In a previous post I looked at how to calculate drag and lift using the HP Wizard drag calculator, and simulated lap times based on various modifications. This time around I want to look at some real-world race cars and use them as examples of drag and lift so I can play “what if”? Such as, what if my Miata was shaped like a NASCAR stock car? What would that do for performance? Or, what if I could make my Miata look like a LeMans prototype?
In the following table, imagine that each body is scaled up or down appropriately and would fit on a Miata chassis. The cars and assumed to be running with windows open, or however they run in their series. For the simulation, I’ll use 2400 lbs, 140 hp, with tires that grip at 1.1g (endurance tires), and 18 square feet frontal area.
For each car, I’ve calculated the lift/drag ratio, which is an indication of how efficient the body is at creating downforce. I’ll also simulate lap times at Mid Ohio Pro course (no chicane).
| Car | Cd | Cl | L/D | Mid Ohio |
| Standard race Miata, no aero | .45 | .45 | -1.0 | 104.60 |
| 1990 Mazda RX7 GTO (spoiler) | .51 | -0.44 | .86 | 102.76 |
| 1990 Mazda RX7 GTO (wing) | .48 | -0.53 | 1.10 | 102.31 |
| 2002 NASCAR | .39 | -0.46 | 1.0 | 101.82 |
| Miata, splitter, wing | .48 | -1.01 | 2.10 | 101.12 |
| Miata, splitter fastback, wing | .41 | -1.20 | 2.93 | 100.11 |
| Audi R8 race car | .54 | -2.60 | 4.82 | 98.12 |
| Mazda RX-729P | .70 | -3.80 | 5.43 | 97.47 |
As expected, an aerodynamic body makes a big difference in lap time. A stock Miata doesn’t have a lot of drag, but it generates lift (positive Cl) while all the other body styles generate downforce (negative Cl). I’m not bashing Miatas, I love them. Virtually every street car generates lift and faces the same problems when used as a race car.
I thought the L/D ratio would be a direct indicator of lap time, but it isn’t. If it was, the RX7 GTO (wing) would have a faster time than the NASCAR body, but the stock car is slightly faster. While I chew on that, let’s take a deeper look at some of the body styles.
RX7 GTO
The 1990 Mazda RX7 GTO is particularly interesting because the spec body kit originally came with a rear spoiler, but this was later replaced with a wing. It’s nice to get these kinds of data points, it helps to correct assumptions and make better estimates.
The earlier GTO version with a spoiler is a bad-ass looking car IMO. It probably has slightly better drag and lift than a Supermiata. They both have airdams and spoilers, but the GTO has side skirts and is a more developed shape. Take a look at what’s going on with the B-pillar vent, I’m not sure what that is, but it’s a good way to use air rushing past an open window.
I’ve previously written about spoilers on Miatas, and have used a theoretical Cd of .46 and a Cl of -.10 for some simulations. I may have underestimated both the amount of drag and downforce a spoiler can produce. I should probably re-run the data using a Cd of around .50 and a Cl of -.40.
Comparing the two versions, the spoiler has .03 more drag, and .09 less downforce. This leads me to the following thoughts:
- I would have expected the spoiler to have less drag than a wing, not more. Perhaps the fastback shape is already very efficient and there’s no need to spoil the shape?
- If you add a splitter to the wing version of the RX7, it would have a Cd of around .47 and a Cl of about -.91. This is pretty close to my Miata’s measured .48 and -1.01. My race Miata also has side skirts and vents behind the fenders, and some other tricks. The difference between the two is probably the wing.
NASCAR, Miata, and other body styles
It’s kind of surprising how good the aero is on a NASCAR stock car, or at least the 2002 version I’m using here. It has a very low ride height, side skirts, a higher rear deck, and a spoiler. Stock cars also do a good job of keeping air out of the cockpit by using window nets, and also curved B-pillars that extract air from the cockpit. The L/D ratio is right in between the two RX7 GTOs, but the stock car’s lap time is faster than both. Wha?
When you compare all of the cars here, you can see that a Miata with an airdam, splitter, and wing has drag and lift values that are good. Race car good. Against a standard Miata, the aero version is 3.5 seconds faster in the simulation. A fastback drops another second, but most people won’t have the time or inclination to go that route. Which is fine, because it’s really the last piece of the puzzle. Or is it?
The Audi R8 race car and Mazda prototype are in an entirely different league. To get anywhere close to this level in a Miata would require a flat bottom (or venturi), diffuser, and other tricks. I intend on building and testing these things in the future, but a round-number goal is something like Cd .50, Cl -2.0, L/D 4.0. This would give a projected lap time around 98.8, which would be almost 6 seconds faster than a stock body Miata, and all this from body shape alone.
Occam's Racer 