the intake manifold. Air is fed into the airbox through two sonic air restrictors mounted on the car’s roof. IMSA can adjust the size of the openings as part of the Balance of Performance (BoP) guidelines. This season the Cadillac teams have had numerous BoP adjustments that have included decreasing the fuel tank, adjusting the aero package and even changing the transmission gear ratios. Meanwhile, IMSA has allowed competing engines to increase boost as a way to even out track performance.
While Cadillac appears headed for a dominating season on the IMSA WeatherTech SportsCar Championship, competition is likely to increase substantially next year. Honda Performance Development and Roger Penske have agreed to field two Acura DPi cars next year. The power foundation will be HPD’s well-tested AR35TT twin-turbo V6. The proposed Acura ARX-05 DPi cars could be unveiled in mid-August at Monterey.