In 1993, the Mazda Miata had a coefficient of drag of .38, and the RX-7 had a Cd of .29. Same manufacturer, same year, both two-door sports cars, and yet the RX-7’s had 20% less drag.
There’s nothing magical about the RX-7 shape, and if you compare its Cd to new cars, it’s only so-so. In The Most Aerodynamic Cars You Can Buy Right Now there are many cars with Cds from .27 down to .22, and a unicorn at .189. (Follow along in this Aero Timeline and see how Mercedes has incrementally improved their aero from high .4s down to .24 complete with wind-tunnel smoke trails.)
But let’s stick with the same year and manufacturer, and see what would happen if you could magically put a RX-7 body on a Miata, and what that would do for performance and fuel economy.
Calculating top speed
To calculate top speed, I’ll use the RSR Bonneville Aero-Horsepower & Drag Loss Calculator. I’ll enter data for a 1993 Miata, with frontal area of 18 sq feet and a Cd of .38. Miatas of that vintage had about 116 crank hp, and if I multiply by .82 to simulate driveline losses, that’s about 95 hp at the rear wheels. (You can argue driveline losses, I’m using figures from Competition Car Aerodynamics.)
First I want to calculate top speed, so I’ll throw some numbers into the calculator until the Horsepower Needed field reads 95. Turns out that 116 mph is the top speed.
Now I’ll drop a RX-7 body on the Miata, and drop the Cd to .29. The top speed goes up by 10 mph to 126 mph. Wow!
However, top speed is rarely important, so I’ll plug in some more common values. I’ll use 60 mph to represent the exit of a corner, and 90 mph to represent a faster section of track. How much power is required to go that fast, and how much power remains?
|Cd||HP to go 60 MPH||HP Remains||HP to go 90 MPH||HP Remains|
At 60 mph, the low-drag RX-7 body has an additional 2.7 hp available over the standard bodywork. Meh. At 90 mph, there’s an additional 9 hp available from the sleek RX-7 body. Wow! Obviously, the faster you go, the more important drag becomes.
Simulating lap time
Drag is obviously important, but more important is lift (downforce). We need the numbers for both drag and lift in order to calculate a lap. I don’t have any published numbers for lift on a Miata, but the Hancha group did CFD testing and I’ll use their lift value of 0.27. In Race Car Aerodynamics (p. 19) Katz lists the RX7 at .24 lift, and AutoSpeeds article on Aero Testing ven breaks that down into front lift vs rear. Let’s plug these values into OptimumLap and simulate lap times at my local track, Watkins Glen International.
So a Miata with a RX-7 body would go over 1.5 seconds per lap faster than a stock Miata. Some people would give their left nut for 1.5 seconds.
Fuel economy and race strategy
In sprint racing, fuel economy is meaningless, but in endurance racing, it can be important. Especially if longer stints will allow you to do one fewer pit stop during the race, or if your car is right on the cusp of doing the maximum allowed stint. OptimumLap shows a 2.5% decrease in fuel economy using the RX7 body. That doesn’t seem like much, but it can be a big difference.
Let’s use my race Miata as an example. It burns about 7 gallons per hour, and with its 12.7-gallon gas tank, it can go about 1:50 before the tank runs out. This is not a problem in AER where stints are 90-minutes long. But in Champcar or Lucky Dog, stints are two hours long, and I end up doing an extra stop. In cases like this, 2.5% fuel economy can be a huge deal.
So not only is the RX7-bodied Miata going 1.5 seconds faster per lap, it’s doing that while burning 2.5% less fuel. Calculate the total number of laps per stint and the driver in the stock Miata can do 42.6 laps per stint. The driver in the RX7-bodied Miata can do 44.1 laps.
Calculating Cd using body shape
A Miata’s drag and lift values aren’t terrible, but the RX7 shows us that it could be a lot better. What body can you make, and how much would that affect drag and lift?
The HP Wizard website has an enormous amount of information, and I have a lot of fun playing with their drag calculator. Basically, you choose from various options and it calculates the Cd of your car. Let’s try this on a Miata.
I set the following options, which gives me a Cd of .376, which is close enough to a Miata’s .38 that I feel satisfied:
- Shape – Well rounded
- Front elevation – Med. height, rounded, sloping up
- Scuttle and Wing – Flush, rounded top wings
- Windscreen – Wrapped ends
- Windscreen peak – Rounded at top
- Canopy plan Constant width
- Rear Canopy – Rounded canopy and boot
- Lower rear quarters – Constant width
- Underbody – Monocoque RWD
- Skin friction – Recessed windows w/ mirrors)
- Internal flow – Typical
- Openings – Closed cockpit
- Wheels – Fenders only, 205/50-15
- Lift induced drag – Production model, sport
- Frontal area 18.09: height (48.2) width (65.9).
Most HPDEs and racing organizations require you to run with open windows, and HP Wizard can simulate that. By changing Openings to “Open window prod car,” the Cd increases to .426. Or if you use the Miata as intended and open the top, then the Cd is .464.
So let’s see about reducing the drag. In the drag calculator I used “Rounded canopy and boot” to represent the Miata, let’s give the Miata a RX-7 top and change the Rear Canopy to “Fastback 10-20 degrees,” and the Canopy plan to “Tapering to rear”. Right away the Cd goes from .378 to .340. As a side note, the worst possible angle for the rear window is 30 degrees – you can see that in the drag calculator – and if you measure a Miata’s rear window, it’s about 30 degrees.
The next thing I see on the Miata is the hard top extends wider than the windows, and catches air moving along the car. The RX-7 doesn’t do that, so I’ll change Skin Friction from “Recessed windows” mirrors, to “Perfectly smooth body,” and now Cd is .331.
The drag calculator doesn’t allow me to address the Miata’s terrible front end, so I can’t get down to the RX-7’s .29. That’s a subject, and a test, for another time.