This article was originally spread out over several different pages. I’m not sure what I was thinking at the time, but I’ve reorganized this as a single (rather lengthy) article now.

For the full story on how I performed these tests, see Testing Miata aerodynamics with $20K worth of computers. This article is essentially Part 2 of that one, so I can deep dive on the test results of the various aero options.

While datapoints like pounds of downforce at 100 mph, or horsepower consumed, are things we can wrap our heads around, it’s difficult to translate that into the only thing that matters: lap time. Therefore, in the following sections, I also include lap time simulations using OptimumLap.

Testing Miata tops

The first thing I wanted to test was the largest knowledge gap, roofline shape. This meant I had to have different options, that would come on and off quickly, using the same brackets. The four options were an open top, an OEM hard top, a Treasure Coast Chop Top (which should approximate a hard top with the window removed), and a fastback of my own design. 

I built the fastback before I had the notion to do this test, or I would have built it differently. The main problem is that I made long brackets along the bottom edge, and this required removing the trunk lid. This meant I didn’t get to test any of the other tops with an OEM trunk lid. Instead, I bolted a plywood cover over the trunk cavity. This new trunk lid is about 3.5” taller at the back than a stock trunk. It’s hard to say exactly what the effect of this was, but it’s likely a reduction in drag and lift, akin to adding a spoiler. So when you look at the data later, note that none of the tops used an OEM trunk.

Open top results

Miatas are meant to go topless, let’s start there and address some burning questions: 

• What happens when you use a wing with an open top? 
• How much does an open top affect a wing’s performance? 
• At autocross speeds, is it better to remove the top or leave it on? 

Take a look at the following table, and you can see that an open-top Miata generates about 40% of the downforce as one with an OEM hard top (Total Cl field). Of all the options, this was the worst at creating downforce.

It might be a little confusing that the coefficient of drag (Cd) is better with an open top than with a hard top. This is likely the result of running the tests with the windows open, which turns the hard top cabin into a parachute.

Top	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78

Let’s plug these numbers into OptimumLap and see what happens. I’ll use three different tracks to represent a range of speeds. These tracks are already in OptimumLap.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Hard top	2:22.96	1:19.95	1:03.34

The hard top is worth about one second at both Watkins Glen and Waterford Hills, and just over a half second on the autocross course. 

For these simulations, the car weight was kept the same. Someone will point out that the top weighs 45 pounds, and that OptimumLap doesn’t factor in the change in center of gravity. Both true. But I can calculate how much weight you’d have to remove from the open top car to match the autocross time of the hard top, and it’s 210 pounds. I’m not sure how high you’d have to place 45 pounds above the car to equal 210 pounds, but it’s probably pretty far up there!

But running an open top car with a wing has two advantages. One, it looks cool. Two, an open top car with this wing beats any top without a wing, every time. That’s kind of jumping ahead in the data, but it’s worth noting.

Let’s get back to the real world and the test at Watkins Glen. Alyssa reported that the car was more difficult to drive with the open top. She had to brake before entering Turn 10, and then had to manage a car that was oversteering badly. With a hard top, she could mash the throttle from the exit of Turn 9 to the exit of Turn 10. That kind of confidence over an 8-9 hour race can mean a lot more than a second per lap.

Chop Top results

Treasure Coast Miata sells their “Chop Top” for budget endurance racing. It’s an economical and lightweight top that does the job of enclosing the roof. This has two benefits: better aero, and you don’t have to wear arm restraints when racing (the car is no longer considered a convertible). I fabricated mounts that attach to the hard top brackets, and with those the total weight of the Chop Top was a scant 7 pounds. 

There is a persistent myth in Miatadom, that removing the rear window from a hard top is aerodynamically better. So I put two small Lexan covers on the sides of the Chop Top, closing in the sides. This made the chop very similar to a hard top without a rear window. Let’s add this data to the open top and hard top. 

Top	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Chop top	-0.20	-0.33	-0.53	0.45	1.19	49.40
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78

As you can see, the chop top allows the wing to work a bit better than an open top, with an increase in downforce. But it’s not as much as you’d think.

However, once you add a wing, the Chop Top performs barely better than an open top. This is interesting, because you’d think airflow over the roof is considerably smoother than an open top. However, it’s what’s happening on the underside of the wing that’s more important, and the Chop Top roof can’t defeat the turbulence coming from the open sides of the cockpit and going beneath the wing.

Next I’ll do the same track simulations, and what I find interesting here is that the Chop Top isn’t really that much different than an open top at any of the tracks. Not enough to really make a difference.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Chop top	2:24.04	1:20.82	1:03.79
Hard top	2:22.96	1:19.95	1:03.34

Nevertheless, for those racing with an open top and a wing, the Chop Top is worth a look for a bit of weather protection and not using arm restraints. In addition, we’ve finally dispelled the myth that removing the rear window is more effective. It isn’t. At least when used in conjunction with a wing. 

OEM hard top

All along I’ve been citing the data for the OEM hard top without really discussing it. It’s the status quo in racing, looks great, and performs its duty.

In the data, the OEM hard top generated more drag and lift than what I expected from published data. This is likely due to the open windows and wide canopy, which turns the cabin into a parachute. The drag is supposed to be around .38 with closed windows, but we measured over .5. Lift is also supposed to be the high .30-somethings, and we measured .55 (with vortex generators, I don’t have the raw data without).

The hard top with airdam, splitter, and wing made a killer combo: .48 Cd and 1.01 Cl. Those are good numbers. Racing numbers.

Fastback results

The front of my fastback uses the Treasure Coast Chop Top, and the rear fastback section bolts on and slopes back at about 14 degrees. So essentially the roofline is the same as OEM to about the rear window. Starting with the Chop Top made building the fastback fairly easy, and it was also easy to add and remove for this test. (You can see construction photos this and other tops I’ve built at the end of this article.)

The Chop Top plus fastback weighed 17 pounds less than the stock hard top, and to equalize the two I bolted 8-pound lead weights to the top of the seat belt towers. This was the only time I made adjustments to the weight of the car, and so the open top and Chop Top configurations were a bit lighter. 

The fastback significantly reduced drag, and helped the wing create more downforce. Compared to the OEM hardtop, downforce increased 129.7%. Another way of thinking of that is that the fastback turned a 60″ wing into a 78″ wing. Or you could say that the OEM hardtop is so bad that it made a 60” wing behave as a 48” wing….

The large gain in rear downforce was offset by a small loss in front downforce. Essentially, the wing was so effective with the fastback that the front end lifted, changing the height and angle of the splitter, reducing its effectiveness. 

Top	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Open top	-0.20	-0.23	-0.43	0.43	1.01	47.16
Chop top	-0.20	-0.33	-0.53	0.45	1.19	49.40
Hard top	-0.17	-0.84	-1.01	0.48	2.11	52.78
Fastback	-0.12	-1.09	-1.20	0.41	2.97	44.81

In addition, the fastback reduced drag by 15%. This in itself is pretty surprising, and not only helps top speed, but fuel economy. Combined, the downforce and drag created a lift/drag ratio that was 50% better than the OEM hard top with a wing. Astounding.

Note that the .41 coefficient of drag is actually quite good when you consider that the wing made the most downforce in this configuration, and just as in all of the tests, the windows were open.

But all was not rosy with this setup. Both Anthony and Alyssa commented that the car understeered badly in this configuration, and was boring as shit to drive. Given time, Jeremiah would have changed the mechanical grip by adjusting the front roll couple, by means of spring and/or stabilizer bar. This would have helped balance the vehicle at speed. 

Let’s do another simulation in OptimumLap. The fastback gains 1.9 seconds at WGI, and about half that at Waterford, which is pretty spectacular.

Top	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Open top	2:24.02	1:20.97	1:03.88
Chop top	2:24.04	1:20.82	1:03.79
Hard top	2:22.96	1:19.95	1:03.34
Fastback	2:21.06	1:19.02	1:03.12

Vortex generators on an OEM hard top

The shape of the Miata’s canopy is abrupt, and if you look at wind tunnel tests, you can see smoke trails that are turbulent, and then separate, as air moves over the top. Vortex generators (VGs) create a thicker turbulent layer of air, which keeps air from separating completely. This should result in less drag, and may also help interaction with a wing.

Most vortex generators you see are cosmetic fakery and don’t create vortices. I bought the real deal from AirTab. Made of thin plastic, they go on with double-sided tape, just peel and stick. The manufacturer says they should be mounted no closer than 4” apart. I set them at 5” on center, and so that made 9 for the roof.

If you do some research on VGs, there’s good data that they work. They’ve been used on semi trucks, RVs, the underside of race car wings, and many places where flow separation can occur. For cars, take a look at the four-part series on Autospeed, where they tested VGs on a Prius and Insight. Even better, check out Hi-kick Racing’s blog on adding VGs to a Miata. VGs decreased his lap time from 1:02.8 to 1:02.1. Here’s a photo from his site.

Nevertheless, my expectations were low. If vortex generators are the cat’s meow, then every cat would have them, right? As you can see in the table below, VGs made things worse. Total downforce decreased by about 20%, and drag increased substantially. Take a look at HP consumed at 100 mph and you’ll see you have five less ponies at that speed. 

Configuration	Front cL	Rear cL	Total cL	cD	L/D %	HP @ 100mph
Hard top, splitter, wing	-0.17	-0.84	-1.01	0.48	2.11	52.78
VGs, splitter, wing	-0.20	-0.61	-0.82	0.52	1.57	57.58

And this is how those values affect lap time in OptimumLap.

Vortex Generators	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, splitter, wing	2:22.96	1:19.95	1:03.34
VGs, splitter, wing	2:24.32	1:20.42	1:03.54

We placed the VGs at the trailing edge of the hard top, but it’s possible that moving them forward may have helped some. Or perhaps we used too many? I followed the instructions and they were supposed to work. 

The double-sided tape was difficult to remove, and so experimentation with the number and location of VGs wasn’t possible. The only lasting impression the VGs made was the adhesive, it was a bitch to remove and so all further tests would use the OEM hard top and VGs combined.

Miata top conclusions

Different tops change airflow over the roof, and this affects how a wing works. It’s unclear how much of this is is based on turbulence, or because of a change in downwash angle as air hits the wing. It’s likely a combination of both, but we didn’t have time to experiment with wing angle and this mystery remains. 

In addition, our data revealed a rolling rake angle that changes ~ ½ degree depending on rear wing configuration, and this impacted front downforce and distribution more than expected. By adjusting chassis rake and splitter angle, it’s likely the total downforce and Lift/Drag efficiency would have been higher, and this may have reduced rolling-rake changes as well. 

We can make the following general conclusions about using a wing with different tops. 

• An open top reduces a wing’s effectiveness by about 2.5x. Or, if you thought you were getting 200 lbs of downforce, you’re getting 80.
• A Chop Top performs marginally better than an open top.
• The OEM hard top is actually quite good with a wing. But don’t remove the rear window. And don’t use vortex generators.
• A fastback allows a wing to perform the best, increasing downforce, while also decreasing drag.

9 Lives Racing Big Wang vs cheap dual wing

The low price and availability of aerodynamic car wings are making them more common in crap-can endurance racing. You can buy a cheap extruded aluminum wing on eBay, Amazon, or other online retailers for $50, but are they good for anything?

I broke the piggy bank and purchased a 53” double-decker wing for $75, shipped. This wing is sold under a variety of brand names like BestEquip, Mophorn, Neverland, etc, and I’ll refer to this as the eBay wing, because that’s where I got it. 

Immediately upon unboxing, I knew I’d have to make some modifications. Like most budget items from China, it came with trunk mounts too low to allow airflow to go under the wing. I trusted the supplied mystery-metal hardware as far as I could throw them, which was directly in the trash. 

The main wing felt light, yet surprisingly rigid. The stiffness is partly from the dual horizontal mounting rails, which allow the wing to be mounted at just about any width. However, the exposed slots and flat underside of the wing can’t be great for keeping airflow attached. In the future, I may add some curvature here.

The upper wing felt like a noodle, and I feared it would vibrate and hit the lower wing at speed. So I riveted on a Gurney flap of 1/4” aluminum angle to stiffen it up. I also added a small stop in the middle of the lower wing to limit downward movement of the upper wing.

The end plates had to go, not only because they were too small, but they didn’t allow the upper wing to pivot into the correct position. According to McBeath in Competition Car Aerodynamics, the upper wing should overlap the lower wing at about 4% of the chord (.3” in this case). In order to accelerate the air, the gap must be larger at the front than at the rear. With those two factors set, the slots in the supplied end plates, which support and locate the upper wing, wouldn’t allow us to pivot the upper wing into a useful angle. 

Since I couldn’t get the correct spacing and wing angle with the supplied end plates, I made my own from 12”x10” sheet metal, and drilled new upper wing mounting slots. All done I paid a little over $110 for the wing and modifications, and a couple hours figuring all that out. 

The car was set up with -1 degree of negative rake, meaning the back was lower than the front. Ideally it should be the other way around, but we got lost debugging what we thought was a clearance issue with a front sensor and didn’t correct the angle. 

I set the lower wing angle to 3 degrees, but because of the chassis rake, the main wing was closer to 2.5 degrees. I set the upper wing to 12 degrees. Measured over the entire chord, this created a total camber of about 14 degrees, which is right in the middle of the values that McBeath cites in Competition Car Aerodynamics. Given more time, I would have experimented with the angle of attack of the upper element, as well as the entire wing. However, not knowing how the wing would perform, I chose middle-of-the-road values from a published text, and figured it was more important to avoid a stall condition than maximize downforce. The wing weighed 7.6 pounds altogether, which is very light.  

The wing I used through all the other testing on at WGI is a 60” 9 Lives Racing “Big Wang”. The standard Miata wing is 64″, but my eventual plan was to end-plate mount this, like a Ferrari F40. I never did get around to that, so the wing is a bit smaller than you’d see on most Miatas. (But as you already saw above, the fastback made it behave as a much larger wing.)

When I took the wing out of the package I was immediately impressed by the sturdy construction. When there are only cockroaches left in the world, there will also be 9LR wings. I made my own 12” x 12” end plates and mounting brackets, and had a local shop weld the mounts underneath. The entire setup was about $500, and weighed in at double the double wing, at 14.4 pounds.

I set the wing angle at 5 degrees, but with chassis rake the actual wing angle measured 4.6 degrees. If you look at the 9 Lives CFD open air data, this is right in the middle of the wing’s working range. I knew the downwash angle would change with every top, and that this would change the effective wing angle. But due to intermittent track closures, I didn’t get a chance to sweep the wing angles, and left it where it was. 

For both wings I used the same wing stands, which I cut from aluminum plate. Many racing series limit wings to roof height, so I made sure the highest part of the wing was level with the roof. I bolted the base of the wing stands through the sides of the trunk gutter, and while this seemed strong enough, I added another L-bracket on top of the rear fenders, and this stiffened things up considerably. 

Wings compared

So let’s see how these wings compared. In this first chart, we’re looking at front and rear downforce using GPS speed alone. It looks like the eBay wing (red) creates more rear downforce as the 9 Lives Racing wing (blue), and when combined with the front, the total downforce is pretty close.

However, the weather had changed during this run, and we had an 11 mph headwind. After correcting the wind speed detected by the pitot tube, a clearer picture developed. See the bar graph below. Here we can see that the eBay wing generated less downforce than the GPS speed would have us believe. This is why you can’t trust testing using GPS speed alone, and why you hire a guy like Jeremiah.

When you add downforce on one end of the car, you can expect to lose downforce on the other end. This is the natural see-saw effect of pushing down on one end. What’s interesting here is that the 9 Lives Racing wing not only made more rear downforce, but it also had more front downforce. How can this be? 

The most likely reason is drag. The eBay wing creates more drag, and this rear-biased force lifts the front end. Whatever the case, front downforce, and thus total downforce, is a lot less with the eBay wing.

Take a look at the following table and you can see the corresponding values for coefficient of lift (which we’ve been familiarizing as “downforce”), and drag. We already saw that the 9 Lives Racing wing practically doubled the total downforce, and here you can see it did that while creating 15% less drag. If you look at the final column in the table, you’ll see that the drag reduction alone equals an extra 8 hp at 100 mph. 

Wing	Front cL	Rear cL	Total cL	cD	L/D%	Front Load	HP @ 100mph
9LR	-0.17	-0.84	-1.01	0.48	2.11	16.76%	52.78
eBay	0.03	-0.58	-0.56	0.55	0.90	4.43%	60.79

If we divide the total coefficient of lift by the coefficient of drag, you get the L/D ratio, which tells you how efficient the entire aero package is. Here you can see the 9 Lives Racing wing contributes to a setup that is over 230% more efficient at creating downforce. 

The data from testing other tops shows that the 9LR wing changes the coefficient of drag by about .03 across all tops, from open top to fastback. By contrast, the double wing changes the Cd by .10. Yowza, that’s a lot.

One final calculation is the front aero load distribution percentage, which gives you an idea of how much the car will understeer (a low percentage) or oversteer (a high percentage). The low values here indicate that with either wing, the car would understeer badly. This is partially due to the negative rake of the chassis and negative splitter angle (both setup mistakes that should have been corrected before testing). However, even with these setup details corrected, the eBay wing would produce a car that understeers more. 

As usual, let’s see what happens in OptimumLap. To spice things up, I’ll also add the data from the 9LR wing with an open top. 

9LR vs eBay	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, 9LR	2:22.96	1:19.95	1:03.34
Hard top, eBay	2:25.71	1:20.99	1:03.81
Open top, 9LR	2:24.02	1:20.97	1:03.88

The 9 Lives Big Wang outperforms the cheap eBay wing in every way. In fact, the 9LR wing with an open top out performs the eBay wing with an OEM hard top on any track that isn’t autocross. 9 Lives Racing is a small, made-in-the-USA business with employees who race cars, and you can feel good about supporting them. 

But if you’re racing in 24 Hours of Lemons on a $500 budget, you might find that a cheap wing suits your janky crap-can just fine. The wing could have performed better with more fine tuning, but it’s clearly a case of “you get what you pay for.” If you purchase this wing, you might want to take similar steps that I did to limit movement of the upper wing, optimize the convergent gap and wing angles, and get the wing about 6 feet above your roofline. It is Lemons, after all.

Wing vs no wing

For most of the test I used a 60″ 9 Lives Racing wing, but I wanted to see what would happen without it. The coefficient of drag went down by .03 for all configurations. This is rather interesting, because usually when downforce goes up, so does drag. But this wing had the same drag in all configurations.

When I removed the wing, total downforce took a nosedive, and for the first time in the test, the car generated lift instead of downforce. This was an appropriate time to test the hardtops and see how they did without a wing.

First we threw the Chop Top back on and did a run. Then we attached the rear section, making it into a fastback again. And finally we tested the OEM hardtop. Unfortunately the OEM hard top still had the vortex generators attached, so I’ve made an educated guess on the Total Cl and Cd values below (these are in italics in the table below).

Configuration	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
Hard top, VGs, no splitter, with wing	0.18	-0.64	-0.47	0.53	0.88	58.75
Hard top, VGs, no splitter, no wing	0.22	0.38	0.59	0.49	-1.20	54.53
Chop top, no splitter, no wing	0.11	0.20	0.31	0.48	-0.64	52.93
Fastback, no splitter, no wing	0.16	0.18	0.35	0.38	-0.80	42.00
OEM hard top, no splitter, no wing			.50	.45

What’s surprising here is that without a wing, the chop top has the best L/D ratio. This is largely because it creates the least lift. Remember that negative lift values are what we’re looking for (downforce), and the chop top’s .31 Cl has the least lift. The fastback creates more lift than the chop top, but it does so with less drag, and in the end, this is makes a faster car.

Unfortunately we didn’t get data for a bare OEM hardtop (without VGs, wing, or splitter), so we don’t know if it’s the shape of the OEM roof, or the VGs that create so much lift. But the total Cl value of 0.59 is quite a bit worse than either the chop top or fastback. Based on the data we obtained doing the wing tests, a bare OEM hard top should have a CD of about .45. It’s hard to imagine the vortex generators adding more than 10% lift, and that would put the total lift around .50. 

Let’s see what happens in OptimumLap when we remove the wing. 

Wing or no	Watkins Glen	Waterford Hills	2010 SCCA Nationals
VGs, no splitter, 9LR wing	2:25.65	1:21.12	1:03.89
Hard top, VGs, no splitter, no wing	2:29.14	1:23.14	1:04.91

Yikes, that sucks! The wing is worth 3.5 seconds at WGI and even 2 seconds at a short track like Wateford? Amazing.

Now let’s just compare the different tops without wings. 

Tops without wings	Watkins Glen	Waterford Hills	2010 SCCA Nationals
Hard top, VGs, no splitter, no wing	2:29.14	1:23.14	1:04.91
Chop top, no splitter, no wing	2:27.74	1:22.85	1:04.63
Fastback, no splitter, no wing	2:26.51	1:22.42	1:04.63
OEM hard top, no splitter, no wing	2:28.17	1:22.87	1:04.80

Here we can see that the fastback is still the fastest configuration, but it’s not a huge difference unless you’re at a high-speed track like WGI. Without a wing, I’d be happy to use a Chop Top at most tracks for the light weight and convenience of strapping in the driver and accessing things like a cool suit, radios, cameras, etc., in the cockpit. And on performance, the Chop Top beats the OEM hardtop, even without factoring in the 38 pound weight difference.

One thing that’s conclusive here is that if the rules allow it, use a wing. This probably even applies to classes like NASA ST6/TT6 that carry a substantial penalty for running a wing.

Airdam and splitter

For most of the test we used a 4″ splitter. This was bolted to a flat undertray, flush with the airdam. I wanted to see what removing the 4″ splitter extension would do. We expected a loss in downforce, but I wasn’t  sure if drag would go up or down. You see it both ways online, with CFD data showing that a splitter reduces drag, and the occasional internet expert claiming that drag goes up. 

Splitters	Front Cl	Rear Cl	Total Cl	Cd	L/D %	HP @ 100mph
VGs, splitter, wing	-0.20	-0.61	-0.82	0.52	1.57	57.58
VGs, no splitter, wing	0.18	-0.64	-0.47	0.53	0.88	58.75

Score one for the CFD team, the splitter reduced drag slightly. When I removed it, the drag went up from .52. To .53. More importantly, we lost a lot of front-end downforce. Our raw data showed a loss of 69 lbs on the back straight, which calculated to a .38 delta in front coefficient of lift. Let’s see what that’s like in OptimumLap. 

Splitters	Watkins Glen	Waterford Hills	2010 SCCA Nationals
VGs, splitter, wing	2:24.32	1:20.42	1:03.54
VGs, no splitter, wing	2:25.65	1:21.12	1:03.89

Obviously, if you’re running just an airdam, and the rules allow it, add the splitter. It’s significant. It’s also worth noting that this was just a plain splitter with no rear curvature or splitter diffusers. Knowing what I know today, the splitter would be twice as effective.

Endurance racing simulations

For endurance racing it’s important to know the amount of energy used per lap, because that determines how far you can go on a tank. You can get this data from OptimumLap simulations. From the energy used, you can determine the length of each driver’s stint, as well as how many laps they can complete in each stint. This can be very important for pit strategy, especially in longer races. Note that simulations are exactly that, and aren’t intended to be exact. But they are useful for making direct comparisons.

The configurations I used for the simulations follow this key. Note that the “B” group has less aero: it uses an airdam, but not a splitter, and no wing.

• 1a – Open top, wing, airdam, splitter
• 2a – Chop top, wing, airdam, splitter
• 2b – Chop Top, no wing, airdam, no splitter
• 3a – OEM hard top, wing, airdam, splitter
• 3b – OEM hard top, no wing, airdam, no splitter
• 4a – Fastback, wing, airdam, splitter
• 4b – Fastback, no wing, airdam, no splitter

In the table below, the Energy value in the table comes straight from Optimum Lap, and I’ve simply taken a value of 16,500 energy units divided by the energy used to get the number of hours per tank. The Miatas I’ve raced have burned about 7 gallons per hour, so these values aren’t far off.

Config	Cl	Cd	WGI Lap	Energy	Hours per tank	Pit stops	Laps in 8 hours
1a	-0.43	0.43	144.02	9087.53	1.82	4	174.98
2a	-0.53	0.45	144.04	9141.08	1.81	4	174.95
2b	0.31	0.48	147.74	9183.12	1.80	4	170.57
3a	-1.01	0.48	142.96	9223.37	1.79	4	176.27
3b	0.5	0.45	148.17	9078.14	1.82	4	170.07
4a	-1.2	0.41	141.06	9031.23	1.83	4	178.65
4b	0.35	0.38	146.51	8911.28	1.85	3	174.05

Take a look at the first two, this is the open top vs Chop Top, and it’s interesting that they turn almost exactly the same number of laps. The OEM hard top beats those by a lap and change.

But the fastback configuration 4a is the clear winner here, turning 178 laps, two more than the OEM hardtop with the same aero. Make this into a 24 hour race and the fastback wins by seven laps. Notice the Energy column, the fastback with wing is not only turning the fastest laps, but it’s using less gas than any other configuration, save the fastback without the wing.

In the non-aero B-group, the fastback (4b) wins by three laps over its wingless brothers. This is partially because the fastback can take one less pit stop. If I re-run the data with a larger gas tank so that all configurations have the same number of pit stops, then the fastback wins by only two laps. 

But once you add a wing, the race is over. In fact the worst combination with a wing (open top) beats the best performing top without a wing (fastback) by a full lap, even though the fastback does one less pit stop.

If you want to check my math, I have a spreadsheet with these values, and for simplicity, I’ve removed a lot of the variables in this table. You have to keep track of things like yellow flags and time taken for each pit stop, which determines the actual driving time per race. The 8-hour race I’ve simulated uses 420 minutes of racing time instead of 480 minutes. Yellow flags at Watkins Glen take longer, and I’ve also subtracted 5 minutes per pit stop. 

For shits and grins, the eBay wing again

Let’s see what happens when we use the airdam, splitter, OEM hardtop (without the vortex generators) and the cheap eBay wing. This is configuration 3c in the table below. Look above and compare.

Config	Cl	Cd	WGI Lap	Energy	Hours per tank	Pit stops	Laps in 8 hours
3c	.56	.55	145.71	9393.39	1.76	4	172.95

The eBay wing (3c) loses to every configuration that uses the 9 Lives Racing wing. Yup, even the open top car with a 9LR wing is going to be the hardtop with a cheap wing. However, the eBay wing beats OEM hardtop without a wing (3b) in both a sprint race or an endurance race. So it’s worthwhile running a cheap wing if you have nothing at all.

The biggest difference is the Energy field. Compared to the lowest drag version (4b), the eBay wing uses 5% more gas on every lap. That may or may not be a consequence, depending on how long you’re in the car.

It wouldn’t matter in a sprint race; the eBay wing (3c) would win against the fastback without a wing (4b). But in an endurance race, it would depend on the tank size and driving stint time. If I change the data so that they all take the same number of pit stops, then the eBay wing wins. If I leave the gas tank size as it is, then the fastback without a wing wins.

Conclusions

I wrote an article called The Dunning-Kruger of Car Aerodynamics, wherein I examine the steps most people go through on their aerodynamic journey. It was lucky that I met Jeremiah online and was able to do these real-world tests, and jump right past the pit of despair.

I’m sort of backsliding on the D-K chart by doing wind tunnel testing now, and who knows, I may slide further backwards towards CFD. But in the future, my ‘Busa swapped fastback Miata, Falconet, will have a full onboard system with strain gauges, pitot tube, and all the works so that I can do the same kind of testing I started with.

In 2024 I’ll do a full writeup of how to install all the parts and hook it up to an Aim dash (ahem, with a lot of help from Matt Romanowski), but this should bring me full circle back to a system akin to what I started with. With those tech gizmos I’ll be able to get real-world data on downforce and drag whenever I want, essentially making any race track into my own private wind tunnel. Fuck yeah.