“My Car Doesn’t Make Enough Power to Use Aero.” Ow!

I don’t know how may times I hear this phrase: “My car doesn’t make enough power to use aero.” After reading a lot of forum and social media posts, there appear to be two main camps people fall into.

• People who believe that adding downforce always adds drag.
• People who set their wing at too much angle.

The first group of people are simply wrong. Perhaps it’s because they are thinking in terms of increasing downforce rather than reducing drag. Lots of things that reduce drag also generate downforce as a byproduct. Airdams, splitters, and spoilers have all been proven to reduce drag. And they increase downforce as well. I haven’t tried a flat underbody and diffuser yet, but when I do, those will surely reduce drag and add downforce as well. If you don’t believe me, believe Kyle Forster.

The second group of people probably don’t understand that most wings work in a very narrow range of angle. The shape of the roof, and the location of the wing, both play a large role in the angle the wing. Whenever I walk the pits I have to bite my tongue. I see a lot of wings with too much angle. I’ve never seen a wing with too little.

The fact is that Miatas don’t have great aero, and the first generation especially can reduce drag and gain downforce without any loss of power. Let’s get into this in more detail.

Airdam and splitter both reduce drag

If you think you it takes more power to use an airdam or splitter, you’re wrong. Miata’s have exposed front tires and a nose that deflects air under the car. Adding an airdam lowers drag and lift.

There’s a great CFD study done by the Hancha group (now also posted by Velox) that shows the effect of various front-end combinations.

In the chart, compare #2 (lowered Miata) to #5 (lowered Miata with Supermiata style airdam).

In the CFD study, the airdam reduced drag by a full 0.1 which is astounding. This resulted in 10 more horsepower at 100 mph. The change in downforce was equally impressive, with a delta of 175 pounds at 100 mph. Shocking stuff. I didn’t get to test these front-end versions myself, so while I wouldn’t put faith into these exact numbers, it’s certainly true you reduce drag and gain downforce with an airdam.

Based on the fact that you get free power and downforce, why wouldn’t you use an airdam? Despite what many people think, you can further reduce drag and add more downforce by adding a splitter.

The splitter in CFD Setup 6 decreases drag by a further .02 over the airdam, and increases downforce by 49 pounds. My actual testing showed a decrease in drag of .01 and an increase in downforce of 68 pounds, and this was with a suboptimal chassis rake. You can believe computers or believe real-world testing, either way, airdams and splitters give you less drag and more downforce.

Spoilers give you something for nothing

You don’t need extra power to use a spoiler. In fact, spoilers often decrease drag, giving you more power. If you have a Miata with a stock trunk and the class rules permit it, use one.

Based on data in  Race Car Aerodynamics, I can make the following generalizations.

• A low spoiler, about 1″ tall, gives you the greatest reduction in drag. It also gives you a bit of downforce. I call this a win-win.
• A spoiler of 3″ tall has about the same as drag as a stock trunk. However, you get about double the downforce you got from the 1″ tall spoiler. I call this getting something for nothing.
• A spoiler taller than 3″ begins to create a small amount of drag over a stock trunk lid, but it adds downforce at a greater rate than it increases drag. As you go higher and higher, this is the gift that keeps on giving. I dropped .55 seconds at Pineview Run (a low-speed 1-mile course) by using a 7″ tall spoiler vs 3.5″. I wasn’t able to increase the height any more than that, but it’s possible that going higher is even better.

Doing any of the above is smart. Using an adjustable spoiler is even smarter, it allows you to experiment and fine tune the balance.

A wing doesn’t require (much) power

I use a 9 Lives Racing wing at 4.5 degrees and this measured to a .03 difference in coefficient of drag, no matter which roof I used. That’s about the same as your two side mirrors combined. If you don’t think you have enough power to use a wing, move your mirrors inboard and stop arguing.

But let’s put some real numbers to it. I’ll use the RSR calculator to see how much power it takes to run a car without a wing, and with a wing. (2300 lb, 18 sq ft frontal area, .45 Cd without a wing, .48 Cd with wing).

Speed	No wing	Wing	HP lost
60 mph	18.1 HP	19.1 HP	1 HP
80 mph	38.6 HP	40.7 HP	2.1 HP
100 mph	71.5 HP	75.7 HP	4.2 HP

So you can see that a wing isn’t a large source of drag unless you’re doing 100 mph or more. And even if you’re clocking less top speed at the end of the longest straight, you’ll be going faster around the track everywhere else.

Wing angle and location

If you run a single-element wing at more than about 10 degrees, you’ve created an air brake. This is because the air separates rather than stays attached, which causes a lot more drag. You don’t create more downforce when your wing is set too high, you create less.

The roofline of most cars creates downwash, and this effectively changes the angle of attack. In the image below, you can see that a wing placed low increases the angle of attack and the amount turbulence. A wing mounted higher up and further back has cleaner air, with less a bit less downwash.

If you’re using a wing and you’ve lost a lot of speed, reduce your wing angle and try again. A lot of wings have the best lift/drag coefficient (the most efficiency) right around zero degrees. As evidence, look up; planes fly around like that all day.

Use aero and profit

There are a lot of ways to add downforce that don’t require more power, and many combinations that work well together.

Spoilers are cheaper than wings, and splitters can get damaged easily. So there’s a lot of sense in running a less expensive, more durable, and more streamlined airdam-spoiler combination. It’s the Supermiata formula, and I think it looks better on a street car.

However, adding a splitter and a wing will make the car go faster around corners. The .02 change to Cd results in a paltry 1.5 hp loss at 80 mph. This is barely worth talking about.

I see that some cars are using a wing and airdam, but without a splitter (I updated this paragraph after reading the comments section). I guess that works for some people, but that combination made my car push everywhere, and boring to drive. Someone who can set up a car could probably dial this out (not me). But I’ll never understand someone using an wing and airdam, and not a splitter, the data just doesn’t support that being a better option.

Likewise there are probably some backwards people who will use an airdam and splitter without any rear aero. If you can get that to work, good on ya, but it didn’t work for me. My street car has an airdam and small splitter and I swapped trunks to one without a spoiler, and the car was loose and hard to drive at the limit.

Whatever the case, aero can make your car handle better, look better, and go faster around the track. Despite what people think, aero doesn’t require power. On a Miata, it’s usually the opposite.

The next time I hear someone say “My car doesn’t make enough power to use aero,” I’m going to punch them in the dick.