The Occam’s Racers team will be racing at Mid Ohio on 10/18-20 with AER. The main team is Mario Korf (me), Pat Cornwall, and Evan Merrill. Pat races regularly with a BMW league, and he’s been on the Occam’s team twice before. He’s a fast driver, a good mechanic, and you can’t ask for a better teammate. Evan was one of the drivers at the Watkins Glen aero tests. His background is sim racing, and this make his racecraft very impressive, and his online speed somehow translates to the real world.
I’ve been looking for a fourth driver for this race, and found one: Sonny Watanasirisuk. If you follow Miatas, you probably know the name, he works for 949 Racing has won many a race and championship, and is a pro driving coach. Sonny will be joining the team primarily as our coach. Who am I kidding? He’s our ringer. I’m super stoked at this development, and I’m sure it will result in some humble pie, as well as a bunch of speed and setup secrets which I will share with you all. (Or not. Muah ha ha ha.)
But first things first. We have a car hasn’t been run since the WGI tests, and we wanted to address the understeering problem we had there. So we put the rear sway bar back on and corrected the negative rake. There were other things to do, like new shackle-style motor mounts, which should keep the header from hitting the trans tunnel. It was making an awful thumping sound and was really disconcerting.
With these and other details done, we loaded up and headed to Pineview Run for a team test day. I’ve been driving my 1.6 Miata at Pineview a lot, and the first thing I noticed about the race car is that it steers way better, probably because of more caster. On the other hand, the brake bias is way off. The front brakes are the larger 99+ Sport model with Porterfield RE4 pads, and the rear are stock 94 brakes with StopTech pads. This makes for a very grabby and front biased setup that will need to be corrected with a prop valve and possibly a pad change before the race.
We each took two turns in the race car, and logged data with an AIM Solo. This allowed us to compere dick sizes lap times, and see where each of us is fast. I had the fastest average lap (I have a lot more laps around Pineview and can drive it consistently), but they both put in laps that were outright faster than I could. It’s good to have teammates that are faster than you are. What was surprising is that while our times were fairly close, we have different driving styles. But more on that another time.
I also invited my 24 Hours of Lemons teammate, Tom Pyrek, down to Pineview Run, and he brought his racing minivan. Tom got into the Miata for a few laps, which was surprising because he doesn’t like Miatas. Evan got to drive his Neighborhood Trolley minivan in exchange, which was pretty dope. Then it started to rain, and the fun really began.
Here’s a couple videos of Evan in his NB Miata and Tom in the minivan. Rain is fun!
In all, the test day went great. We had a lot of fun in the dry and wet, and the race car seemed much more balanced. (No video from the race car, sorry.) We’ll have to sort out the high speed handling when we get to Mid-O.
To get back to Sonny, he recently won the TT6 race at Mid Ohio, and I used his 1:40.7 lap time on the Pro Course to correct some values in OptimumLap. Based on the G values I saw in the video (around 1.3 g steady state cornering with higher spikes, but I’ll settle on 1.3 g) and rough Cl and Cd data (.48, .45), I was able to get the same lap time by changing course grip to 95%.
Based on the data corrections, OptimumLap says my car should be capable of a 1:41.6 on the Pro Course (with Sonny behind the wheel). AER is using the chicane, which adds probably two seconds, so a projected lap time is more like 143.5. This is all preliminary guesswork, but time will tell. Literally.
In a previous post I looked at how to calculate drag and lift using the HP Wizard drag calculator, and simulated lap times based on various modifications. This time around I want to look at some real-world race cars and use them as examples of drag and lift so I can play “what if”? Such as, what if my Miata was shaped like a NASCAR stock car? What would that do for performance? Or, what if I could make my Miata look like a LeMans prototype?
In the following table, imagine that each body is scaled up or down appropriately and would fit on a Miata chassis. The cars and assumed to be running with windows open, or however they run in their series. For the simulation, I’ll use 2400 lbs, 140 hp, with tires that grip at 1.1g (endurance tires), and 18 square feet frontal area.
For each car, I’ve calculated the lift/drag ratio, which is an indication of how efficient the body is at creating downforce. I’ll also simulate lap times at Mid Ohio Pro course (no chicane).
Standard race Miata, no aero
1990 Mazda RX7 GTO (spoiler)
1990 Mazda RX7 GTO (wing)
Miata, splitter, wing
Miata, splitter fastback, wing
Audi R8 race car
As expected, an aerodynamic body makes a big difference in lap time. A stock Miata doesn’t have a lot of drag, but it generates lift (positive Cl) while all the other body styles generate downforce (negative Cl). I’m not bashing Miatas, I love them. Virtually every street car generates lift and faces the same problems when used as a race car.
I thought the L/D ratio would be a direct indicator of lap time, but it isn’t. If it was, the RX7 GTO (wing) would have a faster time than the NASCAR body, but the stock car is slightly faster. While I chew on that, let’s take a deeper look at some of the body styles.
The 1990 Mazda RX7 GTO is particularly interesting because the spec body kit originally came with a rear spoiler, but this was later replaced with a wing. It’s nice to get these kinds of data points, it helps me correct my assumptions and make better estimates.
The earlier GTO version with a spoiler is a bad-ass looking car IMO. It probably has slightly better drag and lift than a Supermiata. They both have airdams and spoilers, but the GTO has side skirts and is a more developed shape. Take a look at what’s going on with the B-pillar vent, I’m not sure what that is, but it’s a good way to use air rushing past an open window.
I’ve previously written about spoilers on Miatas, and have used a theoretical Cd of .46 and a Cl of -.10 for some simulations. I may have underestimated both the amount of drag and downforce a spoiler can produce. I should probably re-run the data using a Cd of around .50 and a Cl of -.40.
Comparing the two versions, the spoiler has .03 more drag, and .09 less downforce. This leads me to the following thoughts:
I would have expected the spoiler to have less drag than a wing, not more. Perhaps the fastback shape is already very efficient and there’s no need to spoil the shape?
If you add a splitter to the wing version of the RX7, it would have a Cd of around .47 and a Cl of about -.91. This is pretty close to my Miata’s measured .48 and -1.01. My race Miata also has side skirts and vents behind the fenders, and some other tricks. The difference between the two is probably the wing.
NASCAR, Miata, and other body styles
It’s kind of surprising how good the aero is on a NASCAR stock car, or at least the 2002 version I’m using here. It has a very low ride height, side skirts, a higher rear deck, and a spoiler. Stock cars also do a good job of keeping air out of the cockpit by using window nets, and also curved B-pillars that extract air from the cockpit. The L/D ratio is right in between the two RX7 GTOs, but the stock car’s lap time is faster than both. Wha?
When you compare all of the cars here, you can see that a Miata with an airdam, splitter, and wing has drag and lift values that are good. Race car good. Against a standard Miata, the aero version is 3.5 seconds faster in the simulation. A fastback drops another second, but most people won’t have the time or inclination to go that route. Which is fine, because it’s really the last piece of the puzzle. Or is it?
The Audi R8 race car and Mazda prototype are in an entirely different league. To get anywhere close to this level in a Miata would require a flat bottom (or venturi), diffuser, and other tricks. I intend on building and testing these things in the future, but a round-number goal is something like Cd .50, Cl -2.0, L/D 4.0. This would give a projected lap time around 98.8, which would be almost 6 seconds faster than a stock body Miata, and all this from body shape alone.
The Pineview Challenge Cup series began last year, and was the club’s first competitive event. I blogged about that on my brother’s site, and joined the club shortly after.
In 2019 they made this into a series of six events, with one final round to determine the winner. I wrote about the simple classing systems in Time Trials on 300 Treadwear Tires, and felt like I would have the best chance in the 300 TW “Street” category.
I decided I would use this series of events to try different things. Each event I would change one thing or another, and see what the effect would be on lap times and feel. So, as you read through this, you’ll see what I’ve done to the car. Track conditions varied with each event, and I’m sure my driving ability did as well. So this isn’t a scientific test, it’s a log.
In the first running of the Pineview Challenge Cup, I put down a couple practice laps in the 1:24 range. This is always a good benchmark for what I consider my “endurance pace” on S.Drives. Meaning I can run this pace consistently, pretty much all day long.
But in the heat of competition, I can drop a fair chunk of time. On this day my fastest lap was a 1:21.7.
Event #1: +6 HP
Fast forward to 2019, and a new Challenge Cup series of events. The first change I made to the car was add some power. Over the winter I installed a Raceland header and a Magnaflow direct fit cat, but I haven’t had a chance to dyno it, or do any tuning. I estimate the car makes 110-112 HP based on a previous dyno run at Overdrive Automotive. It made 106 HP on their DynoJet with a DIY intake, Cobalt cat-back, and Megasquirt PNP2.
Compared to 2018, my best lap improved by .25 seconds. OptimumLap says that adding 6 hp is worth .27 seconds, so that’s really close.
Event #2: Airdam, spoiler, +ride height
For the second Challenge Cup I added a small airdam and splitter. I was worried about the splitter being too low, so I raised the ride height five full turns, or about 5.5″ at the pinch welds. This was probably overkill, but roads in New England suck. A couple years ago the car was lower and also had the R-Package front lip, and it would hit things all the time. So I was a bit paranoid about going too low, and might have overdone it going too high.
I also added a DIY 3.5″ tall spoiler to the trunk. So that’s a lot of variables at once: taller, chin airdam, spoiler. I’ll make a total wild guess on what that does for drag and lift, and increase drag by .02 and decrease lift by .4. The front and rear mods added about 10 lbs.
I got second place because it was just me against a Corvette. I lost.
Event #3: Oil and water don’t mix
My 93 Miata has a 4.3 final drive ratio, and because of that, I bounce off the rev limiter or have to shift briefly into 3rd gear three times per lap. In the future I plan to replace the open diff with a 4.1 Torsen, but until then I need a solution. So I plugged in my laptop and changed the redline to 7500 rpm.
I also had a new DIY spoiler, with an extra flat piece of black plastic to make the height adjustable. I put in Paco Motorsports seat bolsters, and got a bit more aggressive on the seat foamectomy.
I only got one run in when a younger fella blew something in the Porsche he was driving and spun out in his own fluids. He was good enough to pull over and sit for the red flag. But then he got impatient not knowing what to do next, so he drove around to start/finish. He dumped fluids the whole time. ON THE RACING LINE.
He’s a young kid, doesn’t even have his driver’s license, and so his dad brings a car for him. Coolest dad ever, right? I’ll give the kid a pass this time for not understanding the meaning of a red flag. But I left Pineview after doing only one run, I don’t have patience to sit around for a track that will thereafter be slower.
That one run gave me second place behind Mike Filosi in a S2000 and in front of Dennis Rice in a Cayman S. Dennis is about 20 years older than me, but we have a good little rivalry going between us.
Event #4: Bad aero balance
For the fourth event, I switched my trunk lid from the one with a spoiler to one with a luggage rack. I can’t remember exactly why I did this, I must have been traveling somewhere. It was a mistake. Without the spoiler, the car oversteered badly. I probably also have too much chassis rake, and maybe the spoiler was covering up that problem. In any case, I was slower than other events.
The S.Drives are also wearing down quite a bit, and I keep expecting them to get faster because of less tread squirm. There are too many variables in the machine, the driver, and the conditions to know for sure, but I feel like the tires are giving up on me.
I got last place this time, out of 6 cars. A bit humbling, but that’s what happens when you upset aero balance and don’t have the skill to drive around it.
Event #5: Missed it
I missed the 5th event for a late work meeting. I would have placed third or fourth I reckon. Hal and his ‘vette did low 17s, and Mike got second with a high 17, and I can’t touch that. Dennis was fast and did a 1:20.7, he would have given me a run for it.
Event #6: Wet
I’d been waiting all year for a rain event, and this time we finally got a wet track! I took off the S.Drives, which are now down to about 2/32, and mounted the 205 Conti ECS. A lot of people like them in the rain, and they apparently don’t need any heat to work.
In the first session I did a 124.1, which was pretty good compared to the other cars, on any tire. And then I ran a 123.9 in the second set. I got held up by another Miata on crappy all-season tires and had to pass him, so I only got one run in on the second session.
We usually take a break between the 2nd and 3rd sessions, to eat burgers and BS, and then suddenly the sun started to come out. I didn’t need that, I needed a downpour! But the track didn’t dry out completely, so that was fortunate.
In the final session I ran two 123.4s in a row, and that was good enough for first place in the 300 class. My first win! I also beat about half the cars in the 200 class, which was pretty satisfying. I even got close to Josh in his BRZ, he did a 1:22.8 on Z3s. Miatas are good in the wet.
Pineview Cup Challenge Final
I used the Conti ECS last time because the track was wet, but I know they are faster than the S.Drives in the dry, so I kept them on. Previous testing showed they are about 1.5 seconds faster than the Yoks, and so I was hoping for a 1:19 flat.
I decided to remove the top and the passenger seat. This was the finals, you go all out, right? I went over a half second faster than I thought I would. Some of this was track conditions, they were really good, and Josh went under 1:15 in his group, and that doesn’t really happen for him on Z3s (but does on RE71Rs).
I put the top back on for the last session (but not he passenger seat), but by this time the track was slower. I did a couple high 1:18s, which is only a couple tenths from what my car will do with 225 RS4s. The Contis are great tires.
In the end Hal Defrees won the final event, and the Pineview Cup Challenge overall trophy. He won every time he showed up, and never took any penalty points for going off track. That was enough to beat the rest of us, congratulations.
The HP Wizard website has an enormous amount of information, and I have a lot of fun playing with their drag calculator. Basically, you choose from various options and it calculates the Cd and Cl of your car. Let’s try this on a Miata.
I set the following options, which gives me a Cd of .380 and a Cl of .32, which is spot on.
Shape – Well rounded (2)
Front elevation – Low, rounded, sloping up (a)
Scuttle and Wing – Flush, rounded top wings (3)
Windscreen – Wrapped ends (2)
Windscreen peak – Rounded at top (1)
Canopy plan – Tapering to front (3)
Rear Canopy – Rounded canopy and boot (4)
Lower rear quarters – Tapering to rear (1)
Underbody – Monocoque RWD (4)
Skin friction – Recessed windows w/ mirrors (5)
Internal flow – Typical (4)
Openings – Closed cockpit (0)
Wheels – Fenders only, 205/50-15 (0.046)
Lift induced drag – Production model (Cl .32)
Frontal area 18.09: height (48.2) width (65.9).
Windows open and top down
Most HPDEs and racing organizations require you to run with open windows, and HP Wizard can simulate that. By changing Openings to “Open window prod car,” the Cd increases to .428. Or if you use the Miata as intended and open the top, then the Cd is .466.
Now this is interesting because my testing showed that opening the windows was much worse than .428, more like .450. In fact, the hard top with open windows measured worse than a completely open top. I’m still trying to get my head around that. I didn’t test an open top without a wing (no time, and it’s not a racy configuration), so it’s possible the wing actually helped the open top create less drag by smoothing airflow behind the car. Dunno.
In Race Car Aerodynamics, Katz measures the effect of open windows on passenger vehicles and race cars, and a typical sedan loses .067 drag to open windows and .09 to removing the top. So again, this is perplexing why the Miata in particular is so bad with open windows and a hard top.
So I’m just going to stick with a closed-windows model for the rest of this investigation.
Lift and downforce
The HP Wizard calculator can also calculate lift. If I choose the standard production model options, then my Miata model has a coefficient of lift (Cl) of .32. That’s pretty accurate based on other data I’ve seen. Let’s use the calculator’s lift options and see what happens to drag and lift as I lower the car and add downforce.
As usual, I’ll also simulate lap times on a race track (this time VIR). I’ll use a stock Miata (2450 lbs, 4.3, 1.0g tires, etc) and try it with 93 hp, and then I’ll add 50 hp because who has a stock Miata?
VIR 93 hp
Production race car
Speedway Miata (?)
High downforce Miata
Reducing lift (downforce) yields faster lap times, to a point. A Speedway Miata (if such a thing exists) needs a lot of power to overcome the drag. The high downforce version has so much drag that even 143 hp is not enough.
Another thing worth calling out is the diffuser. The Miata underbody is anything but flat, and a diffuser would work better with a flat bottom, but this tool doesn’t allow us to make such adjustments. Anyway, the tool says a diffuser will reduce drag slightly and increase downforce by about .25. If that’s true for a Miata, this is totally worth doing.
Reducing drag with a fastback
In the above simulations, drag turned out to be a lot more consequential than I would have thought. Let’s see about reducing drag with a fastback. Based on real-world tests of my fastback, this could be up to a .07 reduction in drag.
First, I’ll modify some values in HP Wizard and change “Rounded canopy and boot” to “Fastback 10-20 degrees,” and the Canopy plan to “Tapering to rear”. Right away the Cd goes from .380 to .333.
The next thing I see on the Miata is the hard top extends wider than the windows, and catches air moving along the car. My fastback doesn’t do that, so I’ll change Skin Friction from “Recessed windows” mirrors, to “Perfectly smooth body with mirrors,” and now Cd is .323.
That’s a total of 0.058 reduction in drag, which is really good. Not quite as good as my fastback, but I’ve also added some vents for internal airflow. So let’s clean up the internal flow from “Typical” to “Good Design” and the Cd drops to .314, which is a .066 reduction, and getting close to the .07 delta between an OEM hard top and my fastback.
So there’s no magic to a fastback’s drag reduction, just some simple things that are easy to calculate. The fact that a fastback also made my wing produce 20% more downforce was a pleasant surprise.
I’m not really a BMW guy. My brother used to have a chipped 325e (now with WINsome Racing), and we had that car plus our Miata and MR2 at Thunderhill, 3-mile, all on the same day. I loved the torque and sound of the straight six, and I went fastest in that car. But something about it and me never clicked.
On the other hand, Anthony Zwain is a BWM guy, and owns EDGE Motorsports in Mountain View, California. After the WGI aero test, Anthony and I started fantasizing about turning a E30 convertible into a fastback. The convertibles are heavier due to chassis bracing, which is a good thing, and the fastback roof would offset that by being very light (probably 20-25 lbs total). It could be a shooting brake or fastback. The sky’s the limit with a convertible.
You wouldn’t necessarily need a convertible to do this. I can imagine someone simply cutting the C-pillar, jumping on the the rear of the roof until it bent to about 12 degrees, and welding it back again. You’d need new side and rear windows, but those should be Lexan anyway.
Anthony. WINsome. Shut Up. One of y’all should do this.
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.
A low spoiler, about 1″ tall, gives you the greatest reduction in drag. It also gives you a bit of downforce. I call this a win-win.
A spoiler of 3″ tall has about the same as drag as a stock trunk. However, you get about double the downforce you got from the 1″ tall spoiler. I call this getting something for nothing.
A spoiler taller than 3″ begins to create a small amount of drag over a stock trunk lid, but it adds downforce at a greater rate than it increases drag. As you go higher and higher, this is the gift that keeps on giving. I dropped .55 seconds at Pineview Run (a low-speed 1-mile course) by using a 7″ tall spoiler vs 3.5″. I wasn’t able to increase the height any more than that, but it’s possible that going higher is even better.
Doing any of the above is smart. Using an adjustable spoiler is even smarter, it allows you to experiment and fine tune the balance.
A wing doesn’t require (much) power
I use a 9 Lives Racing wing at 4.5 degrees and this measured to a .03 difference in coefficient of drag, no matter which roof I used. That’s about the same as your two side mirrors combined. If you don’t think you have enough power to use a wing, move your mirrors inboard and stop arguing.
But let’s put some real numbers to it. I’ll use the RSR calculator to see how much power it takes to run a car without a wing, and with a wing. (2300 lb, 18 sq ft frontal area, .45 Cd without a wing, .48 Cd with wing).
So you can see that a wing isn’t a large source of drag unless you’re doing 100 mph or more. And even if you’re clocking less top speed at the end of the longest straight, you’ll be going faster around the track everywhere else.
Wing angle and location
If you run a single-element wing at more than about 10 degrees, you’ve created an air brake. This is because the air separates rather than stays attached, which causes a lot more drag. You don’t create more downforce when your wing is set too high, you create less.
The roofline of most cars creates downwash, and this effectively changes the angle of attack. In the image below, you can see that a wing placed low increases the angle of attack and the amount turbulence. A wing mounted higher up and further back has cleaner air, with less a bit less downwash.
If you’re using a wing and you’ve lost a lot of speed, reduce your wing angle and try again. A lot of wings have the best lift/drag coefficient (the most efficiency) right around zero degrees. As evidence, look up; planes fly around like that all day.
Use aero and profit
There are a lot of ways to add downforce that don’t require more power, and many combinations that work well together.
Spoilers are cheaper than wings, and splitters can get damaged easily. So there’s a lot of sense in running a less expensive, more durable, and more streamlined airdam-spoiler combination. It’s the Supermiata formula, and I think it looks better on a street car.
However, adding a splitter and a wing will make the car go faster around corners. The .02 change to Cd results in a paltry 1.5 hp loss at 80 mph. This is barely worth talking about.
I see that some cars are using a wing 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.
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.
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.
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.
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.
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.
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.
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!
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.
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.