Spoiler vs Wing

In this article I’m going to compare spoilers and wings, from cars that have used both, to the effects of trailing car aerodynamics, to when you’d choose one vs the other (or both) based on different rulesets.

First things first: some people get confused about the difference between a spoiler and a wing. Air goes over the top of a spoiler; Air goes on top of and underneath a wing.

This is a spoiler.

This is a wing.

This is a wing, but it’s mounted so close to the trunk that it behaves like a spoiler.

Key differences

If you look at the aerodynamic efficiency of a spoiler, it peaks at around 3:1 lift to drag ratio. So if a spoiler creates 30 pound of downforce, it’s also creating 10 lbs of drag.

Wings can have much higher lift/drag ratios of around 12:1, because unlike a spoiler, air goes under the wing, and it’s the underside that’s doing most of the work. So if the top of the wing is generating 30 pounds of downforce via pressure, the bottom side is generating 90 pounds underneath due to suction, at the same 10 lbs of drag. That’s a simplification to illustrate a point, the actual numbers depend on wing angle, airfoil shape, etc., but just know that the low-pressure region under the wing is what’s important.

The low pressure area is often about the same height as the chord. Meaning, if you have a wing with a 10″ chord, you don’t want to mount it any closer than 10″ to the decklid, or the wing loses performance. Now this is only a guideline, because cars with diffusers might want to mount the wing lower to extract more from the diffuser.

I’ve seen a lot of poorly mounted wings, mostly due to people thinking the top of the wing does the work. Another common error is too much wing angle, from not taking the roofline downwash angle into account. And then there are the low-performance wings that are largely cosmetic; pretty much any wing with a rounded trailing edge is a piece of shit.

As a practical matter, wings are more expensive and complex than spoilers. Initially you need to figure out the height and setback distance to extract the most performance, and then you might have to compromise with trunk access. Then there’s the question of Gurney flap size, after which you’ll probably mess with wing angle ad infinitum. Some people enjoy that kind of thing (guilty), but wings are not ideal for the set-it-and-forget-it crowd.

Spoilers on the other hand are dead simple. They are cheaper, lighter, and easier to mount than a wing. Small spoilers (less than an inch) are great for street cars, as they reduce drag and add downforce for free. But for racing, you want more downforce than a small spoiler can give you.

So yeah, let’s talk racing. What are the effects of drag and lift when using a spoiler versus a wing, and what happens when following a car with one or the other? Let’s start this investigation by looking at some cars that had both spoilers and wings on the same body.

Spoiler vs wing on a Mazda RX-7

A good apples-to-apples comparison is the 1990 RX-7 IMSA GTO race car. The svelte body kit included a small splitter, airdam, and spoiler. I’m still trying to figure out what the B-pillar vent was for, but those sexy extractors at the front wheel arches that blend seamlessly into the side skirts…. so fucking hot. But I digress.

The car originally came with a spoiler, and had a coefficient of drag (Cd) of .51 and a coefficient of lift (Cl) of -.44, for an aerodynamic efficiency of .86. Later the spoiler was replaced by a wing, resulting in a Cd of .48 and a Cl of -.53, for an aerodynamic efficiency 1.10.

In comparative terms, the wing produced 6% lass drag and 17% more downforce, for a 128% improvement in aerodynamic efficiency. OK, but what does that mean for a lap time? Let’s find out.

If you follow my blog you know I like to quantify things in OptimumLap. So I’ll build the exact same car, and then change the aero values for drag and lift. The IMSA car had a four-rotor Wankel that put out 600 hp, which is a bit unrealistic for most of us, so I’ll also run another simulation with the engine detuned to 200 hp. I’ll simulate them at my home track of Watkins Glen, and see what happens. (Note that I chose 200TW generic values for tire grip, so the lap times aren’t meant to represent real-world lap times. The important part is the delta in lap time, by changing the aero.)

Specification	Lap time in seconds
600 hp wing	122.32
600 hp spoiler	123.40
200 hp wing	131.89
200 hp spoiler	132.73

With the 600 hp engine, the wing was 1.08 seconds faster than the spoiler. With the 200 hp engine, the wing was .89 seconds faster than the spoiler. So on average, the wing version is about a second faster than a spoiler.

But I think there’s more that could have been done with the wing. If you look at the following pic, you can see the wing is the full body width of the car, which is the maximum width in a lot of wheel-to-wheel racing rules, but the wing is mounted quite low, and would make more downforce if it were higher. There are reasons to run a wing this low, but that has to do with extracting more from a rear diffuser, which this car doesn’t have.

Borrowing CFD from the JKF Aero course I took, putting the wing higher would result in .043 more downforce and .005 less drag. If I re-run the simulations with those values, the powerful car drops another .46 seconds, and the 200 hp car goes .29 seconds faster. Adding that all together, the wing is faster than the spoiler by 1.54 seconds with the 600 hp engine, and 1.18 seconds faster with the 200 hp engine.

What would you give to be 1.2-1.5 seconds faster than your competitors? I’d give my left nut for that. (But I’m already fixed down there, so they are merely decorative at this point anyway.)

OK, so on this car, you clearly want a wing and not a spoiler. Which is precisely what the IMSA team did, and they dominated. So are there any other cars that had both wings and spoilers on the same body?

Spoiler vs wing in NASCAR

If you think the aerodynamic package on a NASCAR racer is crude, you’d be wrong. The bodywork is highly developed and there are numerous aerodynamic tricks. One source cites drag and lift numbers of Cd .39 and Cl -.46. If you compare that to the IMSA RX-7, you can see that the stock car has a lot less drag, and the downforce value is between the spoiler and wing versions. Put both bodies on the same chassis and a NASCAR stock car would go faster than the RX-7 with a wing. You can read about that here.

So if NASCAR stock cars are sophisticated aerodynamic missiles, why don’t they use wings instead of spoilers? Well, for a brief period of time, 93 races to be exact, they did. This was in an era where the car was dubbed the “Car of Tomorrow,” and amongst many other changes, there was a rear wing.

The airfoil NASCAR chose was for low drag and high speed, and they mounted it low on the trunk. NASCAR is primarily concerned with the spectacle of close racing, so performance was not their driving factor.

So why did NASCAR get rid of the wing and go back to the spoiler? Safety and aesthetics, mostly. The safety issue was this: cars that spun 180 degrees went into the air and flipped upside down. Spinning at 180 mph isn’t something that happens to most of us, but it happens a lot in NASCAR. Also, fans hated the look of the wing and demanded the spoiler back.

There’s also the fact that the racing was better with the spoiler than the wing. Fans want drafting, slingshot passes, and trains of cars moving through the field. There were some good races in the COT wing era, but spoilers made for closer racing than wings.

Trailing car aerodynamics

Racing history is full dominance, where one car is so technically superior that it lines up in pole position every time and is never headed during a race. While those eras are memorable, they are boring to watch. Close racing is much more fun, and so rules are changed all the time to control costs, especially aerodynamic costs, and achieve parity.

If you follow Formula 1 rules, you’ll know there was a big rewrite in 2022. Just like NASCAR rule changes, the purpose was closer racing. The main problem was the “dirty” wake created by the lead car, such that trailing cars lost downforce, and even with DRS, they had a difficult time passing the lead car. Among the changes for 2022 were a rear wing designed to push the aerodynamic wake up and over the car following behind. The front wing, body, and wheels also had wake deflectors and other gizmos so that the trailing car didn’t lose as much downforce. All of this was so that cars could draft each other better.

(If you are reading between the lines here, then you know it’s possible to create an aerodynamic package that makes it difficult for other people to follow you. If I had any concrete information on how to do that, I sure as shit would not be posting this publicly. Am I developing such a package on my race car? Maybe.)

Drafting

Whether you’re talking about bicycles or NASCAR, racing on oval tracks is a game of drafting. The leading vehicle punches a hole in the air, creating a low pressure wake behind it. The person behind the leader can get in that wake and gain straight line speed. How effective is drafting? At the 2023 IMSA race at Daytona, MX-5 Cup cars were about 4-5 seconds faster per lap when they were drafting, rather than driving on their own.

As drafting relates to spoilers vs wings, take a look at the CFD below, comparing spoiler (top) to wing (bottom). Notice how the wake of the spoiler is both higher and longer than with the wing. If we’re racing against each other, and your car has a spoiler, thank you – I’m all up on your ass.

When is a spoiler better?

Wings have more downforce and less drag than a spoiler, and if your racing rules allow one or the other, you’d chose a wing every time. So are the any instances where a spoiler is better than a wing? Kinda.

Racing spectacle

From the spectators perspective, less aero is better. Wings don’t work as well in turbulent air, and so the trailing car loses downforce and stability. This makes passing more difficult, the cars spread out more over time, and it’s just not as fun. Watch NASCAR, Spec Miata, or really any non-aero series and you’ll see more drafting, with closer and better racing.

If I was designing a racing series from scratch, I would absolutely spec a spoiler over a wing. The Superspec Cup series in California (nee Supermiata) does this, and for some reason it hasn’t caught fire like everything else in California. Maybe another series (ahem, Grid Life 18:1 enduro class) could adopt their aero rules? Spoilers are cheap, they make street cars look like race cars, and are safer, as they cancel out the rear-biased lift generated by virtually all street cars.

Street

Beauty is subjective and so this either applies to your or not, but I don’t like the look of most wings on street cars. OEM wings are typically cosmetic and don’t do shit. Big wings look gaudy, and invite too much attention from cops and wannabe racers. Small wings are stupid and useless. The only wings I like the look of on street cars seem to be on Porsche 911s. Well, I like the 9 Lives Street Wang a lot, but it reminds me of a P-car whale tail, so that’s the same damn thing.

On the other hand, I like the looks of a spoiler on pretty much any street car. A low spoiler (less than 1″) reduces drag and lift, so a car will handle better and get higher the car milage than the same car with or without a wing. For a street car that gets occasional track use, a taller spoiler is appropriate. Get one that is adjustable for height/angle and you have the best of both worlds.

Autocross

SCCA national autocross aero rules were written by people who were afraid of or didn’t understand aero, and so they don’t allow wings until you get to the Modified category. Once you get to that category you’re allowed a dual-element wing with 8 square feet of area, which is absurdly large.

On a Miata, this would be a 12″ chord main wing and 6″ upper wing, which is about twice the amount of area you’d see on a typical track Miata. A car set up thusly would have so much rear-aero balance it would have criminal amounts of understeer on a race track.

Now those are the national rules, and at the regional level there’s an Xtreme Street category for track cars with wings. However, the rules allow the same ridiculous 8 square-foot wing as the Modified category.

There are no national or regional rules that have concessions for sane people who want to do both track driving and autocross with normal sized wings. So if you aren’t building a car specifically for the parking lot grand prix, you might be better off racing in the Prepared category. In which you are allowed an absurdly tall 10” spoiler.

I just don’t get SCCA autocross rules, it’s as if they’ve never seen a wing or spoiler? Who drives around with a 8 foot wing or a 10” spoiler? The hilarious thing is they allow a gigantic wing, but then restrict what you can do on the front. You can’t even put an end plate or fence on your splitter.

Anyway, for casual autocross, I’d wager a spoiler is better than a normal-sized wing. Not only because SCCA autocross rules are fucking stupid, but because a spoiler might actually be faster around your mall parking lot. I tested a spoiler and wing at Pineview Run, and I found the spoiler was half a second faster than the wing in A-B-A testing. Pineview Run is a tight and twisty track, with many fast changes of direction; it’s a lot like autocross.

Why was the spoiler faster? Because the wing added 14 lbs, at roof height, at the far end of the car, and this creates a higher center of gravity and more polar inertia. If you’re unsure of what that is, take a broom and hold it out in front of you and “slalom” around your house. Now pull the broom in tight to your chest and do the same thing.

Mass centralization is important for handling, and when you have weight high up at the far end of the car, it makes it more difficult to change direction. So even if the wing was performing statically better (more downforce and less drag than the spoiler), it was slower than the spoiler.

Grid Life Touring Cup

GLTC rules allow a 250 square-inch wing or spoiler for free. Justin Lee and I tested a 250 sq-in wing versus a 9 Lives Racing wing, and it was clear that the larger wing was faster, even if it did require a 3% lbs/hp penalty. Moreover, the 9 Lives wing wasn’t the full 701 sq-in size that the rules allow, or the smaller wing would have fared even worse.

One of the reasons for that is that a 250 sq-in wing has a very small chord. For the most part you can ignore Reynolds numbers (which you can think of as low speed, small chord, or both), but most airfoils don’t perform well at low Reynolds. The following image shows the airfoil efficiency of the CH10 airfoil at different angles of attack, with 200k (brown) and 500k RE (blue). This graph is essentially the difference in efficiency between a 250 sq-in wing and 625 sq-in wing at the same speed (100 kph, 62 mph).

I haven’t tested a 250 sq-in wing vs a 250 sq-in spoiler, but I have a feeling the spoiler might hold its own. A 250 sq-in spoiler has more surface area because it’s using not just the blade, but the entire decklid to aggregate pressure (downforce). Consequently, the spoiler should have a much larger Reynolds number, which would be less affected by low-speed aero losses.

However, as we already saw, a spoiler creates a larger and higher trailing wake, making it easier for cars to follow you. So while a 250 sq-in spoiler might turn a better lap time than a small wing, the wing might actually be better for racing. I don’t know the answer to this question, and I’m unlikely to test it.

Why? Because regardless of which “free” option you choose, a spoiler or small wing, a bigger wing and a 3% penalty to lbs/hp ratio has already been proven far superior, so I don’t know why anyone racing in GLTC would consider a small spoiler or wing to begin with. Just to be contrary? Or because you like losing?

Convertible

If you drop the top on a convertible, it destroys a wing’s performance. If you were making 100 lbs of downforce with the top up, you’ll be at 40 lbs with the top down. That’s not conjecture, that’s hard evidence.

So, if you have a Miata or other convertible, and you’re dedicated to the drop-top, a spoiler is probably better. This isn’t total guesswork on my part, but conclusions drawn by Kyle Forster in the video Do Rear Wings Work on Convertible Race Cars. With the top down, the spoiler lost less performance than the wing, not just because there’s less performance to begin with, but a spoiler just doesn’t seem to be affected by turbulence as much as a wing.

Spoiler and wing together?

So what about using a spoiler and a wing at the same time, is this a Nuts and gum, together at last situation, or the best of both worlds?

Most club racing and time attack rules don’t allow you to use both a spoiler and a wing, you have to choose one or the other. And you’d choose the wing, natch. But if the rules allow it, or if you have a HPDE car that doesn’t have to conform to a ruleset, then using both a wing and a spoiler is like peanut butter and jelly.

A spoiler helps extract more out of the wing in a similar manner as a second wing element. Air kicking upwards can activate the trailing edge of the wing, which in turn can allow you to run more wing angle without separation.

The spoiler will also raise the height of the rear wake, which pushes the rear of the car down. If you have a rear diffuser, that upwards airflow will help extract more from the diffuser as well. Win-win, hallelujah, and let’s see more of that on something other than a Noble M12.

Author’s choice

After all this investigation, you might wonder what the author uses on his two Miatas.

• My street car has a Galvez spoiler. It’s a neat design that mounts easily, but I felt it could be both taller and more rigid. I made a new spoiler blade that is taller and narrower, and matches the profile of the roofline more closely.
• My race car has a DIY spoiler and a 41×16 S1223 wing mounted via the end plates. I have rivnuts in the trunk lid so that I can quickly swap between a small 1” lip or a more aggressive 4” kicker. Or I can run it without a spoiler when the rules call for that.