Nigel S Dobbie's definitive book on Corvette racing

2015 TUDOR Balance of Performance

At the beginning of the 2015 season IMSA the Governing body of the TUDOR United Sportscar Championship set the Balance of Performance for all four Classes. I am only interested in what happened in the GTLM Class. So I have based my calculations on the following scenario:
I took all of the published IMSA Timing data available at for the 2015 TUDOR United Sportscar Championship for all of the rounds involving the GTLM Class.
This year I did not filter out any of the laps except where there were zero data points I grouped the data by Brand and Car (I added two new columns to the data Circuit and Brand). I then wanted to find the fastest theoretical lap by Brand and to do this I looked at all of the sector times to calculate the best sector time for every sector regardless of the driver or the car i.e. Sector 1 for Corvette for example might be Antonio Garcia in the #3, then Sector 2 might be Oliver Gavin in the #4 and so on to get the Best Theoretical Lap Time.
For all of the charts below the bars represent the time in seconds for the cumulative theoretical best lap time by brand and is measured in seconds on the left hand axis (remember lower the bar = faster the car), the dark red line represent the average time for all the brands (6 at Daytona and Sebring and 5 for the rest of the rounds as Aston only competed at the first two rounds). The red line represents the top speed as measured by IMSA at their speed trap, this is the actual best time by brand and is measured in MPH and represented on the right hand axis of the chart.

Race 1 Daytona

Race won by Corvette #3, BMW #25 a close second with both cars completing 725 laps the #4 Corvette finished third on 718 laps. The Corvette was potentially the fastest car but only marginally from the Porsche and Ferrari who were both less than 1/10th of second slower the Corvette. Both factory Porsches collided with each other and therefore removed themselves from the chance of defending their Daytona 24 hours title, had they not they would have been right there at the pointy end of the race. IMSA did a great job of balancing the performance as all five brands were within just over 2/10th’s of a second and top speed was covered by less than 1 mile per hour through the speed trap. Now some would say that the BoP for Daytona is probably the easiest to set as so much of the lap is flat out.

Race 1
No Adjustments to Performance were applied to the GTLM Class before Sebring.
Race 2 Sebring
Race won by Corvette #3. IMSA again did a great job of balancing the performance as again all five brands within 4/10ths of a second over the best theoretical lap and Corvette was actually the slowest and significantly i.e. 2 of the 4/10ths spread of the field. The top speed was also covered by 2 miles per hour for every lap by every car within all five brands. Porsche was potentially the fastest car.
Race 2

8th of April and IMSA issued two separate Adjustment of Performance bulletins, BMW got a 10kgs weight reduction, they also received a 2 liter fuel tank increase. Corvette had their fuel capacity reduced by 3 liters, Ferrari received a 4 liter fuel tank increase but were penalized with an extra 15kgs of weight and a 0.3mm reduction in their air restrictor. Porsche received an extra 3 liters fuel tank capacity.

Race 3 Long Beach
Race won by #25 BMW. BMW potentially fastest, Porsche slowest but all four brands still covered by 4/10ths of a second the 10kgs adjustment definitely helped the BMW as they were more than 2/10ths of a second faster than the other three brands. I would conclude that the adjustment to the BMW’s weight was actually unnecessary and tipped the race in the direction of BMW.
Race 3
Race 4 Laguna Seca
1-2 finish for BMW and Porsche third. Ferrari was actually potentially the fastest car and Corvette the slowest. The Ferrari was potentially fastest by more than a 1/10th from the Porsche, the reason that Ferrari could only manage a 4th place finish in the race was that towards the end of the race the car slowed significantly compared to the top three finishers in the class despite having been fasted earlier in the race. Significantly all four brand were still covered by less than 4/10ths of a second.
Race 4
12th of June and IMSA announced another Adjustment of Performance and this time Corvette received a 10kgs weight reduction and also a 0.3mm engine air restrictor increase, both potentially good for the Corvette and in recognition of their poor performance particularly at Laguna Seca.

Race 5 Watkins Glen

Race won by the Falken shod #17 Porsche followed by one of the factory Porsches and then BMW. Corvette were potentially the fastest car and Ferrari the slowest. The overall gap between fastest and slowest had now stretched to over ½ second. The BMW and Corvettes were closely matched but nearly 3/10ths of a second faster than the Porsche. GTLM field still covered by about 1/2 second. However, this was a wet race so really the data is very inconclusive.
Race 5
Race 6 Canadian Tire Motorsport Park

Porsche wins the race from BMW and then Corvette. Porsche and Corvette both evenly matched for potential best lap time, BMW 2/10ths back and Ferrari a further 2/10ths back. GTLM field still covered by 1/2 second.

Race 6
Race 7 Road America

1-2 finish for Porsche and they had a significant advantage -more than ½ second in potential fastest lap – over Ferrari who finished third with Corvette another 2/10ths back and BMW more than a second further back. The field of four brands now spread over nearly 2 whole seconds!

Race 7
30th of July and IMSA announces the latest Adjustment to Performance and Ferrari and Porsche both get a 10kgs weight reduction and further more Porsche gets to reduce the drag of the Gurney flap on the back of the rear wing to just 10mm, this despite them just having been the fastest cars at Road America.

Race 8 VIR

Porsche 1-2 again with Ferrari again third. Porsche more than 3/10ths faster than the Ferrari and BMWs who were closely matched over a potential fastest lap with the Corvette lagging more than 6/10ths of a second further back. Almost 1 second between potentially fastest Porsche and slowest Corvette.Race 8

10th of September and IMSA announces another Adjustment to Performance which gives 20kgs extra weight to the Porsches.

Race 9 CotA

BMW win a fuel conservation race, finally the extra 3 liters of fuel capacity helped them whereas both factory Porsches and the #3 Corvette had to take on fuel just a couple of laps from the end of the race. Porsche were significantly fastest – despite the extra 20kgs of weight – with a potential best lap time of 2:03.000 whereas Ferrari were 1.65 seconds slower.

Race 9

Race 10 Petit Le Mans

Porsche won the race overall and therefore obviously the GTLM Class. The race was extremely wet and was actually shortened to less than 8 hours when IMSA decided to throw the checkered flag at 7hrs and 51 minutes. The reason that the Porsche and other GTLM class cars did so well has to be down to the Michelin wet weather tire being a significant advantage over the Continental wets being used by the other classes. Porsche was significantly faster – nearly 1.5 seconds – over the BMW who were 2/10ths faster than the Corvette and the Ferrari was slowest by a massive 2.4 seconds when compared with the Porsche. The wet weather and lack of aerodynamic grip of the Ferrari was a factor in their slower lap times.

Race 10


IMSA made a couple of mistakes in my opinion. The BMW weight reduction before Long Beach was one and not adding weight to the Porsche before VIR along with reducing their rear Gurney flap was the other.

Overall IMSA learnt from their mistakes in 2014 and didn’t make too many changes to the cars throughout the season. Personally I would like to see the cars all run with the same weight, same fuel filler restrictor size, fuel tank capacity size etc. I would like to see adjustments to performance being done with just the air restrictor on the cars. This adjusts the horsepower but doesn’t alter the other dynamics that weight adjustment does.

Michelin changed their tire during the season and this positively helped the Porsche and obviously that caused a performance change that IMSA cannot factor into their AoP. I would like to see the tire manufacturers provide a control tire that all teams use all season so that it is then up to the teams to get the best out of it. I know Corvette Racing did not have the time to really test the new Michelin and therefore they were playing catch up for a lot of the second half of the season. Porsche and Ferrari have a big advantage as brands over the other manufacturers as they have so many cars running in the FIA WEC that can be used for data comparision.

For 2016 Ford will be entering their new GT and this will cause an additional headache for IMSA given that the engine is a smaller capacity (3.5L) twin turbo. Porsche will no longer have the Falken shod #17 car as their third bullet as the Team Falken Tires announced they were withdrawing from the GTLM Class.


2 Responses to “2015 TUDOR Balance of Performance”

  1. Great stuff, very interesting.

  2. you’re missing a bunch of interaction that goes on in GTLM in the races with holding back and perdicting BoP pace, and some important technical details. here are some of the significant ones.

    1-Porsche has different tires then every other GTLM/GTE. Their tires are Porsche specific.
    2-Michelin brought out a single stint tire for VIR and Road America only. But of course Porsche tires were only for Porsche. All the other races (all Michelin cars) were run with standard endurance spec tires.
    3-Porsche was propelled forward in 2014 (see BoP data 2014 to 2015 race by race) when they were given a 100 splitter. The Porsche GTE car is only 50mm. they also got a larger rear wing to balance the downforce; IMSA only for both. They just haven’t had their housekeeping in order until this year. They have the strongest BoP in the entire GTLM field by a mile and should be kicking every ones butts all the time the last two sessions.
    4-compare restrictors from GTE in ACO and ELMS to GTLM in IMSA and you’ll find Porsche, Corvette and BMW have a larger change to the ACO base restrictor rules by area then the Ferrari does. In IMSA the Porsche is the biggest gainer comparatively. You can’t compare WEC, ELMS and IMSA straight up.

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