Options on a Spec Miata for Endurance Racing

Evan and I were working on my car this weekend, and, surprise, we started talking about racing Miatas. We see a lot of Spec Miatas converted to endurance racing, and we discussed what different options to power, weight, grip, and aero would do in a 24-hour race.

So we imagined a fantasy team that started with a 1.6 Spec Miata, which they wanted to improve for endurance racing. Since this is fantasy game, let’s pick a silly fantasy name like Hugh Jass Racing. And give them a web page, and channels on twitch and youtube.

IMG_0691 Tom (1).jpg

Here are their current options, with the data necessary to run these simulations in OptimumLap.

  • Stock: This is their car as is. OEM hard top, stock bodywork, lowered suspension, 2300 lbs, dynoed at 120 hp, 4.3 final drive. .45 Cd, -.45 Cl. The drag and lift might be more than this, I’m being kind.
  • Light: Drop 200 lbs.
  • Power: Swap in a 1.8, 140 hp.
  • Grip: Like most Spec Miatas, this team has 15×7 wheels. This option simulates changing grip from 1.1g to 1.2g. This could be 205 RE71Rs on 15x8s, or some other cheaty 200 TW tire. If you don’t think there’s a difference in wheel width, go to the GRM Summer Tire Test and you’ll see the Z3s picked up 8/10ths of a second on a 33-second course by going from 7″ to 8″. (But still half a second behind the RE71R on 8″.)
  • Aero: Add an airdam, 4″ splitter, and 9LR wing. This changes the Cd to .48 and the Cl to 1.01, and adds 40 lbs.

I ran this simulation at VIR. After adding the car data in OptimumLap, I corrected the grip to 105% to get lap times that are about what Evan runs at VIR when the track is clear, and his knowledge of what other teams typically run on similar setups. We also cross listed top speed in OptimumLap with real-world GPS data, and were spot on.

I made this a 24 hour race, with 100 minutes subtracted for yellow flags, and 5 minutes for each pit stop. Here’s how the car options pan out.

ConfigurationLap TimeTotal Laps
Stock2:23.63538.8
Light2:22.40543.5
Power2:20.43549.0
Grip2:20.42551.2
Aero2:20.67551.2

The first thing that strikes me about this data is how boring it is! Adding power, grip, or aero are almost identical in this simulation. Each are equal to about 3 seconds per lap. At the end of the day, the Grip and Aero versions turn exactly 551.2 laps, and beat the Power version by two laps (only because the 1.8 consumes more fuel). However, in the real world, you might choose power, because it’s easier to get around cars that are running a similar speed and slowing you down. If I increase the gas tank size or reduce stints to 90 minutes (AER), then power wins by a smidge.

Lighter is always better, and removing 200 lbs beats the as-is car by 5 laps. But that wasn’t nearly enough to match the other options. In order to do that, I’d have remove 500 lbs. At 1800 lbs, the car completes 550 laps, but really, who can get a Miata that light with a driver? As an aside, removing 100 lbs was a difference of .62 seconds per lap, which is about the difference between a light driver (Evan) and heavy driver (normal people).

Supermiata in the mix

Supermiatas have a good combination of power, grip, and aero. The spec formula is an airdam without a splitter, a spoiler, 2400 lbs, and 140 hp. How would that fare against these other options? Let’s find out.

Image result for supermiata

I don’t have data on the exact aero combination, but this is just for fun, so I’ll estimate a Cd of .45 and a Cl of zero. That’s a .45 shift in coefficient of lift due to the airdam and spoiler, which is ballpark-ish. I’ll use 1.2g of grip, because they run on 15×9 wheels instead of the skinny 15×7 Spec Miatas use. I’ll do two runs, the second one adding a splitter, which my tests showed increased downforce by 0.38 and reduced drag by .01.

ConfigurationLap TimeTotal Laps
Supermiata2:16.29563.5
Supermiata, 4″ splitter2:15.08568.5

The Supermiata build wins by a lot because it has all three: power, grip, and aero. So that was a little unfair, so let’s see what happens when we combine options for the fantasy team.

Pick two. Which two?

The fantasy team has the budget to afford power, grip, and aero, but in the real world, you might have to choose two. But which two make the most sense? I’ll drop the lightweight version since it wasn’t competitive, and see which two options make the best combination.

ConfigurationLap TimeTotal Laps
Power, Grip2:17.09560.2
Power, Aero2:16.85561.2
Grip, Aero2:17.66560.1
Power, Grip, Aero2:13.81573.9

If you have to choose two, the combination of power and aero wins by one lap. If you live in fantasyland and can have all three, you win by a lot. And you even spank the Supermiata.

One thing to notice is that these options are not directly additive. If you add up the benefit of power, grip, and aero individually, it would be 9.35 seconds faster than the original lap time. But when combined, these net 9.82 seconds. The more grip you have from tires and aero, the more power you can use.

How the results are calculated.

Bang for the buck

Given that power, grip, and aero each netted about 3 seconds, what’s the best bang for the buck?

Depending on who’s doing the work, an engine swap is probably $1000 plus some miscellaneous expenses and tuning, call it $1200 without a standalone or $2000 with. Or a lot more than that if you have trouble with the swap and end up putting the 1.6 back in again (ahem, HJ).

Sticky tires like RE71Rs, are the easy option. However, they have less than half the lifespan of R-S4s or other proper 200 TW endurance tires, and cost you more in the long run. Factor in the price of mounting tires twice as often, and it’s about $600 extra per race. After two endurance races you’d start to wonder if an engine swap would have been more economical. After four races, you’d be certain of it.

Aero is the most economical choice. A wing and stands, plus an airdam and undertray is less than $1000. I made my own airdam, wing mounts, stands, and end plates, and spent $600-ish total including a 9LR wing. Doing it properly with swan neck mounts and a CNC cut airdam would double that price, but you’d have a really cool car like the one below. Aero would increase tire life as well, adding more to the economy. (See Race Car Aerodynamics, by Joseph Katz for the full details on that.)

Image result for miata 9 lives racing wing
Miata aero done right.

And aero makes your car look faster. Just fucking do it already.

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