By Jim McCraw
Followers of international sports car endurance racing know that the Chevrolet Corvettes campaigned by Michigan’s Pratt & Miller have been utterly dominant in GT racing over the past nine seasons, chasing off the competition from Aston Martin, Dodge, and others to the point where the two yellow cars have been racing against each other.
But last year GM decided that racing two cars in GT1 against no one wasn’t as good as racing against real competition, so the company announced a switch to GT2 for mid-season 2009.
The switch from GT1 to GT2 meant design and construction of entirely new race cars that conform much more closely to their streetgoing counterparts – in this case, the Corvette ZR1 supercar.
The GT2 Corvettes share the profile of the ZR1, the front and rear panels, the front splitter under the nose, and a standard interior modified for safety. GT2 rules mandate production laminated glass instead of polycarbonate for windows, steel brakes instead of carbon brakes, aluminum wheels instead of magnesium wheels, and a completely flat rear floor pan – no diffuser allowed. The rear wing chord length was reduced from 400mm to 300mm. Overall, the GT2 version has much less aerodynamic downforce, and runs at a race weight of 1,245kg, much heavier than the GT1 car, but with an improved lift/drag ratio.
The new engine is 6-liters, the maximum allowed in GT2. The team says horsepower is reduced from 590 to 470, and peak torque is down from 868Nm to 725Nm. ALMS has reduced the size of the engine’s intake restrictors, using a weight-to-displacement scale, from 30.6mm down to 28.6mm, a 6.5% reduction. So, this is far less power and torque for the chassis and tires to deal with. For the first time, all the team’s racing engines will be built in-house by GM Powertrain in the same facility where the ZR1 supercharged engines are hand-built, rather than by Katech. The engine is mounted in the stock location, as per the rules, with solid mounts, as is the rear-mounted Xtrac transaxle.
Rules also dictate stock frames, which in the case of the ZR1 are aluminum as opposed to the GT1 steel frames. Because the steel rollcages couldn’t be welded directly to the aluminum chassis (fusion welding of the two materials was tried and rejected as unreliable), a very complex solution was worked out using a threaded socket-and-post solution, space-age sand, and aerospace coatings to prevent a galvanic response between the two metals that would weaken the joint. A major concession was given to the Corvette team in that it was allowed to replace the standard rear transverse leaf spring with A-arms and coil springs.
Pratt & Miller chassis engineer Ken Flory says that, because the team knew well in advance of the switch in classes, there was more time for FEM, CAD, and CFD work up front, to improve chassis stiffness, grip level and lift/downforce ratios, and extensive ADAMS modeling of the suspension system. He says that the new car is about 15% stiffer than the GT1 car in torsion, mainly because of the rollcage location, mounting, and gusseting.
The results of the first three races in ALMS GT2 speak volumes for the design and construction of the cars, and the accumulated experience of the team, crews, and drivers. In three races, the cars never qualified below fourth, and scored a third, a third, and a first victory at Mid-Ohio, Road America, and Mosport in those first three outings.
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