Process of Converting C8.R from IMSA GTLM to GTD-Pro Class

[teamzr1]

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 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 differently than GTE for naturally aspirated engines.

There are different amounts of gear ratios permitted in each class, there are 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.

 

[teamzr1]

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 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 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 to 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.

Q: I’m sure you had to go into the process knowing that no manufacturer is going to give up a millimeter in any area or the slightest amount of performance if they don’t have to, especially with such a successful car like the C8.R. Did you feel like they were transparent and open and accommodating?

MK: Working with Chevrolet and Pratt Miller on this process was definitely a collaborative effort. On the feasibility of each one of the things throughout the course of the project, everything that we asked them to change has been changed. So in that regard, it’s been a good partnership between us because some changes required significant modification.

For example, GT3 cars don’t have homologated weight distribution areas like GTE. All GT3 BoP ballast goes into a single location in the passenger side floor. With a GTE, the weight distribution is homologated, and you can put ballast in multiple locations on the car that are identified by the homologation to help balance the car.

So it meant just even applying BoP weight to the car meant that if we didn’t do this adaptation with Corvette, the effects of that BoP weight would be different.
And so one of the adaptations was that we had them design in and modify their cars to accept was a ballast box on the passenger side, which wasn’t as easy as it could sound. It meant moving systems, electronics, fire bottles, and they had to really do a lot of design work on that side. And it also meant they were going to give up some of that flexibility to move ballast around the car.

Another area that’s different is we’re actually using two different configurations of rear wings on the Corvette in GTD Pro, depending on the circuits that we’re going to. We’ve also permitted them to carry over some GTE hardware, because it’s the most realistic thing we could do that that wouldn’t necessarily cause a major redesign of things like the engine so, so we’re using the GTE sonic restrictors to regulate engine power instead of the FIA GT3 air restrictor.
Air restrictors seems like a very simple thing, but when you change that design, it has a pretty fundamental impact on the engine architecture that that was chosen.

In the gearbox, we’re the changing the spec of the differential, and there are a lot of small details that aren’t necessarily straightforward to quantify on what lap time difference would be if they’re permitted to keep that hardware from their GTLM car, but as the homologation authority on this Corvette GTD car, we think that Corvette has the tools they need to race. And then IMSA has the tools we need to balance the car respective to the GTD Pro class.

Q: What are the other areas of interest in terms of tuning or modifying the car to its GTD specification?

MK: On the engine side, we’re using a fixed lambda, so basically, a fixed engine mixture, which is consistent with what we do on all GTD cars. In GTLM, it was more of an open mixture, with the richest mixture being limited.
So, that’s part of how they’ve converted to use IMSA’s VP 100 fuel instead of the E20 specification in GTLM. That’s required different engine tuning which Chevrolet was able to do, and we had to re-dyno test the engine on that different fuel specification with the declared lambda that they run.

And really the last one is the four gear ratio options they had in GTLM, and those are reduced to three gear ratio options for the GTD Pro class. It’s a fairly decent list of things that we need to line up the performance window for the Corvette, and as you can see from the BoP tables, in GTD Pro form, the car has come back heavier to line up to where the class performance targets are.

Q: Coming out of the Roar where they qualified sixth and seventh, but weren’t necessarily close to the GTD Pro class leaders, the Corvettes received an extra 15 kilos of weight, which was a surprise to some, but was given a break on rear wing angle to improve straight line speed.
What can you share, to close, on the final BoP specification for the C8s going into the race?

MK: We tend not to talk about the specifics of any one car’s adjustments, but I can give a general statement that we use the data we have available on all cars within the GTD and GTD Pro classes to analyze how they were achieving lap time.
Each car shows a little different strategy on how they race, and the technical committee took that into consideration with the BoP adjustments.

And I think you’ll see that on a few cars, some have gone up in mass and down on a wing and that’s done to rebalance how a car makes its lap time.
Reducing the drag and increasing the mass changes what we’re seeing for performance in the handling sectors like the infield.

It’s so it’s always difficult to introduce brand-new cars when there’s limited (BoP) carryover like when the Corvette was running in GTLM, and it’s very difficult to start our season introducing cars at one of our more unique racetracks on our calendar–one of our more power- and drag-sensitive racetracks.

There’s a lot at stake with the Rolex 24 for everybody. Everybody wants to win it, and that that makes the technical committee’s job particularly challenging, though, but that’s what we’re here to do.