In my previous post, NACA Wing Shapes and Airfoil Tools, I compared the 9 Lives Racing “Big Wang” to a NACA 6412 wing. The NACA wing was most efficient in the 5-7 degree range, and would begin to stall above 10 degrees. Based on fooling around with camber and thickness in the NACA tools, I’d guess the 9LR Wang will operate best in a slightly lower range. To make more downforce, you can add a Gurney flap or a second element.
How a Gurney flap works depends on who you ask. Some sources say that the flap changes the location where the air above and below the wing meet, making that point further away from the leading edge. Effectively, the Gurney flap makes the wing chord (front to back length) longer.
And other sources say that a Gurney flap works by keeping airflow from separating at higher angles of attack.
Either way, Gurney flaps allow you to make more downforce. Typically Gurney flaps are about 1-5% of the chord length. 9 Lives makes flaps in 1/4″, 1/2″, and 3/4″. Given the 9LR chord of about 10″, the 1/2″ flap is 5%, and this is on the large side. I suspect that the 1/4″ flap is the one to have. This chart breaks it down nicely for you, but doesn’t list the smallest flap.
The chart doesn’t list the lift-drag ratio, but that’s easy enough to calculate from the 9LR numbers. I’ll use just the 100 mph values and make my own chart (below). When I then sort by L/D ratio, you can see that the single-element wing at zero degrees is the most efficient, with a 14:1 lift/drag ratio.
If you crunch the numbers further, you’ll see that the most efficient way to adjust rear downforce is changing the wing angle in the 0-5 degree range. If you need more downforce, keep the 5-degree angle and add progressively larger Gurney flaps. Finally, if you still need more downforce, increase the wing angle until it stalls.
But aero isn’s just about efficiency, but balance. Let’s say you have an airdam and splitter making 200 lbs of downforce at 100 mph (which is what the Hancha Group CFD showed in Miata Airdam and Splitter). If you want the same downforce at the rear, you can generate that by either running the 9LR wing at 10 degrees AOA, or at 5-degrees AOA with a 1/2″ Gurney flap. The latter does so with 10% less drag.
More wings, more downforce
Time attack Miatas use wide and complex splitters, with multiple dive planes and surfaces to create more front downforce. And they turbo or swap the engine, so they don’t care so much about drag. For this specialized application, they need more rear downforce, and a multi-element wing is one way to achieve that.
Multi-element wings effectively increase the camber of the wing, and can therefore be used at a higher angle of attack. The more wing elements you add, the more downforce you get, and with that comes more drag and reduced efficiency.
But I’ll just comment on the dual-element wing. The placement of the second wing is critical, which should create a convergent slot (larger in the front, tapering to the rear), to accelerate air.
According to Katz, in Race Car Aerodynamics, the main wing should be run close to zero degrees, and the second wing at an angle up to, but not exceeding 40 degrees. 9LR just released a dual element wing that can be added to their standard wing and their CFD also shows that greater than 40 degrees is a mistake.
The CFD L/D ratio of 2.7:1 isn’t very good. In fact you can see that the single wing with large Gurney flap makes the same downforce as the dual wing at 35 degrees. And it does so with 1/3 the drag. Perhaps there’s something wrong with the values in the chart. 9LR claims up to 10:1 L/D ratio, and so something is clearly off. I’m also wondering if better numbers will come when they optimize the wing angles and slot position.
In Competition Car Aerodynamics, McBeath also examines dual-element wings. The dual element wing made of 60% more downforce than the single element wing, which is pretty close to the 9LR values above. So maybe the L/D ratio is correct, and there’s just a lot of drag when you add a second wing? My experience with a dual-element wing (a cheap one), wasn’t so great. I would say that unless you’re doing autocross (where drag won’t matter as much) or have a shit ton of power, a single wing is going to be a better bet.
Airfoil Tools is a really neat site. It serves as a catalog of existing airfoil shapes, and allows you to compare them, simulate different speeds and angles of attack, and draw and plot your own wing shapes. Naturally most of these wings are designed for airplanes, but some have been used on cars (upside down) to create downforce. Cars don’t go nearly as fast as planes, and so the ideal shape of a car wing is different, and optimized for Reynolds numbers on the lower side of the spectrum.
The Reynolds number isn’t easy to explain, so I won’t. But it’s worth noting that the airfoil tools calculator has values that can be used to simulate car speeds. For example, a wing with a 10″ chord that is traveling through the air at 67 mph, and at 68-degrees F has a Reynolds numbers of around 500,000. Doubling the speed doubles the Reynolds number. You’ll only need this number if you’re using Airfoil Tools, otherwise forget it.
In Competition Car Aerodynamics, , Simon McBeath frequently uses the NACA wings as examples, such as the NACA 6312,. In NACA 4-digit wings, the first digit is the camber as a percentage, the second digit is the location of camber in relation to the chord length, the third and fourth digits are the percent thickness of the chord. So, a NACA 6312 wing has 6% camber, the max camber occurs at 30% of the chord length (from the front), and the thickness of the wing is 12% of the chord length.
McBeath discusses these form factors and how they allow a wing to generate more downforce.
More camber creates more downforce, but too much and you get separation underneath at the trailing edge.
Moving camber rearward generates more downforce, but further forward is more efficient for low-drag, low-angle applications.
A thickness of around 12% is quoted as being best for flow separation, but a thicker wing doesn’t create much more drag at car speeds, and could be better when run at a high angle of attack. The author suggest around 16-18% thickness as being ideal.
First I want to give a shout-out to 9 Lives Racing who makes a very strong, economical, and proven wing. I don’t know if it originated from a NACA profile, and I’ve tried to find a NACA shape that’s the same, but I can’t find an exact match.
For fun, I’ll plot a wing that looks similar. Start at the NACA 4-digit airfoil generator and plug in some numbers. At first I’ll try 8% camber, 50% chord position, and 12% thickness and I get a wing that looks kind of like the 9LR wing (although 9LR has more camber).
Now I want to take McBeath’s advice and increase the thickness to 16% and I’ll add as much camber as the tool allows (9.5%).
To my eye it looks chubby, especially the leading edge. It’s a lot of effort to build a wing, and it would take a lot of trust to build one based on an online calculator and other people’s suggestions for what might work. A couple proven wing shapes I’m interested in building are the Eppler E423 high lift airfoil, and the Chuch Hollinger CH 10-48-13 high lift low Reynolds number airfoil. And then again there’s the smart move, which is to buy the 9 Lives Racing wing and move onto other projects.
Another thing I discovered using the Airfoil Tools page is that most wing shapes operate best in a fairly narrow range of angle, and that this angle of attack is dependent on speed. Let’s see what the Airfoil Tools page can show us on how wing angle affects downforce and drag.
First I’ll select a wing, and the NACA 6412 is as good as any for this demonstration. Next I’ll set the Reynolds number from 50,000 to 200,000 and the NCrit range between 5 and 7. Click Update Range and look at the resulting graphs.
First up is Cl vs Alpha. Cl is the coefficient of lift, which is downforce to you and me, and Alpha is the wing’s angle of attack. The more wing angle, the more downforce, up until about 9 degrees. After that flow separations occur and the wing creates less downforce.
Next take a look at Cd vs Alpha, which is drag vs wing angle. This wing has the least drag around zero degrees. There are three lines in these graphs that represent different speeds, but you can see that drag goes up quite a bit after 10 degrees.
The next one I’m interested in is Cl/Cd vs Alpha, which is how efficient the wing is at different angles. The wing is most efficient between 5-7 degrees, depending on speed.
So you might think to set this wing at about 6 degrees, so that the wing performs at its best lift/drag ratio over a range of speeds. However, this isn’t necessarily the best setting for your car.
More downforce, at the expense of more drag, is almost always faster. You can read more about that in Downforce vs Drag.
Having an over abundance of rear downforce will make the car understeer at high speed (safer), which means you can tune the car for oversteer at low speed (funner). In the end, the best setting is how you want to adjust the balance.
If you look at the open-air CFD data for the 9 Lives Racing wing, it also operates in a similar range for angle of attack, but with more camber, it’ll probably stall earlier than the 6412 wing. As you can see in the chart below, the 9LR wing makes a tiny bit more downforce at 10 degrees than 5 degrees, but with a lot more drag. It may be starting to stall at 10 degrees, and the downward wash of most rooflines makes the 10-degree setting a bad one.
Also included in their chart is data for Gurney flaps and dual-element wings, which is the subject of the next post.
In this post I’ll examine several aspects of splitter design, starting with length. Some racing organizations regulate the maximum size of a splitter, for example:
Supermiata S2, NASA ST6 – airdam, but no splitter
Supermiata S1, NASA ST5 – 4″ max
Champcar – 12″ max (wow)
In Race Car Aerodynamics, Katz states that the splitter length should be double the chord length, the chord being the distance from the splitter to the ground. So if your car has 4″ of ground clearance, the splitter should be 8″ long. That seems rather long to me, but this rule of thumb may depend on the shape of the nose, and for a vertical airdam, perhaps shorter is ok.
In Competition Car Aerodynamics, Simon McBeath cites a CFD study done on a NASCAR model, using splitters of 2″, 4″, and 6″. In this case the ideal splitter length was 4″, producing the most downforce, and best L/D ratio.
The yellow line represents the total downforce, and you can see that the 100mm splitter is just slightly higher than 150mm.
The blue line shows downforce increasing fairly linearly up to 100mm, and then leveling out at 150mm. So you don’t want to go longer than 6″ (at least not on this stock car).
In the NASCAR CFD study, the splitter added 10% more downforce than using the airdam alone. A Miata isn’t a stock car, and this is all calculations, so YMMV. This is in contrast to the Hancha Group’s CFD work, who’s theoretical airdam produced 34% more downforce with a splitter than with just an airdam. In my real-world testing, I found a 4″ splitter added .38 to the front coefficient of lift (it increased downforce substantially over an airdam alone) and decreased coefficient of drag by .01. More downforce, less drag, so do it.
The same NASCAR CFD study found that extending the splitter rearwards underneath the car had further benefits, and the longer the better. This isn’t surprising, because it’s effectively creating a flat bottom. The interesting part was that lengthening the splitter rearwards made the splitter less effective, because of a build-up of pressure in the engine bay. Overall downforce did increase, but this was because of body interaction. The CFD model was revised to add vents in the hood, and then the splitter and underbody panel both made more downforce. Hood vents are not just for cooling!
Another unexpected result comes from Competition Car Aerodynamics. In Chapter 9 McBeath explains the wind tunnel work done at MIRA using a championship winning Integra Type R (which always makes me want to go all Dor-Dori and shout “Inte-R!” in a Japanese accent). In the wind tunnel, they experimented by putting different ends on the splitter, with ramps of different sizes, and with and without a vertical fence on the end. Each time they measured the result for drag and lift.
The configurations were as follows, and correspond to the image below:
Baseline configuration with no ramp or fence.
High ramp with vertical fence.
Vertical fence alone, no ramp.
Vertical fence, shallow ramp.
In the image above, the total amount of front downforce is the yellow line, and configuration 6, a simple vertical fence, is the winner. Adding a small ramp (configuration 7, which is pictured above), or large ramp (configuration 5) in front of the vertical fence actually reduced downforce and increased drag (the black line). Who’d have thunk it?
McBeath doesn’t reveal the actual numbers (it was a private test, they hold the cards close to their chest), but he did say that the best configuration reduced total drag on the car by 4.8%, and more significantly, total downforce increased by 50%!
I’ll conclude this post with some generalizations about splitter design:
You can make your the splitter as long as your rules allow, but the longer it is, the more it will affect the front/rear balance. Also note that it may work just as well at a shorter length. If you want to choose a length and not experiment with it, 4″ seems a safe bet.
Extend the splitter rearwards as far as the rules allow. But note that this may increase pressure under the hood, and then hood vents may be necessary.
Extend the rearward edge of the undertray as close to the leading edge of the front tires as possible. You can generate downforce from wheel wash.
If the undertray curves upwards at the rear, it will accelerate the air in front of it, creating more downforce and drag. McBeath quotes a value of about 4% increase in downforce and 1.5% increase in drag. For me, the effort wouldn’t be worth it, and I’d just use a flat undertray and perhaps rake it slightly.
Your splitter may create over 200 lbs of downforce, and so if you can’t stand on it, it isn’t strong enough.
Many people use birch plywood for the undertray and splitter, but mahogany marine plywood is better. I suggest Okume or Meranti BS-6566, it’s more weather resistant than birch and 33% lighter. I’d go with 9mm-15mm thick depending on if it’s just an undertray or splitter.
The front edge of the splitter should be radiused on the underside to avoid separation of flow. Sharp on top, radiused on the bottom.
Add vertical fences on the sides of the airdam to shield the tires and increase downforce.
Look at an NA Miata front end and you’ll notice that you can see the front tires exposed to airflow. Rotating wheels are terrible for aero, and this is made worse by the shape of the nose, which forces more air at the exposed tires and also under the car. Under the car, the spinning wheels put the airflow in yaw, making things even worse. For drag and lift reduction, the Miata’s front end is the clearly place to start. Each generation of Miata has gotten progressively better in this regard, and so you can see Mazda agrees.
Mazda’s partial solution to this was the R-package front lip, which reduces the amount of air going under the car, and directs some towards the brakes. But the tires are still very exposed.
The are a number of aftermarket companies making Miata front ends, and many address this problem. The DIY solution is a vertical sheet of plastic, which you can buy at Speedway Motors for $20. I’ll always refer to this as the Supermiata Airdam, because Supermiata did a lot to popularize this style. 9 Lives Racing sells a pre-cut piece that will save you some headaches.
To mount the airdam you should use a sturdy undertray. Some racing rules limit the rearward length of the undertray, and so most of them start at about the front axle. If the rules permit it, most people simply extend the undertray in front of the airdam making it into a splitter. Trackspeed engineering makes an undertray/splitter bracket, which greatly simplifies building one.
Let’s take those drag and lift values, put them in OptimumLap, and see the results at my local race tracks. I’ll use a Miata with 100hp, 2400 lbs, and tires that grip at 1g.
Setup #1 is stock. Setup #2 is lowered to 4″, and this causes more drag, but also less lift, and that’s why it’s faster than #1. The exception is at Watkins Glen, where drag is more important. (Yes, lowering a car has other benefits, but I can’t simulate that in OptimumLap.)
Setup #3 is significantly faster. I don’t know if Hancha used the R-package front lip for the CFD, or if they calculated a chin-style airdam that completely covers the front wheels. Setup #4 adds a splitter to the chin airdam, and you can see it makes more downforce, without adding drag.
Setup #5 is a Supermiata style airdam, and it’s just a bit slower than #4. Add a splitter and you get Setup #6. Adding the splitter to the airdam reduced drag by 6.5%, and increased downforce by 34%. At a short and slow track like Pineview, this is only worth .6 seconds compared to a lowered Miata. At a track like WGI, it’s 3.8 seconds. Astonishing!
But probably not accurate in the real world. Add over 200 lbs of downforce to the front of the car and it’ll oversteer. So a real-world figure would be less. But then of course you’d balance that with a spoiler or wing, and go even faster, right?
If you’re serious about downforce, use a wing; it can generate more downforce, and is more efficient than a spoiler. It begs the question, why would anyone want a spoiler?
Spoilers are usually cheaper than wings.
Some racing rules don’t allow wings, but allow spoilers (Supermiata S2, for example).
A small spoiler can reduce both drag and lift.
Wings are often gaudy on a street car, but spoilers almost always make a car look cool. Not only my opinion, but NASCAR fans as well.
I’m going to build an adjustable-height 70-degree spoiler so I can find out what’s ideal on a Miata. But before that it’s worth looking at the existing literature and products.
How a spoiler works
As the name implies, a spoiler “spoils” the airflow coming over the top of the car, fooling the air into behaving as if the car has a different rear profile. This creates less drag and lift.
Let’s take a look at what the pundits say. In Race Car Aerodynamics, Katz shows two different graphs for spoilers. The first is based on spoiler height alone, at a fixed angle of 20 degrees from vertical, or what I’d call 70 degrees.
I’ve put some pencil marks on the graph and drawn some conclusions.
A low spoiler about 1″ tall reduces drag the most. It also adds a bit of downforce. From a drag and downforce perspective, it’s a win-win!
A 3″ spoiler doesn’t add any drag, and doubles the downforce of the low spoiler. In other words, you get something for nothing!
A taller spoiler adds downforce and drag, but downforce increases more rapidly than drag. The gift that keeps on giving!
So no matter what height spoiler you chose, it has a benefit. Based on theory alone, we should all have low spoilers on our street cars, and taller spoilers on our race cars (rules permitting).
Katz includes another graph on spoiler angle, this time using a fixed-height spoiler. Confusingly, this time the angle is measured from horizontal, not vertical, and the 70-degree angle from the previous graph isn’t included.
Some observations of this data:
Drag increases about linearly with angle.
Lift-drag ratio seems best at a very shallow angle, but this may simply be the low overall height of the spoiler. Also note that L/D ratio is at best 3:1, whereas a wing can be 12:1 or more. Which is why you use a wing if you’re serious about downforce.
Increasing spoiler angle to 60-degrees or more increases downforce, but at a diminishing return.
Spoiler height and angle combined
Next I’ll look at my other favorite reference, Competition Car Aerodynamics. McBeath cites CFD work done on NASCAR spoilers, in which they changed both the spoiler height and angle. Now we’re getting somewhere.
I’ll use the above results to compare spoilers of different lengths and angles that result in a similar total height above the deck. Which in turn allows me to figure out the most efficient spoiler angle.
160mm spoiler, 20 degree angle, 54.7mm total height
80mm spoiler, 40 degree angle, 51.4mm total height
60mm spoiler, 60 degree angle, 52mm total height
It’s a bit difficult to see in this graph, but a 60mm spoiler set at 60-degrees is slightly better than a 160mm spoiler set at 20 degrees, even though the longer spoiler is a little bit taller. In other words, a higher angle works better. But it’s only by a small amount.
Based on Katz and McBeath, here is my conclusion: The total height of the spoiler above the deck is all that really matters.
NASCAR used rear wings for a short period of time and then switched back to spoilers. Not because they could get better performance from a spoiler, but because the series is always looking for ways to make racing both closer and safer, and the wing did neither. In addition, the fans didn’t like the look of a wing. To be fair, the CoT wing was hideous, see for yourself.
So we can’t look to NASCAR for the most effective spoiler design, because we know their priorities lie in close racing rather than outright speed. But it’s worth noting a few things about NASCAR spoilers.
NASCAR probably knows more about spoiler design than any other race series, and they still don’t settle on one design. In fact, the regulations change almost yearly. Looking only at the height, in 2016 it was 3.5″, in 2017 2.375″, and in 2019 8″.
Some years the spoilers were adjustable for angle, some years they were fixed, and there have been different heights, widths, and shapes throughout the years.
NASCAR uses the spoiler to balance not only the overall aero package, but as a way to balance the performance between different cars, and at different tracks.
When NASCAR reverted from rear wings to spoilers, they set the spoiler angle at 70 degrees. In 2019 the fixed angle remains 70 degrees. Interesting.
The 2019 spoiler is flat across the top, but different shapes have come and gone.
The size and shape of Miata spoilers
So now that we’ve looked at spoiler theories and real-world examples from NASCAR, let’s get down to what matters: Miata spoilers.
Miatas have a roofline that is peaked in the middle, and you might imagine that the ideal spoiler shape has a matching convex arc to it. Although like all things aerodynamic, this could be totally false, and maybe the sides should be taller.
The rear edge of the trunk is curved and so a curved spoiler would look more natural, and could be an easier DIY project as well. Also, a curved spoiler would be more rigid than flat. However, some race series say that the spoiler must be flat, with no curvature. Booo!
There’s no reason to “spoil” the air coming along the sides of the car, and so a spoiler much wider than the rear canopy seems like a waste. Although the exposed spoiler ends are probably adding downforce. Albeit not very efficiently, and at probably a different angle than is ideal for spoiling the roofline shape.
This IKON spoiler is an attractive design, with a convex top edge and curved profile. It would be neat to see something like this with a flat extension that’s adjustable for height.
The Rocket Bunny spoiler is flatter across the top, taller, and with a steeper angle. I’d guess it’s slightly more effective than the Icon, but it has a tacked-on look that doesn’t really appeal to me.
And then there’s this JSP spoiler that looks like a wing, but isn’t (air isn’t going to flow under it, hence not a wing). The shape follows the curvature of the sides and roof, and this may be an efficient design. But meh to the looks.
Of course all of these spoilers have a fixed height and angle, so there’s no way to adjust the aerodynamic balance. On the other hand, the Blackbird Fabworx spoiler is large and adjustable for angle. I’m also not a huge fan of the way this one looks, but the beauty lies in the function.
DIY spoiler, testing height
I made my own spoiler, it’s about 3.5″ tall and has some curvature to it that follows the trunk shape. It’s made of plywood and fiberglass, and there are 6mm T-nuts so I can add an extension.
With the low spoiler (without any extension), I ran very consistent 1:22s at Pineview Run. And by consistent, I mean 1:22.03, 1:22.05, 1:22.07, and in my second run, 1:21.99, 1:21.99 and 1:21.93. This was a hot day, and if I compare the times to previous ones, the track was definitely slower than normal.
With a 3.5″ extension (total 7″ height), my lap times were less consistent, most of them around 1:21.5, but my fast lap was a 1:21.03, almost a full second faster. But that one was an outlier, and if I average the five fastest laps, the taller spoiler was about .55 seconds faster than the lower spoiler.
The following table is an average of four back-t0-back runs, two with the spoiler extension, and two without. I’ve averaged the top six fastest laps.
I added .01 to the Cd as a guess, but drag isn’t that consequential anyway. I came about the Cl figure by changing that value in OptimumLap until I got the .55 delta in lap time. It seems absurd to think a spoiler can make a .45 swing in Cl, but that’s what the simulation says.
In Race Car Aerodynamics, Katz cites several examples of spoilers, but none that go as high as 7″. In his examples, the relationship between height and coefficient of lift is nearly linear, and from 0″ to 4″ there’s a change of about .4 in Cl. So if I extrapolate those values from a 3.5″ spoiler to 7″, I’d only expect to see a change of .4 Cl at most, so I don’t see how the coefficient of lift could change by .55. Obviously, there’s human error involved as well (my driving), this is really all for conjecture anyway.
Whatever the case, a 7″ tall spoiler works on a Miata. Now I have to make a taller one and test that.
I have two Miatas, and even my faster one is slow. Adding an airdam, splitter, side skirts, wing, and all the other aero bits add weight, and some of them increase drag as well. The last thing you want to do to an underpowered car is add more weight and drag, right? Maybe.
Drag reduction matters most when accelerating on a straight, but pretty much everywhere else downforce is preferable to drag reduction. Even still, there are times when drag reduction is more important, such as in an endurance racing strategy where you want to do one less pit stop. It also seems logical that at a high-speed track you’d want to skew your aero package towards top speed and reduced drag, especially in an underpowered car.
Or so you’d think. But like most things aerodynamic, what seems obvious could be completely wrong. So let’s examine downforce vs drag on a very fast racetrack that is dominated by long straights and top speed.
I did a motorcycle track day at Portland International Raceway, and I’ll describe it like this: it has a really long and boring front straight, a couple corners, another long and slightly less boring back straight, and a couple corners. If ever there was a track where you’d want to reduce downforce and optimize for less drag, this is probably it.
In Race Car Aerodynamics, the author Joseph Katz calculated lap times for a generic prototype race car at Portland International Raceway factoring in grip and drag. Take a look at the track layout in the chart inset, it’s like I said. The full results are in SAE Paper 920349, but this is what I make of it.
At this track, you might think that you should set your aero for the least possible drag, thereby attaining the highest top speed. But that actually sets the worst lap time, some 6 seconds off the pace. Or you might think to optimize for the highest L/D ratio, and with that you’d at least be within the same second as the fastest cars.
But somewhere in the neighborhood of maximum downforce, that’s where the fastest lap time was. On any other track I’d guess that maximizing downforce is the right thing to do, but I’ve raced down the front straight on this track (which is nearly a mile long), and the results are surprising.
This is a calculation, albeit a very sophisticated one, and it’s based on a race car with a lot of power that can overcome drag. Still, it makes me wonder if we should chase all the downforce we can and not worry about drag reduction at all. Miatas are all about cornering anyway, and we’re used to getting passed on the straights!
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.
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?
HP to go 60 MPH
HP to go 90 MPH
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 even 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 faster than a stock Miata. Some people would give their left nut for a second-and-a-half per lap. I’m betting that with windows open, which is how I’ve always raced, the RX-7 advantage would be even higher. This because the Miata hardtop is quite wide, and acts as an air scoop, especially when in yaw.
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 each day. 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. If I calculate the total number of laps per stint, the driver in the stock Miata can do 42.6 laps per stint. The driver in the RX-7-bodied Miata can do 44.1 laps.
Imagine if Mazda made a RX-7-bodied Miata, without the design compromises of a convertible top. It would be sleeker, lighter, more rigid… and probably fall flat in sales. Ah well, it would have been great on track!