IMSA’s Balance of Performance spreadsheet for this weekend’s Rolex 24 At Daytona lists a model known as the “Corvette C8.R GTD.” It’s a new listing that’s found nowhere other than the WeatherTech SportsCar Championship. And if you know the backstory story, the two factory Corvettes are the only cars among the 59 others in the field that don’t conform to the same regulations as the other models in its class.
Corvette’s mid-engine C8.Rs won the majority of the races and two championships across 2020-2021 in IMSA’s GT Le Mans Class where the cars were built to the ACO and FIA’s GTE formula. Last January, declining GTLM car counts led IMSA to announce its closure at the end of the season along with a move to a new replacement class, GTD Pro, using the popular GT3 regulations as the category’s basis in 2022.
Although it had a year to do so, Corvette opted against building a brand-new GT3 version of the Corvette C8 to compete in GTD Pro. Instead, the brand asked IMSA if it would accept its GTE-spec C8.Rs with a custom, hybrid GTE/GT3 specification to race among the full GT3-spec cars from Aston Martin, BMW, Ferrari, Lamborghini, Lexus, and Porsche.
And with a commitment to creating a new Corvette C8 GT3 for 2024, IMSA officials agreed to work with General Motors, the Corvette team, and Pratt & Miller Engineering — builders of the C8.R — to devise an interim technical solution to keep the popular cars on track through 2023.
GT3 regulations require vastly different aerodynamic, mechanical, and electronic packages than GTE, and it’s here where IMSA technical director Matt Kurdock and his team engaged the Corvette group to come up with a process that ultimately led to modifying the GTE C8.R in a number of ways to dial its performance down to GT3 levels under the ‘Corvette C8.R GTD’ template.
Kurdock walked RACER through the finer details of how the conversion project began and all that has transpired on the way to seeing the Nos. 3 and 4 Corvettes qualify sixth and seventh among 13 entries on their GTD Pro debut.
QUESTION: How did IMSA come up with a process to make a GTE car fit into a class that runs a few seconds slower than GTLM and features GT3 cars that are built from the outset with a lot of driver aids that weren’t allowed in GTE?
MATT KURDOCK: It all started with the review of the two different regulation sets, the FIA GTE regulations that we knew well and then the FIA GT3 regulations that we use in GTD, and just trying to understand where the regulatory boundaries were between the two platforms. There were obvious things that stand out, like GT3 cars are allowed ABS (anti-lock braking), they use a different fuel type in IMSA, and their engines are restricted in a different manner than GTE for naturally aspirated engines.
There’s different amounts of gear ratios permitted in each class, there’s different things you can do in the gearbox, so it was really just listing all those things out and then trying to figure out what was feasible to adapt the car from the GT3 regulations. And then it was providing enough leeway for us to performance-balance the car appropriate to the existing GTD category. GTE cars, generally speaking, are a higher downforce, higher drag car, and a GT3 is a lower downforce, lower drag car. Likewise, GT3 are a higher mass, higher power car, and GTE is a lower mass, lower power car.
So you build these performance windows out of the drag versus downforce, the drag versus power, and the power versus mass. And we plotted all of the current GTD manufacturers in that window and looking at where IMSA is actually running those cars. And then we had the data on the Corvette from its windtunnel testing when it was homologated for GTE, which the FIA does at the Sauber F1 windtunnel. And then we had a wealth of dyno data on the car from homologation of the GTLM engine. So we knew pretty quickly what gaps we needed to make up in terms of performance and aerodynamics and power and weight in GT3.
And what we didn’t know was the big leap in performance between GTLM with Michelin’s confidential tires versus GTD and their commercial tires. So what we didn’t have a whole lot of information on was that tire performance offset from GTD to GTLM, but found it was closer than anticipated. So we worked in collaboration with the FIA on assessing all the aerodynamic data that we had on the GT3 cars and then the GTE cars and had a very open discussion with Corvette and Pratt & Miller about the things we had in mind to move the Corvette into the GT3 performance windows we were seeking.
Likewise, we had pretty, fairly transparent conversations with the GT3 manufacturers been telling him the kinds of things we were evaluating, and the process we were going to do to get the Corvette into GTD Pro.