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A splitter makes the majority of its downforce through suction. If you’re fuzzy about what that means, read Top 10 Reasons Your Splitter Sucks. In that article, I beat to death the fact that it’s the underside of the splitter that does most of the work. To make a splitter work better, you need to expand the air under and behind the splitter. The most common way to do this is with splitter diffusers (aka splitter tunnels or splitter ramps).

Splitter diffusers dump air into the wheel wells, which is a convenient area with a lot of volume. But it’s also an area of chaos, with spinning tires that squirt a jet of air perpendicularly across the splitter diffusers, and suspension parts moving rapidly up and down. Getting air to move efficiently through this mess isn’t easy, and so pressure can build in the wheel wells, which reduces the effectiveness of the splitter.

So anything you can do to get air out of the wheel wells will help the splitter make more downforce. As a byproduct, you’ll often get better brake cooling as well. The most common way to extract air from the wheel wells is by installing louvers on the top or sides of the fenders.

You can also cut a slit and push the fender inwards. Or some people pull the rear of the fender away from the body, and extract air from behind the fender.

However you do it, fender vents have been proven to be highly efficient and add downforce with very little drag. Race Louvers test data shows that a top vent alone is worth about 14-17 lbs of downforce at 100 mph, and only a pound or so of drag. Adding another vent behind the fender results in more gains.

But what’s interesting is that adding a third vent doesn’t seem to add any more downforce. Despite the fact that Race Louvers is a business that wants your money, they advise you not to buy a third vent. So there’s definitely a point of diminishing gains with fender vents, and I’ll address one solution to that in the future.

In this article, I’m going to review some of the popular Miata fender vents available, and some DIY solutions for the frugal. Most of these are universal in nature, and can be used on other cars as well.

Race Louvers

I’m going to start with Race Louvers fender vents, because Al from Race Louvers has done a lot of wind tunnel testing and publishes PDFs on his site. Much of this article is based on the testing and data in his articles and videos with AJ Hartman.

Al’s extensive wind tunnel testing shows that louver blades should extend both above and below the bodywork. This applies not only to fender vents, but hood vents as well. Louvers that were only on top of the bodywork, or were only recessed below the bodywork, didn’t work as well as louvers that extended both above and below the skin. Moreover, it appears that the depth below is more significant than the height above. This discovery seems lost on virtually every other manufacturer.

At $289 for the curved version you see above, or $279 for the straight version (less visually interesting), these do not come cheap. But then again, other louvered vents aren’t going to perform as well as these, so you get what you pay for.

On a NB Miata, these tested at 17 lbs of front downforce and virtually no drag. If that doesn’t seem like a lot of downforce, note that any gains you make in front downforce are typically matched with 100-150% more rear downforce. So you could think of this as a means of adding 42 lbs of total downforce.

Now this is just from a single pair of vents on top of the fenders, and more venting can be done behind the fenders. Race Louvers doesn’t have a rear vent for Miatas, but Left Lane Designs does.

Left Lane Designs

Left Lane Designs makes rear fender vents for NA and NB Miatas. NAs use two vents, one on either side of the style line, while NB Miatas have just a single big vent. These are priced at $279 and $289 respectively.

Adding the Left Lane Designs vents to the Race Louvers fender vents could get you as much as 50 lbs of front downforce. At least that’s what Race Louvers got on another test using similar vents, and I think that data can be safely applied to Miatas and other cars. Again, add 100-150% because now you can add more rear downforce.

Singular

For $249 you can get Singular fender louvers, which are the same vents that are on Ryan Passey’s HyperMiata. These laser-cut aluminum vents are visually interesting, in that they have some curvature to the vent shape, rather than the more rectangular venting on the Race Louver vents. This curvature follows the countour of the Miata fenders, and while I haven’t found any data on them, it’s a great look. (I’ll be testing these in a wind tunnel soon.)

One interesting feature of these vents is that for an additional $40 you can get a partial cover so that these vents are legal for Grid Life Touring Cup. GLTC limits the size of fender vents to 45 square inches for some reason, and it seems as arbitrary and quizzical as SCCA Time Trials limiting splitter diffusers to 84 square inches, or NASA ST4-6 limiting hood vents to 3/8″ in height. Whatever people, whatever.

R Theory

For slightly less dosh than the Race Louver or Singular fender vents, you can get a similar looking vent from R Theory. However, the similarities are only skin deep. Literally.

If you watch their wool tuft testing video, you can see that the louvers do not extend into the fender. Recall from the Race Louvers data that it’s probably more important to extend below the surface than above. The Singular vents are similar in this fashion, but at least those install below the surface, while the R Theory appear to install on top of the fenders (which not only looks less professional, but probably increases drag as well).

While the $192 price is significantly lower than Race Louvers or Singular, I couldn’t justify the loss in performance of using these. If you want fender vents because they look cool, go for it. If you’re primarily looking for performance, I’d look elsewhere.

Spiked Performance

Spiked Performance sells an inexpensive aluminum fender louver for $169. Like the R Theory vents, they install on top of the fender skin. Spiked vents measure about 33 square inches, and appear to be the smallest here. Based on that, I would expect them to be the least effective (but still worthwhile).

Napp Motorsports made a great installation video, and it looks quite easy with the supplied cutting template and a few rivets.

DCN Performance

I’ve never heard of DCN Performance, but they have NB Miata fender vents that are inexpensive and appear to be properly engineered. You can buy all three vents directly from DCN and Top Miata.

I didn’t see a US supplier, but I did find the lower pair of vents available on Etsy and eBay. At $150 for the lower pair, these are worth looking into.

Amazon plastic shit

Finally there are tons of universal plastic shit vents you can buy on Amazon or eBay. I’m not going to review any of them because I would rather put a stick in my eye than put these on my car. If you’re too cheap to buy decent vents, don’t buy plastic crap, make them yourself.

DIY options

If you think it’s ridiculous to spend nearly $600 on fender vents, or support crappy products made in China, I’m with you on both counts. This whole site is about DIY creativity, and I neither shill for nor take kickbacks from anyone. Let’s DIY us some fender vents!

Fenders holes

If you’re a cheap bastard or just an avid DIYer, then you can make your own fender louvers by just cutting into the sheet metal and bending louvers into it. Follow a pattern like Race Louvers and make them go both above and below the fender skin.

Another option is to say fuck the louvers, and cut a big gaping hole in the fender. In the course I took with JKF Aero, the CFD showed that a big louver-less hole made more front downforce than a fender vent with louvers. So if you’re after the maximum front downforce, and can accept a slight drag increase and loss in rear downforce, get that cutting wheel out grind you a big ass hole in your fenders.

Note that you should put a 3/8” Gurney flap on the leading edge. For hood louvers, you’d want up to 1” high flap, but apparently fender vents should use a smaller leading edge flap.

Fender cuts

Another easy modification is cutting a slit into the fender and pushing the bottom inward. On a NA Miata, a lot of people make a subtle cut on the belt line. But some people make a more aggressive cut higher up, and vent the entire rear of the tire.

I tested fender cuts versus Singular fender vents in the wind tunnel. That data, plus hood vents, wings, fastback, canards, and many other aero options are in my 54-page Miata Wind Tunnel report, which you can purchase for $35. Buying this report helps support this web page, aero investigations, and future wind tunnel testing. Thanks!

3D printed

If you have a 3D printer, Beavis Motorsports has a free STL file you can use to print a “time attack style” fender vent. These sit up quite tall, and you’d think they would be really draggy, but Race Louvers tested a 1 7/8″ tall time attack vent (not this exact vent, but something similar) and it made 14 lbs of downforce and 1 lb of drag, which was the second best of the vents they tested.

I don’t love the look of these, but that’s purely a subjective thing. If you do, then something like this is a good option.

Installation advice

After reading all of the PDFs on the Race Louvers site, and watching too many of his videos, I’ve distilled the following advice, which applies generally to all fender vents.

• Don’t place top-mounted fender vents too far rearward. The vents should end 6″ before the windshield to avoid high pressure air from the windshield interfering with the vent.
• Airflow over a hood or fender is generally straight back, and louvers should be installed perpendicular to this flow. Don’t angle the louver blades more than about 15 degrees or it will reduce their functionality. (The louvers should be more vertical than swept back.)
• Many people have removed their fender liners completely, which is often a mistake. But if you still have fender liners, you’ll get the most downforce by cutting a hole in the fender liner near the louver. If improved cooling is your primary goal, remove the front of the fender liner.

I’ve been playing Gran Turismo since the first version on the PSone, and I still grab my controller a couple times a week and get in 26.2 miles to earn my “daily disappointment” (a roulette prize wheel that always seems to land on the most worthless prize).

GT7 touts itself as a racing simulator, but its aero modeling sucks. I understand why they did this, to protect the game’s leveling system, called Performance Points (PP). In the real world, downforce benefits you in fast corners, the faster the corner, the more benefit. In order to balance that in the game using their PP system, every car would have a different PP value at every track. That’s too complicated, so Gran Turismo simply made aero less effective.

How much less effective? I’m glad you asked! To find out, I built four cars with exactly the same weight, power output, suspension, ground clearance, etc., but very different aero.

I had to buy ECUs, power limiters, and ballast to make them all equal weight and weight distribution, and then I put them all on Sport Medium tires.

I chose the Subaru BRZ and Toyota GR86 because there are a few different versions of the car, but one could do this experiment with a Mustang, C7 Corvette, or other car that has both street and racing versions.

I used Watkins Glen for the race track, because it has fast corners where aero is effective. At least in the real world. In the Gran Turismo world, aero is virtually meaningless:

Build	Front / Rear DF	Grip @ 75 mph	Lap
No aero 524pp	0 / 0	1.18g	2:06.845
Street aero 545pp	200/300	1.22g	2:07.006
Group 4 532pp	200/350	1.21g	2:07.076
Group 3 549pp	500/800	1.29g	2:06.730

As you can see, the car with the most downforce (1300 lbs), set the fastest lap, but only by a tenth of a second! In second place was the car with no aero at all, and it came in at 524 points versus 549 points for the Group 3 car. In the real world, you’d see a few seconds between a car with no aero and a car with a simple splitter and wing.

In conclusion, aero is meaningless in Gran Turismo, and that makes me sad. But are the aerodynamic values in GT7 totally worthless? No. I discovered that the downforce values listed for each car are fairly accurate, which means we have a large catalog of data that is otherwise very difficult to find!

Downforce values in GT7

In GT7 all cars with aero are given front and rear downforce listed in pounds. For example, a Toyota GT-One has 600 lbs of front downforce and 900 lbs of rear downforce. But in order for those values to have any meaning, we’d need to know what speeds those figures are based on. After some investigation, it appears to be at about 130 mph (210 kph).

I’ve come to this conclusion by comparing the real-world downforce values for several cars to the downforce values in the game. To get the real-world values, I went to Muslanne’s corner and noted the downforce values for every car (at 150 mph) and threw them in a spreadsheet. I then compared that to the same car in Gran Turismo, factored in the speed, and came out with 130 mph as being very close.

Another car that helped me figure out this value was the Porsche 911 GT3 RS, which reportedly makes 895 lbs of downforce at 124 mph. By factoring in a speed change to 130 mph, I got 984 lbs of downforce, which is close enough to GT7’s figure of 1000 lbs.

So here’s a smattering of cars in GT7 and their approximate coefficient of lift (cL), which is inverted here to mean downforce. If you’re not familiar with coefficients, check out my article on Thinking in Aerodynamic Coefficients to see examples of various cars and how much downforce and drag they make. Note that you can take any car in GT7 and divide the total front and rear downforce by 900 and get a close approximation to the (inverted) coefficient of lift.

Car	Front	Rear	Total DF	cL	% F	Drive
Formula	1500	2000	3500	4.20	42.9%	MR
GT-R LM	1400	1400	2800	3.43	50.0%	FF
RB X2019	1000	1600	2600	3.20	38.5%	MR
Valkyrie	1000	1500	2500	3.05	40.0%	MR
Gr 2, GT500	750	1150	1900	2.33	39.5%	FR
GT-One	600	900	1500	1.84	40.0%	MR
BRZ GT300	500	800	1300	1.37	38.5%	FR
Group 3	450	700	1150	1.21	39.1%	FR
911 GT3 RS	350	650	1000	1.06	35.0%	RR
Amuse S2K	350	550	900	0.95	38.9%	FR
Group 4	200	350	550	0.60	36.4%	FR
RX7 GTO (real)			525	0.57		FR
Street car aero	200	300	500	0.59	40.0%	All
RE-RX7	150	350	500	0.59	30.0%	FR
RCR Civic	200	250	450	0.53	44.4%	FF
Miata touring	70	350	420	0.50	16.7%	FR
LP 750-4 SV ‘15	150	250	400	0.42	37.5%	MR
C7 ZR-1	150	250	400	0.42	37.5%	FR
Gr 3 road car	100	200	300	0.32	33.3%	All
930 Turbo	100	150	250	0.27	40.0%	RR
Cayman GT4	80	160	240	0.26	33.3%	MR
911 GT3 (996) ‘01	30	120	150	0.16	20.0%	RR
C8	50	80	130	0.14	38.5%	MR
M2C	20	40	60	0.07	33.3%	FR
911 RS SC (993) ‘95	0	60	60	0.07	0.0%	RR
NSX Type R	25	25	50	0.05	50.0%	MR
WRX ‘99	0	40	40	0.04	0.0%	FR
GR Corolla	0	20	20	0.02	0.0%	FR
E30 M3, 190 Evo	0	0	0	0.00	0.0%	FR

I’ve also added the front aero load distribution (% F) and the drivetrain layout to the table. Many people say you should match aero distribution to weight distribution, but I don’t subscribe to that theory, and neither does Gran Turismo (or the real-world values in Muslanne’s database). You can see that aero distribution is typically around 40% front (meaning more rear aero), regardless of drivetrain layout. Even the front wheel drive Nismo GT-R LM car has a 50/50 aero distribution, despite having 65% of the weight on the front tires.

You’ll also notice that some cars that probably do have some downforce, such as the E30 M3 and 190E DTM cars are listed as having no downforce in GT7. And the Miata Touring car can somehow make 350 lbs of downforce, despite being a convertible. (In my real world testing, a convertible top made 40% of the downforce as a hardtop.) There’s also no concept of lift in the game, and as you might know, most street cars make lift, and cars with zero lift are rare. Anyway, it’s a game afterall, and I’m sure there are many other oversights and some mistakes (and that goes for this article as well).

I started this website five years ago, with the aim of expanding my personal knowledge of motorsports aerodynamics, and conveying that to others in a non-technical way. At the time I was mostly concerned with filling gaps in the Miata knowledge base and making vanity posts about endurance racing, but the site has grown to be more about aerodynamics and DIY projects in general.

If you have been following my blog for more than a minute, you figured out that I’m not a professional aerodynamicist. I regularly drop Fuck bombs, I threaten my readers with dick punches, and I neither work for, nor show allegiance to, any manufacturer.

I write for the joy of it, and to blow off steam from my day job, which is also technical writing, but boring stuff I leave at work. Aerodynamics is a passion project, a side gig, and like a lot of modern media, created by an enthusiastic amateur, rather than a professional.

I feel like my role in this industry is the narrow middle-ground between math-heavy SAE papers and bromance novels. I try to mix data with humor, and be entertaining. I’m sure that this comes at the expense of being truly accurate. I also don’t have the education or patience to achieve that level of excellence.

As such, I don’t pretend to know all the answers. Honestly, sometimes I don’t even know the fundamentals! So I offer ideas and conjecture when I’m uncertain. I absolutely make mistakes, and when people comment on them, I do my best to correct them.

In a game where most players keep their cards close to their chest, I’m playing with an open hand. By openly documenting my ideas and DIY projects, I share my successes and failures so others can learn from that. By openly providing data, I give other people the means to unfuck whatever errors I make. This is all about expanding knowledge, and five years on, this is still true.

In the end, we live in a world full of too many thinkers, talkers, and shit talkers. The world doesn’t have enough doers. And because of that, I’m going to keep experimenting with, and writing about, aerodynamics. As a result, I’ll continue to move quickly and fail often, so if you find errors, I beseech you to make comments so that I can correct them. Sniping from a distance is cowardly and helps nobody.

Of the future mistakes I’ll be making, I’m really excited about my Rosetta Stone Project. The concept is a single source that translates between three ways of testing that often give different results: CFD, wind tunnel, and track testing. For the “stone” I’ll be using the the ubiquitous Miata (with 9 Lives medium aero kit). By getting data from all three areas and comparing them on the same “stone,” I hope to refine the existing CFD model, as well as fill gaps in wind tunnel data (errors from the boundary layer and wheels that don’t spin). In turn, the track testing data from steady state and coast down testing should help refine the drag and downforce numbers. This is a big project, maybe not entirely achievable, but I dream big.

And of course there’s all the mistakes, financial and otherwise, that I’ll make swapping a motorcycle engine into my 94 Miata race car. Expect quite a few articles on Falconet, and a lot of tears as I cut new teeth. But if it works out like I imagine, it’s going to raise some eyebrows, set a new benchmark for DIY aero, and smash a few lap records.

Both cars will be tested in the A2 wind tunnel, and both AJ Hartman and Johnny Cichowski will accompany me. I won’t shill for a manufacturer, my role is more of a canary in the coal mine, so I tend to ruffle feathers. It will be interesting to see how we fly together. (That’s definitely enough bird analogies already, but maybe I’ll get someone to take video and see who is the first one to take a peck at the other.)

My Veloster N may or may not make another trip to the wind tunnel. While I have some aero parts still to refine, there’s just no market for a car with such low production numbers. But I will be offering canards for sale after I modify the design some more. I also want to make a wicker/kicker that turns the OEM wing into something more performant. Or make an active aero spoiler. These are Veloster N specific parts for the few; I don’t see myself making generic parts or wading into the aftermarket pool anytime soon.

As I reflect on this last 5 years, I’ve been largely a one-man band, the only elf in the shop, a lone wolf, etc. As much as my intentions are to benefit the entire community, I’ve done fuck-all to engage with any of you. As I kick off this next chapter, I’m going to start a new series of articles starring YOU! I want to hear what my readers are working on, and how your passion for aerodynamics has literally shaped your car. This has me more excited than either of the projects above, and so thank you ahead of time for anyone who will participate. See this page to submit your information. And if you have an idea for a guest post on any subject, contact me and let’s publish that shiz.

And with all that said about the past and future of this site, let’s keep playing this game of DIY aerodynamics. With our cards played openly on the table, and with data, of course.

Professional aerodynamicists mostly work in terms of coefficients. When they converse with each other, they talk in terms of points, decimals, and percentages. When I started writing about aerodynamics five years ago, I also worked solely in terms of coefficients. Not because I’m qualified to do so, but because I’m a cheap bastard who uses a free tool for racing simulations.

The only way I know to quantify lap time differences in aero setups is to use OptimumLap, and it requires coefficients for inputs. Thus, when I published the results of my aero testing at Watkins Glen in 2019, it was all numbers that were perhaps difficult for the average person to understand. I probably lost readers who don’t think in those terms.

These days I try to report things in pounds of downforce. I usually cite this at 100 mph because I like round numbers, and also because the A2 wind tunnel spreadsheet defaults to 100 mph. I’m still more comfortable talking about downforce in terms of (negative) coefficient of lift, but for the bulk of my audience who measures things in bananas and bald eagles, pounds of downforce is good.

But I also want to convert some of you to using coefficients, and also to using OptimumLap. So this blog post is my take on thinking in coefficients. (I’ll do a how to guide for OptimumLap in the future.)

Coefficient of drag

First let’s talk about the coefficient of drag (cD). Manufactures regularly publish drag numbers and I would guess that most car enthusiasts know that .20s is amazing (EVs), .30s is good (most cars), .40s is OK (SUVs), and .50s is terrible (trucks).

What people may not know is that manufacturer-supplied drag coefficients aren’t super accurate, because they cheat. They may adjust ride height and other things to achieve the best drag coefficient, so don’t take those numbers as gospel. Independent wind tunnel tests are perhaps better, but also not 100% accurate unless it has a rolling floor. Anyway, you can look up the drag coefficients of most cars (Ecomoder has a good list), and so I won’t spend a lot of time on that.

One thing I do want to mention is that you rarely see drag coefficients for cars with open windows. That’s how we track and race our cars in the USA, and it affects cars greatly. Every car is going to behave differently with open windows, so you’ll need to add somewhere between .03-.05 to your cD.

In my Veloster N, opening the front windows increased drag by 15%, and reduced rear downforce by 7%. The faster you go, the more aero balance shifts forward. This makes the car more prone to oversteer at high speed. In short, opening the windows is doubly bad for performance, so keep that in mind, as well as the drag penalty.

In the following table, the cars are all assumed to have closed windows except for the track car, which is a very rough estimate for a car with open windows and some aero bits added. I’ve listed pounds of drag and horsepower consumed by drag, just to give some hard numbers at 62 mph (100 kph). This is a rather low speed, but it comes up again when I examine coefficient of lift.

Type	cD	Lbs @ 62 mph	HP @ 62 mph
EV	0.25	110.72	8.55
Commuter	0.33	127.1	11.29
SUV	0.40	141.43	13.68
Track car	0.50	161.9	17.1
Truck	0.60	182.36	20.52

Coefficient of lift

While most people can wrap their heads around coefficient of drag (cD), they may have never seen values for coefficient of lift (cL). And that’s understandable, because manufactures seldom mention it. But they should! For going around a race track, cL is way, way more important than cD.

That’s because cars lift at speed, and most of that lift is over the rear. As the car goes faster and faster, it loses grip. If you want to go faster around a track, you need to reduce lift, or add downforce, which is essentially the same thing.

Note that OptimumLap inverts the coefficient of lift. So if your car makes downforce, use a positive value; if it makes lift, use a negative value.

So how much lift do most cars have? In the following sections I’ve divided cars into various groups based on their cL. These are general guidelines, to give you an understanding of how much lift or downforce typical cars have.

Low-drag cars (cL .20 )

A car that is streamlined for low drag usually has a lot of lift. The reason for that is the roofline is gradually curved and longer than the bottom of the car, so this creates low pressure above the car, creating lift. As such, most EVs have a lot of lift, but so do some sports cars that lack rear spoilers or wings.

The C6 Corvette below, with a cL of .21, has significantly more lift than most cars.

Typical street cars (cL .15)

Most passenger cars have a cL of around .10 to .20, with the average somewhere around .15. I think I’ve written elsewhere on this site that the average was around .3, but now that I have more data, I see it’s about half that value.

The best sports sedans (BMW M3, Mercedes AMG models) are in the single digits, and some even get close to zero lift, but not quite.

Ideal balance (cL 0.0)

A car with a cL of zero over both axles is ideal; the handling doesn’t change at any speed. To do get that balance often requires a big spoiler or wing, to cancel out the lift, which is mostly in the rear. You won’t see those accoutrements on family sedans, and so this category (with a few exceptions) is for true sports cars.

Cars that have around zero lift include the Audi R8, Koenigsegg CCR, Nissan GTR, and Pagani Zonda F, as well as most Ferraris and Porsches. You don’t really see many sedans that achieve zero lift, but oddly I know of two hatchbacks that do, the Ford Focus RS and Hyundai Veloster N.

Supercars (cL to -0.3)

There aren’t many production cars that have a cL in the negatives, and the ones that do are supercars. Dodge Viper SRT 10, Ford GT, Honda NSX-R, Mercedes MacLaren SLR, and Porsche 997 GT3, to name a few, and also most cars from Lotus.

The production car with the most downforce (that I have data on) is the Lamborghini Murcielago L670-4 SV.

Production cars end at around cL -.3, but this is where proper race cars begin. I don’t have numbers for the latest hypercars, like the Aston Martin Valkyrie, but that surely produces race car downforce.

DIY track cars, race cars with spoilers (cL -0.5)

Many DIY track cars with splitters and wings have a cL approaching -0.5. They should be better than that, but are limited by the average builder’s understanding of aerodynamics and average level of execution. Professional race cars with airdams and spoilers also come in around this number.

For example, NASCAR stock cars circa 2002 had a cL of -.46. These cars make look crude, but are highly developed aerodynamic missiles. This is about as good as you can do with an airdam and spoiler.

Another race car with a spoiler was the 1990 RX-7 GTO, which had a cL of -.44. The Mazda looks really slippery, but it doesn’t achieve the same level of aero as a stock car. The interesting thing about the RX-7 was they ran it with both a wing and a spoiler.

My Hyundai Veloster N with the a flat splitter (3″ lip, 4.5″ height) and a big ducktail spoiler tested at a cL of -.43. I guess Hyundai got a few things right, because I was going for the crudest aero I could and somehow landed next to some pro race cars with spoilers.

Winged track cars (cL -.5 to -1.0)

To get better than cL -0.5, you pretty much need a wing or underbody aero. Let’s start with the RX-7 with a spoiler and see what happens when they swapped to a wing. Drag decreased from cD .51 to .48, and lift went from cL -.44 to -.53. That’s a big difference in both directions, and resulted in a 128% increase in aerodynamic efficiency (more on that later). I don’t know if the rules limited wing height, but mounting the wing higher would probably increase downforce even more. In any case, the wing bumped the car above cL -0.5, which is where most race cars with wings reside.

Another car with a similar cL was a GT2 car used in a scientific paper (pdf), which had a cL -0.5. Interestingly, the majority of downforce (85%) was from the rear wing, and it seems like they should have been able get more front downforce to balance that out. But GT2 is a highly regulated class, and so maybe there’s nothing they can do within those rules.

Most Miatas with an airdam, splitter, and wing will also be in this range. The 9 Lives Racing medium downforce kit typifies this build. I don’t trust CFD until it’s been validated in a wind tunnel, but these do seem like reasonable numbers.

My Hyundai Veloster N outfitted with a DIY splitter, canards, and wing also made its way into this category, with a wind-tunnel tested cL of -.68. The wing was rather short at 135cm (53″), and I didn’t optimize the angle or height (I was testing many other things). I also had the splitter at 4.5″ ground clearance, which was too high. But even if I had a chance to lower the splitter and optimize the wing, I’m not sure the car would break the cL -1.0 barrier. That’s serious race car territory.

Serious race cars (cL -1.0 to -2.0)

The average car enthusiast building a track car is going to have a difficult time getting below cL -1.0. To get into this category requires careful attention to details that the average DIY builder doesn’t know or care about.

Still, it can be done, and my fastback Miata with various tricks managed cL -1.2 as tested at Watkins Glen.

Prototypes (cL -2.0 and less)

Any car that achieves a cL of less than -2.0 is likely a pro-level race car or a time attack build. It’s just very difficult to achieve this level of downforce. Katz cites a generic prototype racer at cL -3.0. The Audi R8 Lemans racer was around cL -2.6. At the top of my list is a Mazda RX-792P that was reported to be around cL -3.8.

Time attack cars don’t race wheel-to-wheel, and so their aerodynamic parts can be wider than the car. I’m quite sure these cars are well under cL -2.0.

It’s also possible to achieve a cL of under -2 by sucking the car to the ground with a huge fan. The Gordon Murray T.50 does just that, and has a cL in the -2.2 range.

The following table is based on a car with 2 meters of frontal area (21.53 square feet) and different cL values representing various cars. I chose a speed of 62 mph (100 kph) as OptimumLaps lists this in its vehicle report. It’s also a sensible cornering speed, and about where aero feels like it’s working. (Aero is always working, but this is around the speed where people feel it working.) The downforce value in the table is the cL inverted, which is how you’d put them into OptimumLap.

Type	Downforce	Lbs @ 62 mph
Sedan	-0.15	-31
Sports	0	0
Hypercar	0.25	51
NASCAR	0.45	92
Track	0.8	164
Race	1.3	266
Time attack	2.2	450
Prototype	3	614

Lift/Drag ratio

I started by discussing drag and then moved onto lift, so now let’s divide lift by drag to get the L/D ratio. The L/D ratio tells you how effective the car is at making downforce, and it’s often referred to as aerodynamic efficiency.

If this is a positive value, then the car makes lift, and the bigger the number, the worse it is. Most street cars and Spec race cars (Spec Miata, SE46, 944 Spec, etc) have a positive L/D ratio. Any car with a negative L/D ratio makes downforce, and the mo negative, the mo better.

In the following sections, I sort cars into rough categories that represent different L/D ratios. I’ll ignore anything that doesn’t make downforce. Fuck those cars.

DIY track cars (-1:1)

Cars with a L/D ratio of negative one to one make as much drag as they do downforce. Most track cars with DIY splitters and wings are somewhere in this range with a cD 0.5 and a cL -0.5. Add canards and it might be cD 0.6 and cL -0.6. Add a dual element wing and it might be cD 0.7 and cL -0.7. You see where I’m going with this, as long as the drag and downforce values are about the same, the car is in this category of mediocre aero efficiency.

My Hyundai Veloster N with a ducktail spoiler, better splitter, and some aero tricks thrown at it was about -1:1.

Race and TT aero (-2:1)

Any car that achieves a L/D ratio of -2:1 or better I’m going to put into the very generalized category of race aero. It’s difficult for a typical home builder to achieve this for a number of reasons. One of which is that in the USA we have to run with the windows down, and this adds drag and reduces rear downforce, making it even harder to achieve aero efficiency. But it can be done if you know what you’re doing, especially if you consult a professional or talk to Kyle Forster.

I don’t have a lot of data on time attack cars, but because they are usually unbound by engine restrictions, they make enormous amounts of power. As such, they can get away with adding downforce and mostly ignoring drag. I would imagine a lot of time attack cars are in this 2:1 category as well.

Professional aero (-3:1)

To get a L/D ratio of -3:1 you need to obsess about drag, and choose aero components that return a high L/D ratio. No, I’m not talking about choosing a wing for its efficiency, please stop with that nonsense. I’m talking about choosing components that give the entire vehicle the best aerodynamic efficiency (often this is running wing angle that’s close to stalling).

I don’t think you can get to this level without a lot of research and testing, and by that I mean a combination of CFD, wind tunnel, and track testing. I’m sure there will be a lot of time spent managing and manipulating vortices.

This is the level I’m trying to reach with my race car. I’m purposely not putting an image of a car here, because I’m reserving that for Falconet, after I come back from the wind tunnel and figure out how to squeak over the line with a L/D of -3:1

Prototype aero (-4:1)

It’s difficult to achieve this level of aero efficiency, but it can be done, or even exceeded. In fact, Le Mans prototype cars are limited to L/D of 4.0, so you know that it’s possible to do better than that. But you or me? Nah.

It’s been a little while since I announced that I would be teaming up with Spec13 to Busa-swap my 1994 Miata. I can pretty much assure you that looking back on this years from now, that this was both a necessity and a mistake.

A necessity because the motor allows me to do things I couldn’t have done with any other engine. And this has nothing to do with the power output, ram-air system, 11k redline, or sequential shifting, and everything to do with weight and packaging, and how I can exploit that.

A mistake because I was just about to get out of Miatas entirely, and concentrate on other things in my life. But the availability of this motor swap is creating so many opportunities for radical change that there will be no going back to a standard Miata! This is a one-way trip to the dark side of aerodynamic madness. So, what’s happening?

The engine and transmission are out, and the PPF has been removed. The Spec13 kit doesn’t use the PPF, and so the diff is supported by a rear bracket (I got mine from K-Power, but Spec13 will also offer a version of this).

I bought a V8 Roadsters subframe, which isn’t necessary, but should create more space to mount the engine. Not that the Busa engine needs a lot of room, it’s tiny. What excites me about the tubular subframe is what I can do with all the extra space.

The area in front of the engine block will be used for a radiator and extractor hood. That won’t take up much room, and I should be able to run the headers in front of the engine, put a hole in the firewall, and make a side exit exhaust.

This is a lot of work, and it’s extra work considering that Alyssa scanned the engine bay and is designing the header pipes right now. The engine sits longitudinally, and the pipes exit like a standard Miata under the car. Well… that’s what the swap kit comes with, but like I said, I want to make a side exit exhaust, and so my headers will likely wrap around the front of the engine.

The reason for that is threefold:

• Side exit exhausts are cool looking. I like the idea of a motorcycle muffler set into the door and visible on the passenger side of the car.
• A side exit makes it easier to make a flat bottom, because I won’t have to worry about heat management. I may still need to cool the diff with a NACA duct, but that’s a minor thing compared to cooling an exhaust.
• With one less item in the transmission tunnel, I can dump air into the empty chamber and make a front diffuser. Huh?

Because the Hayabusa transmission is within the engine case, this leaves the trans tunnel mostly empty. Typically you have a PPF, driveshaft, and exhaust sharing the trans tunnel, but with two out of three of these removed, there’s a lot of air space in the tunnel.

My plan is to diffuse air from the center of the splitter into the transmission cavity and make downforce in front of it. This is an area that a standard car can’t exploit, and because the diffuser is so far forward, it should create front downforce.

The Toyota GP-One used a front diffuser, but sends air to the sides, rather than into a central tunnel.

The Miata chassis doesn’t allow me to do the same thing, which is why I’m sending some air down the center tunnel. But I’ll also be sending some air to the sides via what I’m calling rocker vents.

Rocker vents

There are many options for fender vents, from expensive laser cut aluminum, to budget eBay plastic, to the cheapest solution – simply cutting a hole in the OE fenders.

Vents on the top of the fender are important, but so are vents behind the front wheel. The budget solution is to make a horizontal cut and then push the fender into the wheel well. Or some people will pull the bottom of the fender away from the body and vent air between the chassis and quarter panel. And finally some people will cut into the quarter panel and install Left Lane Designs fender vents.

All of these are good solutions to venting the front wheels and making the splitter work better. I like the following picture, which is a great example of NA Miata fender venting.

That’s good venting, but it’s not enough for Falconet’s front diffuser, so I’m cutting into the chassis to get even more extraction. My front wheel extractor vents will start about 3” further inside and exit into the bottom half of the doors. Say what?

I’ve been looking at DTM cars and Japanese GT500 cars (similar chassis), and they all have extreme front vents that exit in the rocker panel. Unfortunately I can’t do that without seriously compromising the unibody. My car has a full cage, but there are no tubes that connect the cage to the shock towers (yet), and so unless I weld those in, I’ll have to limit the amount of venting I can do.

I’d like to start the vent as near to the center of the car as possible, but I can’t get too aggressive with it, because if I cut into the footwell, I lose space for the dead pedal. I can live without that, but if I go further, I’ll lose space for the clutch pedal as well. So unless I mount the clutch lever on the steering wheel or shifter, there’s a limit to how centrally located the extractor vent can go. (The motorcycle transmission allows clutches upshifts, so this isn’t an insane idea.)

So for the time being, I’m going to stop the fender vent at the footwell, keeping all of the sheet metal in the cockpit intact. In the image above, that inner wall with the holes in it is the vertical wall that makes the side of the footwell.

I had originally thought I’d cut out the bottom door hinge completely and continue the extractor vent into the rocker panels. But I don’t want to turn my car into a noodle. There’s a vertical support at the hinges which ties into the rocker panel, and so I’m leaving all of that intact. This makes the exit of the duct much shorter and a harder angle than I’d like, but I’m just not feeling brave enough to cut a hinge into the middle of the car and then re-brace it.

This is already a lot of work, and you might be wondering, what’s all of this venting going to be worth in terms of downforce? I honestly don’t know. It’s not just fender venting we’re talking about, it’s a front diffuser.

I will A/B test the rocker vents in a wind tunnel on 6/20, but without spinning tires, I don’t know how accurate it will be. But I can at least compare the rocker vents with other fender vents I’m testing, and see how they compare. I took a bunch of pictures and will do a how-to post in the future.

I can tell you that the idea of the whole aero package occupies my thoughts from the moment I wake up. I’ve drawn and re-drawn several ideas, and I’m not ruling out a front wing molded into the bodywork.

The Motorsports Safety Foundation (MSF) is an instructor accreditation organization that is trying to standardize HPDE driving instructors nationwide. I have a MSF Level 2 certification, and felt that program (taught by Hooked on Driving, in my case) was great, and I learned a lot. MSF was supposed to have an instructor event this year, but that somehow fell through. Mass Tuning either picked up the ball or already had it rolling, so kudos to them.

Who is Mass Tuning? A bunch of cowboys, that’s who! I’m joking of course, but in the realm of HPDE, they feel like the disruptors. In just a couple years they’ve grown massively to become the HPDE organization with the most events in the northeast. Then they started adding time trials to their HPDE events, and this year they are even starting their own racing series, Northeast GT.

So given their rate of expansion and innovation, it doesn’t surprise me that Mass Tuning is also making waves in HPDE instruction. On March 2nd and 3rd (this coming weekend), they are hosting the first Instructor Fest (register). I believe you have to be a coach or aspiring to be one, but it’s FREE, so get on it!

You can see the full agenda below, but the important part is that I’ll be teaching a class on Saturday, on how to read a speed trace. The session is a little odd, because it’s a hands-on exercise using pens and paper. You won’t learn how to navigate a difficult app (ahem, Race Studio), instead you’ll learn the following things which you can apply to any motorsports data app:

• Look at the squiggly lines on a speed trace and know what they mean
• Recognize common errors in braking, cornering, and acceleration
• Describe “backing up the corner”
• Explain why vMin may be the most important data point
• Contrast two laps for for time gained or lost

Why data? Because most HPDE organizations don’t have a data coaching program, and don’t have enough right-seat coaches for intermediate drivers. Those drivers tread water in a pool of conflicting information, with little guidance after they’ve been soloed. I’d like to be one of the people that turn that situation around. As part of that, for the past two years I’ve been data coaching for the Niagara Porsche Club of America, and I can pass along my experience so that you can read data as well.

Even if you have no desire to be a data coach in the future, it’s important to understand data. I mean, everyone uses data already, from knowing what gear they are in, to what braking marker they use, and of course their lap time. If we did everything by butt dynos and “you know, I think that felt faster,” we’d get nowhere in life. Objective measures are the only way we can quantify anything, and data is the foundation of that.

So if you’re not doing anything this weekend, and are in the vicinity of Thompson Speedway, register for Instructor Fest. See you there!

Agenda

12:00pm – Lunch Meet & Greet

1:00pm – Instructor’s Meeting

1:15pm – “Survive Your Student” by Nick Fontana (Corner Faster)

2:15pm – “An Introduction to Data Coaching: Reading a Speed Trace” by Mario Korf (Occam’s Racer)

3:00pm – Break, Break, BREAK

3:15pm – “You, Me and the Journey to MSF Level 3” by Eric Kaul (Motorsport Safety Foundation)

4:15pm – “Trends in Causes of Claims & How to be a Safer Instructor” by Steve Katz (OnTrack Insurance)

5:00pm – Dinner & Cash Bar Social

10:00pm – Checkered Flag
 

Sunday:

8:00am – Breakfast Meet & Greet

9:00am – Instructor’s Meeting

9:15am – “Insurance Concerns for Instructors and Tips to Avoid Incidents with Students” by Ryan Staub (Epic Brokers)

10:15am – “Data Guided Coaching” by Matt Romanowski (TrailBrake.com)

11:00am – Break, Break, BREAK

11:15am – “A Guide to HPDE Instructing” by Matt Leblanc

1:00pm – Lunch Meet & Greet

2:00pm – “Taking Your Instructing Over the Top” by Bill Stevens (BnS Racing Services)

2:45pm – “Communicating with your Student” by Paul Balich

3:00pm – Questions & Answers

4:00pm – Checkered Flag