GLTC Watkins Glen Simulations

Grid Life is coming to my home track of Watkins Glen International, April 28-30 2023. This is the first time Grid Life will come to this iconic track, and I hope to race in one of the events.

Unfortunately my Miata doesn’t have anywhere near the 12.5:1 lbs/hp limit of Grid Life Touring Cup, and I’m not an A-class driver besides. The end result is I wouldn’t be competitive. Still, I thought it would be fun to run some simulations and see my car’s potential. There have been a few rule changes to balance out engine performance, and I thought this would be a good opportunity to smoke test those as well.

For these simulations I’ll use five different Miatas. Each has an identical aero kit that results in .48 Cd and -1.0 Cl (values I’ve measured on my car), with standard values for air density and rolling resistance. I’ve spec’d the same tires at 1.2g of lateral grip, and 1.1g of longitudinal grip.

Per GLTC rules, all the builds take a 4% penalty to weight because they run a splitter and large wing, but being Miatas, they get some back by having no ABS (-2%) and running wheels 16″ or smaller (-1.5%). The end result is that all the builds take a .5% weight penalty to the standard 12.5 lbs/hp ratio.

Now here’s where it gets interesting: each Miata has a different engine, and this changes the final lbs/hp ratio. Per the latest rules, small-bore engines less than 1999cc get a 1.5% break and 1999-2500cc get a 1% break. Turbos are penalized 1.5%, K24 swaps are a 2% penalty, a 5-liter is a 4.5% penalty, and so on.

I’ll do five simulations based on different engines. Note that I went out and created each engine in OptimumLap using dyno charts I found online, and so the torque curves should be pretty accurate.

  • N/A – 170 hp, 2103.75 lbs. This represents a well tuned 1.8 BP. NB2s would have more torque, as I used the dyno graph from my NA8 and bumped it by 141%.
  • Turbo – 200 hp, 2512.5 lbs. I pulled this dyno from someone on Miata Turbo. The 1.5% penalty for using a turbo is offset by the 1.5% benefit for under 1999cc.
  • Ecotec – 192 hp, 2388 lbs. I used the dyno chart from an Ecotec swap I found somewhere.
  • K24 Z3 – 200 hp, 2562.5 lbs. I used the dyno from KMiata’s Z3 blog post. I could have used the A2, but then I’d have to detune it more. Notice the power output is the same as the turbo, the 50 lb weight penalty comes from the K-swap.
  • 5.0 Mustang – 220 hp, 2887.5 lbs. Fox Mustang old school V8 Miata.

It’s worth noting that four of the cars came in at less than 2725 lbs, and would be limited to a maximum 245mm average width tire. The 5.o Mustang swap is slightly heavier, and thus could run a 255 tire. To simulate that, I’ve given it a very small bump in grip (from 1.2g to 1.212g).

Before we look at the lap times, understand that I’m not trying to predict an accurate lap. OptimumLap is a tool that’s good at predicting the differences in changes you make to your car, but can’t factor in weight distribution, elevation, camber, surface friction, or other variables. So how did the different engines shake out? Take a look at the speed trace:

  • 2:09.11 – Turbo (orange, like 949 Racing)
  • 2:09.20 – Ecotec (green, cuz eco)
  • 2:09.39 – N/A (red, because mine is)
  • 2:09.82 – 5.0 Mustang (blue, for USA int’l racing color)
  • 2:09.85 – K24 Z3 (purple, for Miata royalty)

The first thing you might notice is that the red car (N/A) is considerably slower on the back straight. However, because aero works better on lighter cars (the percentage gain in grip is higher), the red car has higher min speeds in the fast corners. I thought the N/A would be last, but it’s right in the middle.

The fastest builds are the turbo and Ecotec, but there’s not much between them. The Ecotec has a great powerband, but the little turbo has a fatter torque curve and wins overall. The slowest cars are the K24 and Mustang swaps, about half a second adrift. These engines are penalized the most, so it’s not surprising.

But what is surprising is how close the times are on such an extreme speed track. I’d say the rules parity is very good right now. Naturally, there are some caveats here, because OptimumLap is a single-point mass calculator, and can’t factor in elevation changes or camber.

And that’s a problem because all but one of the corners at Watkins Glen has favorable banking. If the simulated lap times seem slow, that’s why. If I adjust the grip by 110% and re-run the simulations, two seconds disappears. Notice that the K24 and 5-liter swap places, but the other cars remain where in the same order:

GLTC cars have yet to race at WGI, and so I don’t know if 2:09 or 2:07 is more accurate, that’s not really the point of this experiment anyway. I’ve seen some NASA TT5 cars do 2:08-2:09, so that’s in the same ballpark, though.

Now it’s time to see how my Miata (black in the following speed trace) would do against these cars. While the engine is down on power, my car has better aero than the average Miata. On the back straight I’d give up 7 mph, but over a lap it would be only about a second slower. Not that bad, actually.

Grid Life will be at several tracks this year, including Gingerman, Mid-O, Lime Rock, and Laguna Seca. I have those track maps in OptimumLap, so I ran those simulations as well. The parity across the classes is pretty good, and the order stays mostly the same. However, there is one surprise winner.

One final comment is that these simulations were all done on Miatas with aero, and aero favors light weight. But air has resistance, and it takes power to overcome that. A heavy, torquey car with maximum tire width and a lot of mechanical grip exploits that imbalance. I don’t have the data for such cars so I can’t run those simulations, but if you look at the race results, that’s also a winning formula. Therein lies the success of GLTC, a diverse selection of cars and evolving parity in the rules.

I’m really looking forward to April 28-30, although I’m not sure if that will be as a participant or spectator. The weather probably won’t be great, but it’s going to be a heck of a party.

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