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“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.

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 that shows the effect of an airdam.

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.

Image result for blackbird spoiler

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).

SpeedNo wingWingHP lost
60 mph18.1 HP19.1 HP1 HP
80 mph38.6 HP40.7 HP2.1 HP
100 mph71.5 HP75.7 HP4.2 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.

Putting a wing on this car at 5 degrees AOA would be a mistake. I’d run the wing at zero AOA.

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 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 a wing without 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.

Aero vs BTM in NASA ST6/TT6

I was chatting with a guy on Facebook the other day, he does time trials in NASA TT6, and was wondering about gearing ratios. I ran a couple simulations for him in OptimumLap with different gearing. It wasn’t very much different, shorter gearing was a hair faster, but required more shifts, which OptimumLap doesn’t account for. In the end we got to conversing about ST6/TT6 Miatas and whatnot.

The NASA ST/TT series is a really cool system based on power-to-weight ratio, which is then further modified by your car and its mods. For example, a Miata has A-arm suspension, and must take a -0.7 penalty to the power-to-weight ratio because that’s an advantage of cars that have Macpherson struts or whatever else.

Aero is another place where points are assessed, and if your car has stock bodywork, then you get a bump in power-to-weight ratio. They call this BTM aero, which means Base Trim Model, or in this case, Boring Typical Miata. You get a bit more horsepower if you run your car with stock bodywork. Here’s a brief look at the aero modifiers, and how they affect power to weight.

  • +0.4: BTM aero (more power for stock aero)
  • 0: Non-stock aero (R-package lip, airdam, etc)
  • -0.4: Altered roofline shape (fastback, whatever)
  • -0.4: No windshield (and by that, no roof)
  • -1.0: Wing or spoiler (a big HP penalty for rear aero)

The guy I was chatting with on FB told me that at a recent race there were 11 Miatas in ST6 and/or TT6 and every one of them was running BTM aero. I was kind of surprised by that, because Miatas have shitty aero, and it’s a lot easier to tack on an airdam than it is to add a bunch of power.

Image result for spec miata
BTM aero = slight bump in HP.

Maybe some of them were NBs? NB Miatas had better aero than NA, and I suppose if you have a NB, then BTM might be the way to go. I don’t have drag and lift data for NBs, so I can’t compare. But on a NA Miata, you’d have to think at least an airdam would help.

Image result for NA miata airdam
An airdam would decrease drag and lift, and would get a HP penalty.

So I ran some configurations in OptimumLap, using 2460 lbs and adjusted horsepower to the class limit based on different aero modifiers. I used Watkins Glen and Summit Point (main) for the two tracks.

Here’s a brief description of each car configuration:

  • BTM – Stock NA aero. I’m using .5 and .55 for drag and lift, which are close estimates based on my testing. This version of the car has the fewest penalty points, and ergo the most power, at 131.6.
  • Airdam – .45 drag and .5 lift are pretty accurate numbers. People may wonder why they are so high, it’s the open windows that destroys aero. The airdam loses the BTM modifier, so HP drops to 128.8.
  • Airdam + Chop Top – Treasure Coast’s “Chop Top” had the best L/D ratios of all the tops when used without a wing. It didn’t work well with a wing, so we won’t bother with that configuration. It’s unclear if this would be penalized for changing the roofline shape, so I took the penalty and HP is 126.2. But it would have 128.8 HP if it’s considered standard roofline shape.
  • Airdam and fastback (AD/FB) – A sleek combination with the lowest drag of .38, as verified in testing, and a lift of -.35. The roofline modifier of -.4 means this has only 126.2 HP.
  • Airdam and wing (AD/Wing) – Using a wing in ST6 is a full 1-point penalty, and brings HP down to 122.4. May as well run the airdam because you’re no longer using BTM aero. The Cd and Cl figure here is based on my tests with splitter on and off. Taking the splitter off reduced downforce by .38 and increased drag by .01.
  • Airdam, fastback, wing (AD/FB/Wing) – Using all the aero at once is the largest penalty, and gives an even 120 HP, 11.6 less than BTM aero.
ConfigBTMAirdamAD/CTAD/FBAD/WingAD/FB/Wing
Total Points-.7-1.1-1.5-1.5-2.1-2.5
HP131.6128.8126.2126.2122.4120.0
Cd, Cl.5, -.55.45, -.5.48, -.31.38, -.35.49, .63.42, .82
WGI140.89140.31140.61139.03138.42136.98
Summit84.0883.7983.7583.0881.8481.02

As you can see, at both Watkins Glen and Summit Point, aero beats power in a simulation. And possibly in a time trial where there’s no traffic. However, in a real-world race, you might choose power, thinking it’ll be easier to pass a slower car that’s holding you up.

Another thing to note is that while the versions with the wing went the fastest, it might be hard to balance that much rear downforce with nothing up front. Splitters aren’t allowed in ST6/TT6, and the front aero load distribution would be pretty light. So while the simulation says the wing is faster, in the real world, it might not be. You also have to factor in that a wing adds about 20 lbs high up and at the far end of the car, and I can’t calculate what that would do in OptimumLap. In the real world, mass centralization is a valuable thing.

All said, I think the airdam/fastback combination looks pretty good, it’s a full second faster than BTM aero at Summit Point, and 1.8 seconds faster at Watkins Glen. The Chop Top also does pretty well, and if NASA doesn’t consider it to be altering the roofline shape, then it would run 140.17 and 83.54. Not quite as fast as the fastback, but simple to do.

Another reason to choose aero over BTM is maybe your car makes 120-odd horsepower and you don’t want to shell out for the engine mods that would be required to hit the class limit. In this case, aero could be a cheaper way to run faster lap times.

Image result for supermiata spoiler
This Supermiata would have to be tuned down to 122.4 HP. A spoiler has the same penalty as a wing.

A note on these simulations: I used a 7500 rpm redline (sorry about that, I didn’t change it to 7000 and ran all of them at my redline), and 4.1 final drive, with tires that grip at 1.3g. I don’t run Hoosiers (or whatever), and lack that data. So if the lap times at these tracks are not super close to real world, that’s partly why. The comparative differences between the configurations is the point.

Part 2: Static HP

I posted the results of this blog on the Miata Race Prep group and got some interesting replies and requests to run more simulations. Time is money, but I guess I work for free?

Dan Howard suggested I keep the HP static and adjust the weight. This is because 130+ HP is pretty unrealistic in a NA Miata, and that’s a good point. So I’ll reduce HP to 114 (which is about what my 1.6 makes these days) and re-run the data and adjust the weight. I’ll remove the Chop Top from the comparison this time.

Note that this makes the BTM aero version very light, and slightly below 2150 lbs. That should incur another -0.1 penalty, but I’ll cheat a bit in favor of BTM because it lost last time. Also, if the full aero package added just 2 lbs, it could get a slight 0.1 bump in power. But it’s winning, so we’ll leave the playing field level.

ConfigBTMAirdamAD/FBAD/WingAD/FB/Wing
Total Points-.8-1.2-1.6-2.2-2.6
Weight21432189223423032348
Cd, Cl.5, -.55.45, -.5.38, -.35.49, .63.42, .82
WGI142.12141.26139.68138.73137.05
Summit84.6384.1883.3082.0081.07

If you compare the times to the previous table, you’ll see the same relationship, with aero winning out over power. In addition, notice that lap times are overall a bit slower, because reducing weight isn’t as effective as increasing power in OptimumLap.

I’m not sure what the deal with that is, because in the real world, I notice about a .8 second difference in lap times at Pineview Run with a 200 lb passenger. If I do a simulation in OptimumLap and add 200 lbs, it’s only a difference of .33 seconds. So for whatever reason, OptimumLap simulations don’t make weight as consequential as power, even if the car has the same lbs/hp ratio. Anyway, it’s a fucking simulation, and I wouldn’t bet a race on it.

Part 3: Spoiler and Airdam

Damn the power or weight, the winning aero simulation so far has been with a fastback and a wing! But it’s pretty clear that it would be hard to balance all of that rear downforce without a splitter. So what about using a smaller wing, or a wing with less angle of attack? That makes sense, but I don’t have that data. But I have some spoiler data, and that would balance an airdam nicely.

The data I have is from some back-to-back tests I did, and based on some pretty tight lap times, the spoiler made a difference of .55 seconds, which I calculated as .45 in coefficient of lift. This is me driving, and naturally there’s noise in this data, but whatever. I found that number pretty shocking, but a graph in Race Car Aerodynamics has similar results, and so maybe it’s accurate. Accurate enough for a simulation, anyway.

For this configuration, I’ll use a Cd of .46 and a Cl of -0.1 I’ll use the de-powered 114 hp Miata from Part 2, which¬†means the car weighs 2348 lbs.

  • WGI: 140.96 (BTM 142:12)
  • Summit: 83.67 (BTM 84.63)

If the real world is like the simulation, then the airdam/spoiler package is about a second faster than BTM aero at both tracks. The spoiler also beats the airdam alone, which surprises me, because the spoiler costs a full point, the same as a wing, and this incurs a large weight penalty.

Autocross. I don’t F@%#king get it.

I recently went to my country club, Pineview Run, to do some low-speed aero testing. I didn’t check the schedule ahead of time. My bad, the track was rented out to an SCCA autocross group. Todd said I could run when the autocross was finished, so I sat around and watched for a few hours.

Lots of neat cars, including a Miata with no windshield and aluminum panels screwed down over the passenger compartment. It was on slicks and looked like a shitload of fun. There were a lot of other neat cars, as well, and it was great to see the parking lot full!

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Neat cars come to autocross.

Pineview limits entries to 50 cars, and these were split into two groups of 25. They don’t use the whole track. Instead, they grid the cars on the front straight and do a timed run from a standing start near Turn 2. The timing ends before the final corner, and then the car goes back to its grid spot on the front straight, ready for another run.

Pineview pro driver Takis was the fastest, and completed the course in 50 seconds. He’s a regular at Pineview, and is capable of 1:10s, so that means there’s about 30% of the track that isn’t being used. On some sections of the course, they also put cones up. Why? It’s not a parking lot, it’s a short and twisty track that already has corners. Is it not an autocross without cones? Or is it that every autocross requires a dumb slalom section? Whatever the case, I don’t get the need to place (and thereby replace) cones on a track like this.

Enough of the cone rant, let’s do some math. The fastest car was doing around 50 seconds, and the slower cars were doing over 70 seconds. If the average car takes a minute to circulate the track, and they let out the next car say 10 seconds later, that’s 70 seconds per car. Multiply that by 25 cars on grid and each car gets a run about every 30 minutes. That group of 25 had to swap with another group of 25 who were all waiting in the parking lot. So there’s a shit ton of waiting in the parking lot, and even when you’re on grid, you have 30 minutes between runs. Are you fucking kidding me?

Well at least it isn’t very expensive. For SCCA members, it’s only $50, so maybe that’s the appeal? But again, let’s do the math. Based on track time, they are getting maybe six laps per day, so that’s about six minutes of track time, or $500 per hour. That’s ridiculous. Pineview recently ran a $129 open track, which is about an hour and a half of track time per car. So autocross is 6x more expensive than a track day?! For realz?

I saw a lot of talking and comparing of notes, it’s definitely a talker’s sport. And there is some degree of competition, I suppose. I guess that’s why people do it? But it seems like a total fucking waste of time and I wouldn’t wait around for 30 minutes to take one lap that I couldn’t even string together with another lap.

It would be a lot more efficient to let them do a couple laps at a time. Heck, space the cars out and get more cars on track at once. Oh wait, that’s what the Pineview Challenge Cup is. That makes too much sense for this group.

Pick up your cones so I can get on track!

After the autocross suckers packed up and left I got to run as many laps as I wanted, with no run groups. I went out, did a few laps, came in and adjusted the wing. Went out, did some more laps, removed the splitter. Went out, did some more laps, removed the wing. Etcetera. In the time it took for one of the autorcrossers to wait for another run, I’d done twice as many laps as they’d done all day. And I was stopping every few laps to make adjustments!

I don’t get autocross. I have race team members that do it, they’ll have to explain it to me sometime. I think one of them said he likes picking up cones.

Options on a Spec Miata for Endurance Racing

Evan and I were working on my car this weekend, and, surprise, we started talking about racing Miatas. We see a lot of Spec Miatas converted to endurance racing, and we discussed what different options to power, weight, grip, and aero would do in a 24-hour race.

So we imagined a fantasy team that started with a 1.6 Spec Miata, which they wanted to improve for endurance racing. Since this is fantasy game, let’s pick a silly fantasy name like Hugh Jass Racing. And give them a web page, and channels on twitch and youtube.

IMG_0691 Tom (1).jpg

Here are their current options, with the data necessary to run these simulations in OptimumLap.

  • Stock: This is their car as is. OEM hard top, stock bodywork, lowered suspension, 2300 lbs, dynoed at 120 hp, 4.3 final drive. .45 Cd, -.45 Cl. The drag and lift might be more than this, I’m being kind.
  • Light: Drop 200 lbs.
  • Power: Swap in a 1.8, 140 hp.
  • Grip: Like most Spec Miatas, this team has 15×7 wheels. This option simulates changing grip from 1.1g to 1.2g. This could be 205 RE71Rs on 15x8s, or some other cheaty 200 TW tire. If you don’t think there’s a difference in wheel width, go to the GRM Summer Tire Test and you’ll see the Z3s picked up 8/10ths of a second on a 33-second course by going from 7″ to 8″. (But still half a second behind the RE71R on 8″.)
  • Aero: Add an airdam, 4″ splitter, and 9LR wing. This changes the Cd to .48 and the Cl to 1.01, and adds 40 lbs.

I ran this simulation at VIR. After adding the car data in OptimumLap, I corrected the grip to 105% to get lap times that are about what Evan runs at VIR when the track is clear, and his knowledge of what other teams typically run on similar setups. We also cross listed top speed in OptimumLap with real-world GPS data, and were spot on.

I made this a 24 hour race, with 100 minutes subtracted for yellow flags, and 5 minutes for each pit stop. Here’s how the car options pan out.

ConfigurationLap TimeTotal Laps
Stock2:23.63538.8
Light2:22.40543.5
Power2:20.43549.0
Grip2:20.42551.2
Aero2:20.67551.2

The first thing that strikes me about this data is how boring it is! Adding power, grip, or aero are almost identical in this simulation. Each are equal to about 3 seconds per lap. At the end of the day, the Grip and Aero versions turn exactly 551.2 laps, and beat the Power version by two laps (only because the 1.8 consumes more fuel). However, in the real world, you might choose power, because it’s easier to get around cars that are running a similar speed and slowing you down. If I increase the gas tank size or reduce stints to 90 minutes (AER), then power wins by a smidge.

Lighter is always better, and removing 200 lbs beats the as-is car by 5 laps. But that wasn’t nearly enough to match the other options. In order to do that, I’d have remove 500 lbs. At 1800 lbs, the car completes 550 laps, but really, who can get a Miata that light with a driver? As an aside, removing 100 lbs was a difference of .62 seconds per lap, which is about the difference between a light driver (Evan) and heavy driver (normal people).

Supermiata in the mix

Supermiatas have a good combination of power, grip, and aero. The spec formula is an airdam without a splitter, a spoiler, 2400 lbs, and 140 hp. How would that fare against these other options? Let’s find out.

Image result for supermiata

I don’t have data on the exact aero combination, but this is just for fun, so I’ll estimate a Cd of .45 and a Cl of zero. That’s a .45 shift in coefficient of lift due to the airdam and spoiler, which is ballpark-ish. I’ll use 1.2g of grip, because they run on 15×9 wheels instead of the skinny 15×7 Spec Miatas use. I’ll do two runs, the second one adding a splitter, which my tests showed increased downforce by 0.38 and reduced drag by .01.

ConfigurationLap TimeTotal Laps
Supermiata2:16.29563.5
Supermiata, 4″ splitter2:15.08568.5

The Supermiata build wins by a lot because it has all three: power, grip, and aero. So that was a little unfair, so let’s see what happens when we combine options for the fantasy team.

Pick two. Which two?

The fantasy team has the budget to afford power, grip, and aero, but in the real world, you might have to choose two. But which two make the most sense? I’ll drop the lightweight version since it wasn’t competitive, and see which two options make the best combination.

ConfigurationLap TimeTotal Laps
Power, Grip2:17.09560.2
Power, Aero2:16.85561.2
Grip, Aero2:17.66560.1
Power, Grip, Aero2:13.81573.9

If you have to choose two, the combination of power and aero wins by one lap. If you live in fantasyland and can have all three, you win by a lot. And you even spank the Supermiata.

One thing to notice is that these options are not directly additive. If you add up the benefit of power, grip, and aero individually, it would be 9.35 seconds faster than the original lap time. But when combined, these net 9.82 seconds. The more grip you have from tires and aero, the more power you can use.

How the results are calculated.

Bang for the buck

Given that power, grip, and aero each netted about 3 seconds, what’s the best bang for the buck?

Depending on who’s doing the work, an engine swap is probably $1000 plus some miscellaneous expenses and tuning, call it $1200 without a standalone or $2000 with. Or a lot more than that if you have trouble with the swap and end up putting the 1.6 back in again (ahem, HJ).

Sticky tires like RE71Rs, are the easy option. However, they have less than half the lifespan of R-S4s or other proper 200 TW endurance tires, and cost you more in the long run. Factor in the price of mounting tires twice as often, and it’s about $600 extra per race. After two endurance races you’d start to wonder if an engine swap would have been more economical. After four races, you’d be certain of it.

Aero is the most economical choice. A wing and stands, plus an airdam and undertray is less than $1000. I made my own airdam, wing mounts, stands, and end plates, and spent $600-ish total including a 9LR wing. Doing it properly with swan neck mounts and a CNC cut airdam would double that price, but you’d have a really cool car like the one below. Aero would increase tire life as well, adding more to the economy. (See Race Car Aerodynamics, by Joseph Katz for the full details on that.)

Image result for miata 9 lives racing wing
Miata aero done right.

And aero makes your car look faster. Just fucking do it already.

DIY Miata Fastback

When I built my first fastback, the design was pretty organic, mostly a game of seeing what would fit with the Treasure Coast chop top. At the time it seemed a simpler way to define the roof shape, but it’s probably easier and cheaper to start from scratch.

Treasure Coast chop top.

I started by laying a piece of wood between the wing uprights and playing with the angle. In the pic below you can see I’m using a pair of vice grips clamped to the wing uprights. I moved them up and down until it looked right. I settled on around 15 degrees. The ideal angle is closer to 12 degrees, but that would create problems seeing out the rear window.

I cut silts in the wood to to fit the shape of the chop top. This was just to copy the shape of the roofline, I’d fill the sides later.

Then I built a wooden “transom” to hold the back end up. I also wanted to taper the sides in at the same 15-degree angle.

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Playing with angles and shapes.

I added a strip of angle aluminum along the body, which acts as the base of the fastback. These rails meant I could no longer use the stock trunk, which became a slight problem when I did the testing at Watkins Glen. Not a functional problem, but I wanted to test OEM bodywork, and that was no longer possible with this modification.

Mocking up the side shape with skateboard laminates.

The sides are made from skateboard laminates. They are thin and bend easily, and I had some on hand. Once I was satisfied with the basic shape, I glassed the outside seam to hold it together.

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Fiberglass on the outside seam to hold it all together.

Then I pulled it off and glassed the inside as well. You can see the graphics on the skateboard laminates, which are printed already.

Fiberglassing the seams inside.

I put in three Lexan windows. I filled the bodywork gaps with red race plastic, which was a mistake, as they expand and contract with heat. I’d eventually replace the red parts with sheet aluminum.

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The tapered sides may be a large source of the drag reduction, first because of the shape, and second, because the sides of the OEM hard top scoop air into the cabin, while the fastback does not. You can see how much narrower the fastback is in the hips, it’s all that red plastic.

To help deflect air getting into the cabin, I added small Lexan air dams where a B pillar would be, to help direct air going past the windows along the bodywork, and added vents at the roofline and at the trailing edges of the windows (these are not shown here).

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Finished, more or less.

How did it work? Better. In the race, the engine was running badly, to the tune of at least 5 seconds off pace, and yet I repeatedly passed a BMW e30 on the back straight at WGI. He would pull car lengths on me accelerating out of every corner, and I’d reel him in and pass him on the back straight. A year later I’d do aero testing at Watkins Glen and find out that the fastback is even better than I had imagined.

I recently saw a picture of the VW EV and from the back, the top looks somewhat similar to my fastback. I like the box cavity. Functionally, it should decrease turbulence and wake on the fastback. I may try that that on the next one.

DIY Cold-side Cowl Ram Intake

Backstory: I added a Megasquirt PNP2 to my 1993 street car in 2017. I replaced the airbox with a cone filter, and ran the IAT sensor through a tube just after the air filter. This allowed me to get rid of the restrictive AFM flapper valve, but I kept the stock crossover tube with the resonator, which is supposed to fill in a midrange flat spot.

From left to right: stock crossover tube, IAT, cone filter.

I was pretty happy with that setup because I picked up 4 mph on the uphill front straight at New York Safety Track. Overdrive Automotive in Johnson City dynoed the car, and it made 106 HP. I don’t know what the car was making before, but this is pretty good considering it’s only a Cobalt cat-back, a cone filter, and a Megasquirt PNP2 running on the base map.

106.5 HP on the base map. Messing around without a wideband O2 made less power.

This winter I added a Raceland header and a Magnaflow direct fit cat. I felt like the intake needed more air than just the turn signal intakes, so I put a dorky hood scoop on the headlight. And because all those intakes on the front were probably pressurizing the engine bay and keeping the radiator from working efficiently, I added a pair of $8 hood vents.

Intakes and vents abound. Probably didn’t work effectively.

The Raceland header doesn’t have a heat shield and I could feel a difference in the engine compartment. I was curious, so I bought a dual element thermometer and put it in different places in the engine compartment. I drove the car around like that, with one sensor at the air filter and the other at ambient temperature. I then put the outside temp sensor in the cowl behind the firewall and the temperature was very close to ambient.

$15 temp gauge was pretty useful.

The temp sensor only reads Celsius, converted to Fahrenheit, this is about a 48 degree difference. And this is running the car, not sitting at idle. A rule of thumb is that every 10 degrees F is equal to about 1% in power, so if I could move my intake from the engine compartment to the cowl, I should be able to get almost 5% more power.

Cowl intakes are nothing new, they take advantage of the high pressure zone where the windshield meets the hood. In the image below you can see the arrows mostly point upward indicating lift or negative pressure, but the cowl has significant positive pressure, which brings in cold air. This is a good place for an intake.

Image result for miata hood pressure map

I didn’t want to re-use the existing intake crossover. While this might be good for midrange, once the engine compartment heats up , the plastic crossover tube is like a sponge and retains heat, which ruins the whole idea of reducing intake temperature. So I needed to replace the plastic with metal. At the same time, I decided I should move everything over to the cold side of the engine bay, away from the headers.

The first thing I did was see if it would actually fit. I would have to remove the charcoal canister and water bottle for sure, and probably relocate the main fuse box. But it looked like it would fit, so I ordered the parts:

I then started making room for the parts, and as I did, I noticed that the alternator wheel had cut a hole in the back of the crossover tube’s resonator. So much for filling in the midrange flat spot! Note that since the MS PNP2 is a closed loop system, this didn’t make the engine run excessively lean, as this isn’t an introduction of unmetered air, as when running a AFM or MAF.

Well that explains the hole in the midrange.

Next I test fitted the parts and found I could snake the aluminum intake tubing underneath the strut tower bar. But only just barely. In the end, I removed the STB, because I’m skeptical of how much it helps, and I wanted a bit more room.

The intake tubing was a bit long, but after trimming the 180-degree elbow, I was able to fit the entire 18″ long tube and filter right up against the cowl. 949 Racing tested different length intake tracts and IIRC a 1.6 made the most power with a 19″ long intake (1.8s should be 21″). Close enough.

Removed the strut tower brace, fits better.

Then it was time to finish up some final details.

  • Cut a hole in the cowl to feed the air filter. Since the hole is slightly above the filter, I made a small shield to deflect air down towards the filter.
  • Put the fuse box bracket and fuse box back in. Can’t believe if fit.
  • Clamp all the hoses. Also strap down the intake from moving, using safety wire. It’s a tight fit next to the throttle wheel, and I don’t want the two to make contact.
  • Tape up all the holes in the cowl area and screen it off from debris.
  • Get rid of the headlight intake scoop and turn signal intakes.

There’s only one final detail, which is to put a new washer bottle on the hot side of the engine. The old airbox location is perfect for a universal washer bottle, which will complete the project. Oh, and I have to dyno it again and report back.

Part 2: Ram Air

That intake lasted about a week and then I decided to change it into a ram air intake. I’d already done one version of this on a 1.8 using the stock airbox and the turn signal. I posted the results some years ago. I measured the manifold pressure using a DIY water manometer (aquarium tubing and a measuring stick), and watched the intake pressure move 4″ of water at WFO. This worked out to about 1% more power at 100 mph.

This time I decided to use the cowl rather than the turn signal for the intake, but once again I butchered a stock airbox to make it work.

Stock airbox inverted, cut to fit and sealed with aluminum tape.

I started by cutting the bottom off of a stock airbox and turning it upside down. I then cut matching (ahem) holes in the airbox and cowl. Effectively, I made the entire cowl into an airbox and I’m using the stock airbox mostly to hold the air filter.

Stock airbox on the cold side using ram air. Washer bottle relocated.

I fastened a clear cover to the airbox, so I can see how dirty the air filter is without opening it. Inside the airbox, everything is sealed up and pressure should build. From experience, I know this will be almost negligible, but it will be fun to measure this and find out.

Third Gear No Brakes

I don’t know a lot of car drills, and in fact I only do one: “Third gear no brakes.” Leave it in third gear (or fourth on a higher speed track) and don’t touch the brakes, that’s all there is to it. I learned this exercise from Keith Code back when I was a motorcycle journalist for Moto Euro. We did an article on the California Superbike School, and Keith made us do this drill for two sessions at Sears Point (Sonoma), in the rain.

Third-gear-no-brakes is a great way to focus on entry speed, and you absolutely have to use reference points. You will eventually scare the shit out of yourself, but after that, you’ll be surprised how fast you can go.

For example, here are two laps, the red is me, the black is my friend Jim. We are in the same car on the same tires, and he is .2 seconds faster than me. But if you look at the traces, he’s shifting and braking, while I’m staying in 3rd gear the whole time, never touching the brakes.

If you look at the time graph along the bottom, you can see I make up most of the time in the middle of the graph. This is the “knuckle”, a triple-apex corner. I have to shut off the throttle right at the end of the corner, and that long sloping line is me coasting downhill, waiting for the blind hairpin. At the same point, Jim’s trace looks like a mountain, with strong acceleration upwards and hard braking coming back down.

The other place I make up time is in the S-trap, a low-speed switchback just before the front straight. This is at about the 4000 foot mark on the graph, and you can see we’re just crawling here, but I maintain my speed and this makes a big difference.

Jim isn’t a bad driver, he’s taken racing classes, and has raced wheel to wheel. He has strong inputs behind the wheel, and an aggressive driving style. But he slows down too much and my Miata doesn’t have a lot of power to overcome that.

This is what third-gear-no-brakes looks like from the cockpit. It’s not very exciting. I edited out the part where I went three wheels in the dirt!

Pineview Run, 1:28.9 in 3rd gear, no brakes. ’93 Miata, Yok S.Drives