today is Jun 08, 2023

Both Corvette Racing and BMW faced high expectations for the new season of IMSA WeatherTech SportsCar Championship, with fans and competitors anticipating their entrance. In Corvette’s case, it was entering GTD Pro with a modified version of its GTLM car. For BMW, it was bringing a new GT3 car, the BMW M4, to both GTD classes.

The results for both at the Rolex 24 At Daytona were underwhelming at best. Both suffered from a lack of straight-line speed, an absolute killer on a track where straight-line speed is everything. There are a variety of reasons both cars had difficulties at Daytona. For Corvette, part for the problem was the sanctioning body didn’t want this non-GT3 car to come into a class of homologated GT3 cars as an overdog, and had it run 8 degrees minimum rear wing angle (down from the initial 11 degrees). Part of it was Corvette Racing not having a good understanding of the customer Michelin tire when the car had been racing on a confidential tire specifically suited to the C8.R.

For BMW the answer was the nature of Daytona International Speedway — the car had been homologated and tested on European circuits that have almost nothing in common with that track. No European track has the amount of full-throttle time that Daytona has. And in trying to get the car to work at Daytona by finding ways to gain speed on the straights, the teams created new problems.

Come the Mobil 1 Twelve Hours of Sebring Presented by Advance Auto Part, through, both cars were not only competitive, they were fighting for the win. Much of the GTD Pro race was a battle between the No. 3 Corvette of Jordan Taylor, Antonio Garcia and Nicky Catsburg and the No. 24 BMW Team RLL M4 of Philipp Eng, Marco Wittmer and Nick Yelloly until the latter had a gearbox problem in the final hour, leaving the Corvette to take the victory. The No. 96 Turner Motorsports BMW driven by Bill Auberlen, Robby Foley and Michael Dinan looked like a sure bet to win in GTD until it had power-loss issue in the final quarter of the race. Paul Miller Racing was on pace with its brand-new M4 GT3 in its debut as well.

So what changed? The track, for one. Sebring and Daytona are very different circuits; IMSA doesn’t bother specifying a minimum wing angle at Sebring like it does at Daytona, for example. Both cars received Balance of Performance adjustments in between — the M4 lost 25 kilograms from its Daytona spec, and Corvette received a 1mm increase in the size of its engine air restrictor between practice and qualifying at Sebring (although the restrictor was still 0.8mm smaller than what the car had at Daytona). Both Corvette and some of the BMW teams did some testing on the track, with BMW finding some answers in the wind tunnel as well.

“Back in Europe, we’ve actually had a fair few tests,” says Yelloly. “We’ve been working really hard to make sure we put everything together. Obviously, BoP-wise, we had a little help. But I think a lot of the pace has been coming from working back in Europe and in conjunction with RLL. We’ve done plenty of days, trying to dial in different setup options as we didn’t do a load of performance running in in the testing leading up to the debut. That looks like it’s paid off.”

Some of the BMWs experienced broken floors at Daytona. That came from trying to find any extra top speed they could on the bank, and the compromises they made trying to get there. IMSA allowed a -2.2 degree wing angle after the Roar Before the 24 test, and the teams tried it trying to find some more top speed.

“You can’t just lower your wing and have the car handle,” explains Auberlen. “So we lower the rear wing and, to move the center of pressure rearward so the aero balance would maintain, we dropped the rear of the car 12 mil. It had never gone down that low in history. And what they found out in the wind tunnel after all our floors broke was it caused an odd harmonic frequency that put these bending loads on the floors that pulled them all off. So they’d never experienced it because they’d never done that with the wing and the ride height. And there we were. So you have to test, but we never had that option of decreasing the rear wing.”

BMW is on its third iteration of GT3 car. It’s first was based on the Z4 two-seater. Then it went the completely opposite direction by making a GT3 car based on the M6, a much larger car. The new M4 GT3 is somewhere in between, but still presents a lot of frontal area compared to many of the other GT3 cars and it takes power to push it through the air. That’s where the BoP adjustments came in — giving the cars more speed and allowing them to use more sensible levels of downforce.

“They gave us power, which is what we needed. They took weight off, which is what we needed. Also, our old car, the M6, needed an exclusive BoP for Daytona, because it’s such a big car — it could never achieve the top speeds, so it needed a lot more power there just to be even with other people. We were never even, but just to get close. I think this M4 is in that same family; to go down the high banks and hit 185mph, it needs a proportionate leap in the amount of power, and we didn’t have it. So now when we come to a slower track, we’re more in line and we got a power boost and a weight reduction,” says Auberlen.

For Corvette , the BoP adjustments certainly helped. But it was testing that made the biggest difference.

“Daytona was our first race in this class,” notes Taylor. “We only had one day of testing last year before jumping into Daytona. It didn’t go that well from a pace perspective, and then obviously the mechanical problem. But about a month ago we did two days of testing here and found some big gains with the setup. Just understanding what this tire needs to go fast.

“We started with our old GTLM setup and found out here at Sebring, it was way off. So if we didn’t have those two days of testing to develop that and come to this race strong, I think we would have been in big trouble this weekend. So hats off to all the guys of Pratt Miller and Corvette Racing for doing that work and knowing which things to twist and turn to make the car go fast.”

The result was something more resembling parity. Neither Corvette nor BMW sat on pole for GTD Pro — that was the Risi Competizione Ferrari. Garcia qualified the Corvette in third — notably, that was a tick behind Foley in the Turner M4, which qualified second in GTD behind the Wright Motorsports Porsche. The BMWs didn’t even qualify in the top five; Yelloly was seventh in GTD Pro, the 11th-quickest GTD car overall. But there they were, running 1-2 in the evening at Sebring, each employing their individual strengths — which, given a mid-engine V8 in the Corvette and the twin-turbo inline 6-cyclinder at the front of the BMW, were naturally different.

“They were very good on brakes, very good interaction,” said Catsburg after a long triple stint where he was battling with the No. 24 BMW. “It seemed like we had a bit better rotation in some of the medium-speed corners, but overall, it was tough. They were tough competitors and I had some some cool moments; we had side by side and very respectfully, so I had good fun.”

From Yelloly’s perspective, it was about where on the track the different cars worked well.

“I feel high speed we are better and slow speed, they are better,” he says. “Also, when it’s really hot, potentially, they look after their tires a little bit better. But they have that setup. They’ve had this car for a long time. I know it’s now a GTD not a GTLM but they know the car inside out really, whereas we’re still learning so we definitely have room to improve still.”

Taylor feels that one of the things he and his Corvette Racing team got a handle on was a better understanding of the Michelin customer tire and especially learned a lot about keeping the tire working over a stint, given the heat in the afternoon at Sebring, where everything was put to the test.

For all involved, it comes down to experience that just didn’t exist at Daytona. Not just from the perspective of the engineers and drivers, but the sanctioning body as well, as the IMSA technical department learns what the cars’ strengths and weaknesses are and how to keep them competitive with the other GT3 cars, but not let anyone run away with an easy win.

Has everyone figured it out? We’ll see at Long Beach and WeatherTech Raceway Laguna Seca, each of which presents very different problems and solutions to going fast compared to Daytona and Sebring.

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