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What Miatas Can Learn from a Porsche 914

A Miata and a Porsche 914 are somewhat similar in size and shape, and there’s some interesting studies and CFD data on the 914 that could help fill some knowledge gaps that exist in the Miataverse.

Image result for porsche 914
Not a Miata. But not that far.

They have similar frontal areas, but the 914 has a better coefficient of drag (.363 vs .38). I suspect this is partly the front end, which covers the tires completely from airflow. The 914 canopy might be better as well, since a vertical rear window is better than one that slopes back at 25-35 degrees, which is the worst possible angle, and about what a Miata hard top has. But in many ways, these cars are not so different, so let’s take a look at the 914 in more detail.

Published drag data

I’ve been digging around and found a few pieces of interesting data on a Porsche 914. In the May 1978 Porsche Panorama magazine, there are drag figures for a Porsche 914 in various configurations.

ConfigurationCdChange from stock
Windows closed.3630
Headlights up.380-.17
Windows open.381-.18
No roof.389.-26
No roof, windows open.447-.84
No roof, windows open, headlights up.464-1.01
  • One of the things I didn’t test at Watkins Glen was the effect of pop-up headlights (because I removed them from the race car). Miatas have slightly larger headlights, but at least this is a ballpark figure I can use for simulations in the future.
  • I also wasn’t able to test the difference between windows open and closed. Again, because race car. However, the windows open drag figure published here is very good compared to a Miata. I think it’s the Miata’s wider rear canopy that’s the culprit. The 914 canopy doesn’t stick out into airflow the way the Miata does, and it’s probably a significant drag factor when the windows are open. The 914 also has turn signals mounted on fender bulges, which redirects airflow along the car. But more on that later.

Scientific experiments on a 914

Chris Cassidy works at UCSD, in the Aerospace and Mechanical Engineering department. He also raced a Porsche 914 in autocross, and sponsored a project where his students used his car to do aerodynamic experiments to reduce drag, which would make his car faster in autocross. Much of this blog focuses on his work, thank you Chris and students!

Chris had several groups of students over a few years, each group doing a slightly different project, from preparing a scale model, to validating the model, and then doing computer analysis. His second group of students did the CFD vs water tank experiments, summarized in the table below.

FloWorks is CFD. LDV is a scale model in a water tunnel.

From the scale model and flow visualizations, they concluded that the CFD model was accurate enough. And from this, they could test a few different configurations, and see how drag affects HP at 50 mph.

Horsepower gains, via reduction in drag.

Now I’m grateful to Chris for allowing me to use his data, but I think the primary supposition is wrong, which is that drag reduction equals speed in autocross. The students concluded that the standard car with no airdam or spoiler would be the fastest, because it had a 2.1 horsepower advantage due to drag reduction.

However, I’ve done plenty of computer simulations, and I can tell you that at low speed, drag reduction isn’t that important, certainly nowhere near as important as creating downforce. If Chris wanted to go faster, he should have had his students work primarily on downforce. But let’s not skip ahead, let’s check out what they did.

CFD studies on a 914

Chris’s third group of students put 40 different configurations into CFD analysis, totaling over 400 hours of computer simulations. You can check out his website for all the results, but I’ll hit some of the higher points and relate this to a Miata.

First, take a look at the following image. The body styles are listed in order by the amount of HP gained in each configuration, which roughly (but not exactly) corresponds to the least drag. Not surprisingly, the most streamlined version, #15, gains the most HP. Being wing shaped, it also has the most lift. In the downforce column, the lower the number the better (a positive number is lift, negative number is downforce). The last two configurations are missing from this image, but are on the poster. All of this makes me wonder which one would be fastest on a race track….

Before I get to finding out that burning question, I’ll need to figure out the coefficients of drag and lift from Newtons.

Figuring out drag and lift

OptimumLap uses coefficient of drag and lift, and Chris’s students used drag force and downforce in Newtons. How am I going to convert this? I wish I paid more attention in math class, but here’s a formula I can almost get my head around. Solving for coefficient of drag Cd, Fd is drag force, p is the density of fluid, V is velocity, and A is surface area.

Cd = (2*Fd)/p*V^2*A)

For each configuration, the velocity is the same (50 mph), as are the fluid density (air) and frontal area. (17.8 sq ft). So it looks like the Cd is directly based on the drag force. This means I can set up a simple ratio, and get the relative Cd.

Likewise, I need to convert downforce in Newtons to a Cl value. I’m sure there’s a formula for this, but if I call the stock body .30 lift (a reasonable guess), I can figure out the lift of other body styles by a straight ratio using the downforce value.

OptimumLap simulations

Let’s see which of these is fastest on an autocross track using a 914 with stock weight and power (94 whp) and tires at 1.1g grip. I’ll also throw in Watkins Glen to see if anything different happens at high speed. Some of the configurations in the big image above make no sense (like the car resting on the ground), and so I’ll use only some of them. I’ve reordered these from mild to wild:

ConfigurationCdClAutocrossWGI
#33 – Stock.3630.3065.08150.17
#5 – Rear spoiler.385-0.3764.44148.30
#4 – Front spoiler .413-0.0564.77149.77
#29 – Slotted pillar.3200.3065.06149.34
#23 – Big wing.405-0.2364.59149.20
#34 – Racing open.3030.5365.28150.10
#22 – No glass.429-.4864.36148.86
#10 – Slant back.3190.7365.48151.30
#14 – Scoop front, slant back.2990.4365.18149.57
#15 – Full streamliner.2701.1065.82152.42

The students theorized that the stock configuration, with the least drag, would be fastest, but it doesn’t come out that way in OptimumLap. Configuration #22, which had the most drag, won on the autocross course. It also placed second at Watkins Glen, but got beat by the spoiler, Configuration #5, which has less drag.

A conclusion you can draw from this is that at low speed, ignore drag and go after as much downforce as possible. At a high-speed venue, go after the best L/D ratio. Most tracks are somewhere in between these two, and in this case, I would maximize downforce as long as it doesn’t require an extra stop in an endurance race.

There are some strange discrepancies in the data, like why is an airdam and a spoiler worse than just a spoiler? And why didn’t the wing perform very well? I don’t have the answers, and this is just CFD, so we have to take all of this with crack-rock sized grain of salt.

What can we apply to a Miata?

Well that was fun to see what happens to a modified 914, but what can we apply to a Miata?

Spoiler alert

The CFD study here shows that a spoiler is very effective on a 914. I’ve written about Miata spoilers before, and mine works really well. There’s really no downside.

Directing airflow below the window

Although the 914 front end directs air under the car just as a Miata does, it covers the front wheels nicely. But that’s not the part of the front end that’s the most interesting, take a look at the turn signal bulges. They look like a styling feature, but in fact they work to pull air down the side of the car, and away from the windows. These days you see this done with canards, but the 914 does this with a very simple forward bulge for mounting turn signals.

Image result for porsche 914 drag coefficient
The airflow denoted by the green line may help keep air out of the cockpit when you open the windows. It would be nice to induce that on a Miata.

Most of us race with open windows, and keeping air out of the cockpit will reduce both drag and lift. For top-up-windows-down driving, it should also mean less buffeting inside the car. I may have to build some of theses bulge-thingies and yarn test it, this could be a great way to reduce the amount of air going into the open window.

Streamlines show many interesting details, including the downwash along the door.

Roof spoiler

I’ve always wondered if the Miata roof spoilers were anything more than cosmetic.

Image result for miata roof spoiler
Cosmetic or performance?

If you look at configuration #24, they added a small roof extension to the 914. It reduced lift but increased drag, both by a tiny bit. Performance-wise, it was a wash. Maybe I spent too many years in California, but I like the way they look, and I certainly wouldn’t fault anyone for using one.

Miscellaneous drag reduction

In the CFD study there were a number of small things that reduced drag. If you did all of them, it could prove beneficial for endurance racing.

  • Removing the 914’s mirrors (configuration #19) reduced drag quite a bit. If I use the RSR calculator to calculate the power used at 50 mph, then the mirrors contribute about .035 to drag, which is a lot. Note that 914 mirrors don’t have the rounded aerodynamic shape that Miata’s have, and so it’s not apples to apples. (For more on mirror aerodynamics, you have to check out this lesson in Flow Illustrator.) A more accurate figure is probably half, or around .018.
  • Covering the rear wheels with flush covers (configuration #35) reduced drag by .02. If you use the HP Wizard tool, you get a similar value (.022). This is something I’ve been meaning to do to my brother’s Yaris, because the rear wheels don’t do much except keep the trunk off the ground. We can run skinny wheels with less offset, and get them flush underneath a side cover. However, I don’t know if it’s as practical to do rear wheel covers on a Miata, since the wheels are wider than the bodywork. Rear wheel covers that aren’t flush with the body would be configuration #17, and you can see that did nothing for drag reduction.
  • Slotting the pillar (#29) reduced drag by .043, which is a very significant amount. This directs air into the vacant area behind the rear window. We don’t see this much on street cars, I guess because they are ugly. But slots like this or guide vanes could be useful when you have an abrupt back end and turbulent wake. On a Miata, I’d figure out how to do this with air rushing past the window, and duct that internally to fill in the wake behind the hardtop, or behind the bumper.

I Don’t Want a 1.8 Swap

Just say no to 1.8 swaps

The other day I was chatting with a guy about his bitchin Miata with ITBs. He put them on his BP engine and got a lot of power and cool sounds. Forced induction would probably have been cheaper, but I totally understand wanting NA power and better throttle response. I also understand doing things just to be different, and thought I found someone who was a kindred spirit. So I asked him about putting ITBs on a 1.6, and what kind of power did he think I’d get from that? He said the 1.6 head doesn’t flow enough, and I should be swapping in a 1.8 anyway.

I’m so fucking sick of that. Everyone says don’t tune a 1.6, swap in a 1.8. But I don’t want a 1.8! I already have a 1.8 in my race car, and I don’t want one in my dual-duty street car. It won’t make sense to most people, but I’d rather have a 1.6 making 125 hp than a 1.8 making 140 or whatever. Here’s why.

NASA TT6

949 Racing built a Miata for NASA ST/TT race series. The NASA classing is based on power-to-weight ratio, and Miatas typically fall into the TT6/ST6 class, which is 18-20 lbs/hp depending on how you configure it. 949 chose the NB body because the rules give a slight bump in power for using base-trim model bodywork (BTM), and the NB has better aero, than a NA. This calculates out to a car that weighs around 2460 lbs and puts out about 138 hp. These are easy numbers to get from a BP 1.8 Miata engine with tuning. However, they swapped a 1.6 engine into the NB chassis. Huh?

Emilio says “We don’t tune 1.6s, we swap in 1.8s,” but there he went and made himself a liar. In this case, it made a lot of sense. The 1.6 engine is lighter and shorter, and this gives the car a better front/rear weight balance. In theory it should have worked better, but they later abandoned the 1.6 engine project because it was going to be too expensive to hit the horsepower number. But theoretically, a 1.6 should be better than a 1.8 if making the same power.

And so I’ve been thinking about competing in NASA TT6 as well, but I’m not going to build an engine that has 138 hp. First, that’s an expensive build, but more importantly, I want to use aero, and that means the car isn’t allowed the same power to weight ratio. If I use all the aero allowed (airdam, roof, wing), then the engine can make a maximum of 125 hp. I’m guessing my 1.6 has 110-112 as it sits, and so I can probably hit 125 without breaking the bank.

Also, I have a low milage engine. My dad bought the car when he was 69 and never drove it hard, and so the engine has had a low-stress life. Tuning this engine makes more sense to me than swapping in something of unknown origin.

Taking scalps

One of the joys of driving a momentum car is embarrassing more powerful, expensive cars. Hunting down BMWs, Corvettes, Mustangs, and Porches, and passing them (or harassing them into giving you a point-by), is great fun. For lack of a better phrase, I’ll call this “taking scalps”.

You don’t earn a scalp for passing slower cars, only faster ones. But when you’re in a Miata, pretty much every other car on track is a scalp for the taking. If you’re driving a 1.6 Miata, then 1.8 and later Miatas are scalps, as well!

Accentuating the 1.6-ness

I want to build the ultimate expression of a NA 1.6 Miata. The early 1.6s differ slightly from other Miatas in several ways, and my goal is to exaggerate all of these differences.

  • Higher redline – The 1.6 revved to 7200 rpm, which was later reduced to 7000 in the 1.8. I have a Megasquirt PNP2 and programmed my redline to 7500 rpm. I won’t go higher than that without modifications.
  • Higher compression ratio – The 1.6 had a 9.3:1 ratio, compared to the 1.8 NAs which were 9.0:1. I got a spare head and decked it, which should bring the compression up to around 10.3:1, which is close to what the later 1.8 BP engines have.
  • Lighter clutch and flywheel – The 1.6 has a smaller clutch and flywheel, which means less rotating mass and a revvier engine (at least without load). I bought the lightest aftermarket clutch and flywheel I could find.
  • Shorter final drive – The NA6 4.3:1 diff ratio is shorter than the 4.1 ratio used in the later 5-speed 1.8s. My original plan was to exaggerate this difference further by using a ring and pinion from a 4.78:1, but I’m fairly certain this will annoy the shit out of me on the street. For my county club track, a 4.1 makes a bit more sense (fewer shifts), so I’m on the fence for which way to go here.
  • Hotter cams – The 1.6 has a slightly more aggressive cam profile than the 1.8s. I went one louder and bought Kelford 264 B-grinds with 9mm lift. It will be interesting to see how much power these add.
  • Smaller brakes – The early cars came with smaller disc brakes. On a street-oriented car that does the occasional time trial, I don’t see the need for larger brakes (and more rotating mass), so I’m leaving them stock. I’ll duct the rotors, but that’s it.
  • Smaller wheels – The NA6 always came with 14” hoops, and it wasn’t until 1995 that 15” was an option. If I’m being serious about performance, then 15s are a better choice. They are available in wider widths and have better rubber selection. But if this is an exercise in nostalgia and 1.6-ness, then 14×7 RPF1s are obviously that way to go.
  • Less power – Do you ever floor the accelerator, and then pat the center console or the steering wheel, urging your Miata to giddy-up? The 1.6s have less power and torque, and it leaves you always wanting more. Somehow that’s part of the charm. A naturally aspirated 1.6 is never going to overwhelm anyone with power, so this is a goal that’s impossible not to achieve.
  • Less weight – Miatas got heavier through the years, and to keep with the theme I’ll reduce weight where I can. There’s no secret recipe, just concentrate on the ounces, and the pounds will take care of themselves.
  • Simpler – The earliest Miatas were the simplest, with fewer electronics and conveniences. Mine has a manual steering rack, manual mirrors and windows, no ABS, and came with only a single airbag (now gone because I replaced the steering wheel). Anything I do to further modify this car will stay in the same theme of keeping things simple, pure, and analog.

People say that the later Miatas are better cars, but the earlier ones are better Miatas. When you add up all the differences between the 1.6 and 1.8, you get a car that has a slightly different character. Those differences are what I’m accentuating. For sure I could make a better car, but I’m building a better Miata.

And that’s why I’m not swapping in a BP 1.8, using forced induction, or whatever else is the cost effective and logical way to get more power. Yeah, I’m tuning a normally-aspirated 1.6, get over it. And all of y’all are scalps to me!

Occam’s Racer is Looking for a Teammate

The Occam’s Racer team could use another teammate or two. We currently have a rotating group of drivers, but most of them are essentially arrive-and-drives that live far away, or are too busy doing other things to be a regular part of the team.

Photo by TJ Keller

I need help working on the car, loading and unloading for races, and all the other things required to make a race weekend happen. Currently I do 95% of this stuff myself, and it’s a lot of work. Both of my knees are held together with metal plates, and the pain of lifting heavy objects regularly reminds me that I can’t keep doing this on my own. Often times I think to myself that I should quit racing, buy a new ND, and just do laps at the country club.

But for the time being, I’m still racing, and I need help. In exchange for your labor, you get the team rate for driving, which is a straight division of all consumables.

The ideal teammate lives near Ithaca NY, knows how to work on Miatas, has some Miata spares, and has lots of free time. But that’s being really picky, and I’d settle for someone who has mechanical aptitude and free weekends.

You should have some wheel-to-wheel racing experience. AER requires that you’ve raced in five endurance races, and I intend to keep racing with AER. But if you don’t have that much experience, I may also race in Lucky Dog Canada, Lemons, and Champcar, so there’s some chance to build up to the AER level.

My racing philosophy is primarily to have fun, and that means we get along with other teams in the paddock, and that starts by being courteous on track. I’m more impressed by drivers who have situational awareness and good decision making than outright speed. You won’t be faster than Evan, anyway.

Use the Contact form and I’ll get back to you. It’s a long winter, but there’s a lot to do.

Lemons Aero Study: Star Wars Ecotec Miata

A guy recently contacted me on Facebook to ask my opinion on aero modifications for his Miata. Cameron and his team live in the Great Lakes area and race Lemons. Their Miata has a Star Wars theme, complete with R2D2 behind the seat, with a head that spins, too. The car has an Ecotec swap. Cool build, right?

Star Wars Miata. Red Nine. Get it?

My first response was to get an airdam and splitter, but the team had already done that. They ripped it off shoveling mud, so it’s not in the pic, but the team intends to put it back on. He was wondering about also adding a wing, and how much would that help?

My reply was that a wing would certainly help. From my aero tests at Watkins Glen, I know that a car with a wing will beat a car without a wing in just about any simulation I run. However, a wing with an open top or chop top is about 2.5x less effective than one with a hard top.

Cameron wanted to know what this would do for lap times at Gingerman, his home track. Luckily that track is in OptimumLap, and Cameron provided me with some data based on a recent race, and that their theoretical best lap was 1:46.65. What would happen if they added aero?

I have Miata models in OptimumLap, but nothing with an Ecotec, so I used Cameron’s dyno sheet to make a new model.

In case you wondered what an Ecotec Miata puts out, it’s a shit ton of torque.

Next I input weight of the car, final drive ratio, and set the tire grip value to 1.15g. This is a bit higher than I use for 225 RS4s, but the point was to get data close to his best lap. I made some guesstimates at drag and lift. His car has a partial roof, so I’ll use my Chop Top data and fudge it slightly because he’s using a stock front end, and not an airdam. Call it .5 drag and .34 lift.

Based on all this data, the simulation shows a 1:47.10, which is reasonably close to what Cameron said was their theoretical best lap, and was in fact their actual fastest lap in the race. So let’s see what happens when I add the airdam, splitter, and wing that Cameron asked about. At this point I can use the data I have (I tested almost exactly the same setup), and Cd is .45 and Cl is -.53.

StandardAirdam, splitter, wing
1:47.101:44.72.

So that’s 2.38 seconds faster using aero. Pretty substantial considering the wing isn’t going to work very efficiently with that janky top.

My advice was to extend the roofline and support it at the rear with the wing uprights. (While still allowing R2’s head to spin.) I sent him a crude drawing that looks like this: the red is the extended roofline, the blue is the wing and uprights.

Extend the roofline and support it with the wing uprights.

The roof extension would provide clean air to the underside of the wing, which is what a wing wants. The roof mod might look silly, but this is Lemons where silly is de rigueur. With this modification he might arrive at values of something like Cd .45 and Cl 1.0. This would theoretically drop lap times by a full second.

StandardAirdam, splitter, wingExtended roof
1:47.101:44.721:43.63

OptimumLap assumes a perfect aero balance, but in the real world you can’t just keep adding rear aero and expect faster times. This is just a simulation, after all, but it’s good to know if the trouble and expense is worth it on a Lemons build like this. In this case, I’d say yeah.

Mid Ohio, Part 5: Driver Analysis

The race is over, but we’re still living the race weekend over and over. In this installment, let’s see how the drivers did compared to each other.

Fastest laps

Evan made a video of each driver’s fastest laps. Let’s take a look at Evan first, because he set the fastest laps of the weekend, and we’ll use him as a reference for where the rest of us can improve.

Evan’s fastest laps overlaid.

From his triple-video overlay, it looks like he’s pretty consistent. However, if you look at the Aim Solo data, his laps vary by .4 seconds above and below the reference lap. On the back straight, his top speed varies by 2.2 mph.

The bottom of the graph shows the time gained or lost compared to the reference lap, which is black in this case. On the lap denoted by the blue line, you can see he had .4 seconds in hand at around 2200′ again at 6000′ and .2 seconds at 9800′. As good as Evan was driving, it looks like he could have gone one second faster.

Team fastest laps

Fastest laps from each of us.

Evan also made a neat overlay with everyone’s fastest laps. Here are some things I see in the video.

  • T1 – Everyone except me initiates the turn sharply, my hands are slower at first, and I must continue to steer through the corner. I’m also a chicken shit in fast corners, and I lose most of my time here.
  • Keyhole – We are all very similar through this turn. If you watch other laps in our full race videos, we often take different lines, sometimes diamonding it, sometimes driving a rim shot. Oddly, it doesn’t seem to make a difference in lap time or trap speed.
  • T4 – Pat and Sonny are equal through here, but Evan trails the brakes the best and this is where he makes up most of his time. From here on, he is GONE.
  • T9 – Pat and Sonny are still dead even on the approach to the corner, but by the time they pass under the bridge, Sonny has a tenth.
  • Carousel – Sonny takes a wider entry line that seems to work really well.

Team race laps

I don’t have data on Sonny’s best qualifying lap, which he set on Friday. So let’s take a look at the best race laps over the weekend. These are not the same laps as those in the video. The chart below is color-coded thusly:

  • Evan is Green (think money) – 1:44.637
  • Sonny is Orange (think 949) – 1:45.273
  • Pat is blue (like his F150) – 145.355
  • Mario is red (like his NA6) – 1:47.579
Best laps from Sunday’s race.

I’m no expert on analyzing data traces, but this is what I see:

  • Everyone goes through T1 (1000′ mark) pretty well, except me. I decelerate too much, and this severely impacts my speed down the following straight.
  • The chicane begins at the 2000′ mark, and Evan (green) is faster and later on the brakes than everyone, and gains time here.
  • The Keyhole is at the 3000′ mark, and Pat (blue) has a higher minimum speed, but can’t get on the gas as early at the exit. In the end, we all come out of the Keyhole the same. Notice the time bar on the bottom is flat from 3000-6000′, meaning we are all going about the same speed.
  • Sonny (orange) does something interesting in T4. He doesn’t gain time, but it might be a move that’s useful in traffic. Maybe he had traffic? In any case, it didn’t affect him.
  • Evan ekes out a little bit on everyone in the esses.
  • Turn 8 is at about the 8000′ mark, and it’s not really a corner, you just pin it through there. I may have experienced traffic at this point, because there’s no reason for me to slow down, and I did.
  • Turn 9 is about about 8500 feet, and this is where Sonny is better than Pat. On every other corner they are pretty evenly matched. Meanwhile, Evan flat sails through this corner and his resulting top speed at the top of the hill is where the hash marks are. You can see the speeds in the upper left hand corner. Evan’s going 90 mph, which is 3-6 mph faster than the rest.
  • Turn 10 is at about 10000′ feet, and everyone else goes through it really well. Except me, I park it.
  • The Carrousel is like the Keyhole, we might take different lines, but everyone gets through it at about the same speed.

Lies, damn lies, and statistics

It was illuminating to look at our fastest laps, but an endurance race is won on consistency, not hero laps. Let’s crunch some numbers. Better yet, let’s get someone else to crunch the numbers. Evan’s friend did some statistical analysis of our race laps, and it’s so nerdy I have to share it.

In the following box charts, the lower the box, the faster the lap, the shorter the box, the more consistent the driver. The horizontal line in each box is the median lap time. The mean (average) lap time is called out in the inset, and the horizontal line connecting the boxes is a way to visually compare that. Very slow laps (out laps, in laps) were removed from the data set.

Let’s start with Saturday. Evan is capable of low lap times nobody else can match. Sonny is a wee bit faster than Pat. All three of them can lap through traffic without dropping into the 1:49s, which is where most of my laps are. Our standard deviations are similar, meaning we were all driving with about the same consistency, but Pat was the most consistent on Saturday.

On Sunday the track was a bit slower, but we all drove better. Evan’s average lap is about the same as the day before, but look how much more consistently he’s driving. Sonny had the fastest average lap time, and he improved his consistency a bit as well. Note that we should disregard Pat’s laps, because he only did six, and hadn’t found his groove in that stint yet; he’s over a second off his normal pace.

When I look at the Sunday data, I feel a lot better about my driving. Pat has raced at Mid-Ohio a couple times before, and this was my first time. On average, I’m only 6/10ths off Pat’s faster Saturday times. I can live with that.

Also on the plus side, my standard deviation says I’m the most consistent driver, by what looks like a decent margin. I think this is because I drive with more in reserve. Some examples of this from the weekend are that I avoid contact with other cars (Sonny), I don’t put four wheels in the grass (Pat), and I don’t hit tire walls (Evan). But let me not congratulate myself too much. As we saw in the video and Aim data, I’m driving tentatively in the fast sections.

The next chart is basically the same as before, but sorted by driver, rather than day. This chart is a good way to look at driver improvement. (Again, we should disregard Pat’s Sunday stint.)

I’m told that the ANOVA p-value is a test of the difference in the mean lap time for each driver. If the p-value is less than 0.05, then there’s a statistical difference. What all this means is that I drove significantly better each day, and they didn’t. Another way to look at it is that they were all on pace rather quickly, while I took longer to learn the track.

The following histograms give another way to view the laps. Each bar represents half a second. Evan (blue) is other-worldly. Pat (yellow) and Sonny (green) both have a rhythm centered around 107-seconds. My Saturday is poop, but I like what I’m seeing on Sunday.

On Sunday I did a lot of 108-second laps, and all of my laps are within a narrow 3.5-second window. If you compare my Saturday to Sunday (red), it’s like two different drivers were in the car: Doctor Jekyll and Mister Consistency.

Racing incidents matter

Endurance racing isn’t just setting lap times, it’s staying out of trouble. Our average pit stop was 3 minutes and 26 seconds, but two racing incidents made them longer.

After Sonny and the BMW clashed, we lost two minutes in the pits. Most of those two minutes were because he caught us unprepared, but that’s what happens when you take an unplanned stop. We also spent some time looking over the car for damage. Those two minutes are the equivalent of adding 2.5 seconds per lap in his stint. On average lap time, this made him the slowest driver of the day on Saturday. As a side note, the impact bent the right rear wheel. That’s the wheel hub that would later break and end our weekend. We’ll never know if the impact damaged the hub and caused it to fail, but it can’t have helped.

When Evan hit the tire wall at pit entrance, it took us an extra minute and twenty seconds pulling the bodywork straight and checking over the car. If we add that time to Evan’s stint, each lap was 1.5 seconds longer. That single mistake at the very end of his stint made Evan the slowest average driver on Sunday (I’m not counting Pat’s handful of laps). Yes, this made Evan even slower than me! Racing incidents matter.

In conclusion, our two fastest drivers were also our two slowest drivers. The obvious take away here is that one should avoid contact at all costs.

Driver weight [updated 11/6/2019]

When I originally wrote this section, I compared data traces of Evan and I, and showed that 65 lbs was very significant for acceleration and resulting lap times. However, after going through the data some more, I think this was probably time of day. Evan’s first run on Sunday was cold, and the engine would have made more power. There might have also been a headwind. Whatever the case, there’s just no way 65 lbs amounted to .4 seconds on the back straight.

I then went and looked at many other runs, comparing the same driver on a full tank and then an empty tank (same stint), which is a difference of about 100 lbs. Even with that larger weight amount, it was hard to find any correlation between weight and speed on the back straight. Go figure.

OptimumLap simulation

Another way we can look at the difference weight makes is to do a simulation in OptimumLap. On the pro course (no chicane), the simulator says my car should do a 1:41.57, and I’ll add two seconds to that for the chicane, so call it 1:43.57. (Incidentally, this is the same lap time Evan would do if he dropped the one second we saw in the Aim data.) So then, what happens when I add 65 lbs?

WeightLap
125 lb driver1:43.57
190 lb driver1:43.93
– difference0.36

According to the simulator, a difference of 65 lbs is worth .36 seconds. I typically trust OptimumLap, but in back-to-back tests at Pineview Run, I’ve noticed that the time difference is in OptimumLap is about half of what I’ve logged in the real world, and I would expect about .7 seconds.

So let’s split the difference and say 65 lbs is worth .65 seconds. That makes every 100 lbs = 1 second, which is a rule of thumb I’ve heard before. It may not be accurate, but it’s easy to remember.

6/10ths of a second would be a big deal at the pro level, but not at my level. I have a lot of things to work on before I start using weight as an excuse. But Pat could argue that he’d be faster than Evan and Sonny if they weighed the same. Start your excuses Pat!

Mid Ohio, Part 4: Race 2

I was up before sunrise, waiting in the parking lot for the Summit guys to deliver my parts. It was dark when they drove in, and I was the only one in the parking lot. I greeted them with a creepy “Where’s my drugs, man?” and they chuckled and handed over the parts. I got back to the RV and woke everyone up, telling them we need to get the brakes done, and why is the radiator water all over the floor?

We checked everything in the cooling system, swapped an overflow bottle, blew out all the water we could find, etc. We started the car and pressure-tested the system. But we just couldn’t figure out how all the radiator water dumped out. We vowed to keep an eye on the temperature and check the water level in the pits each time, and just trust it.

The brakes were another issue, Summit gave us the wrong parts. I later went back to them to figure out how this happened, and apparently the Hawk product info says that these brakes work on a 1994-2005 Miata. I politely explained that this is not the case, and that 94-97 are different than 2003-2005, at least in the USA.

I’ll get my money back, but the point is we didn’t have fresh brake pads, and must use the 3.5 hour old StopTech Sports that nobody is really trusting to go the distance. For braking power and feel, both Pat and Evan liked them more than the Porterfield R4E. More StopTech converts? In any case, Sonny gaves us new team orders: coast slightly before braking zones and use less brake throughout the race. As if.

Race start

Evan. I don’t think I’ve ever seen him awake this early.

Evan has never started a race, and so we gave him his wish. He started in 32nd position. The Audi R8 in first was going slow and bunched everyone up, which made for a tight group as the green flag dropped. There was a brief heart-attack moment as one of the cars had a mechanical, but everyone avoided the incident well. See for yourself in the clip.

A good tight start… WATCH OUT!

We can set fast laps, but not in traffic, and on the 5th lap, we’d lost four places. But as the cars got spaced out, Evan got his head down and after 54 blazing laps, we were in 23rd position!

Evan passing a GT4 Clubsport ($165,000, 385 hp) around the outside.

Evan is our fastest driver, but he’s also our youngest driver, and young people do stupid shit. For some reason Evan thought a good place to make up time was on the pit exit (WTF), and so he came in too hot. He had to make a split second decision between running over the RFID gate or hitting the tire wall. He chose the wall.

Evan stuffs it into the wall.

We lost some time in the pits pulling the bodywork straight and checking the car for damage. Initially we thought there might be radiator damage because of all the water on the car, but a quick car wash is one of the benefits of hitting tires. I learned that earlier this year in a Lemons race when a teammate did the same thing (but while racing, not while pitting, for fuck’s sake).

Sonny went next and started in P30. I went through his footage and found a cool clip. In the video below, Sonny points by a faster car quite early in the Carousel, which shows what an aware and courteous driver he is. Keep watching the full lap for a surprise ending for the faster car.

Don’t let me hold you up.

Sonny banged out 50 fast laps, safe and consistent, and brought us up to 22nd position overall. This put us nine places ahead of where we were yesterday at the same time, and things were looking really good when I got in the car.

A few cars passed us in the pits, and so I was out in P27, but was feeling my oats and got us up to P21. I had some good racing in this stint, real wheel-to-wheel action that was fun and intense. I’m still replaying some of those in my head.

I had planned to run the car out of gas in my stint, but I thought I saw Pat with his helmet on, indicating that they were ready for me to come in (my radio wasn’t connected). But when I pitted, Pat didn’t have his helmet on, Sonny wasn’t prepared, and Evan was nowhere to be seen. Must have been some other team that was ready! Still we managed a decent pit stop of 3:22, and when Pat got in, we’d only lost two positions.

Six laps later, all hell broke loose. To be more specific, the right rear wheel hub broke loose.

Something broke, I’m in the sand trap….

Pat was going 83 mph through T1 when the hub flange broke. This took out the brake disc, the entire wheel, and spun Pat around 720 degrees. Luckily there’s a big sand trap there and he slid safely to a halt. That could have been a lot worse, and I really only care that Pat is safe and unhurt. Damage can be fixed, and I’ll replace the rear hubs every year now.

Post-race thoughts

Some thoughts in no particular order.

Miata hubs. Miathubs!

Everyone knows Miatas have weak hubs, and most of the time it’s the front hubs that fail. Evan examined my hubs before the race and declared they were crap. So I ordered a pair of Miatahubs at the very last minute. Justin Lee was also racing at Mid-Ohio (in the Finish First Racing Scion FRS, Class 2), and so he hand-delivered the new Miatahubs to us, and helped us install them on Friday morning.

Miatahubs aren’t cheap, but I was feeling pretty pleased with myself now that the car has a bulletproof solution for the front hubs, and they should last forever. Of course it was the rear hub that broke.

Aero vs power

Nobody had less power than we did, but we routinely out-handled and out-braked most cars in the field. A lot of that comes from a better aero package that give us more downforce and less drag. Case in point, here I am in a dead heat with a 1.6 Miata, the other slowest car on the track. I eventually pass him because my car has less drag, and I brake later.

Neck and neck with a 1.6 Miata.

The other NA/NB Miatas in the field were on RE71Rs, while we were on RS4s, and they probably had more mechanical grip than we did. But aero works better the faster you go, and that’s where we could take them.

Hankook RS4 225/45-15

The tires worked predictably throughout the weekend. At first we were on RS4s that had about 16 hours on them I’d guess? They were only about half worn, and didn’t wear appreciably the entire day. On day two we started on stickers, and we put down times that were 1/10th slower on average. The difference was down to warmer weather and has nothing to do with the tires. Anyway, Hankook RS4s are great tires, and I see no reason to use anything else for endurance racing. The fastest NA Miata team was on RE71Rs and we saw them changing tires after only 6.5 hours. No thank you.

No full-course yellows

We didn’t have a single FCY the entire weekend. Compare this to a race at Watkins Glen, where you can expect over an hour of parade laps each day. We had plenty of accidents, but the wrecking crew managed the entire event without stopping the race. They cleared the incidents quickly, and the flaggers were on their game. It was orchestrated perfectly time and again. Thank you Mid-Ohio staff.

Speed differential

I’ve heard some people complain that the speed differential in AER is extreme. The Audi R8 car was about 10 seconds faster than us, but it was easy to see coming and stay out of the way. There were other fast cars, and they did occasionally dive bomb us in to T1, T4, and T11, but you learn quickly to accept that.

So you drive your mirrors a bit more, and there’s the occasional “holy crap” moment when a car suddenly appears next to you. But most of the Class 4 and 5 cars were really well driven, respectful, and safe. The speed differential was actually a shit ton of fun. Those cars take different lines, and so you can still play with them in the tight sections, and even show them your tail from time to time.

Here’s a clip of me going though T1. I purposely don’t track all the way out so I can point by the horsepower car behind me. I stay on my line going into the chicane, he takes a different line, and we trust each other to dance the dance. It might look like a close call, but it isn’t.

Speed differential not a problem. But look at them go!

Mid Ohio, Part 3: Race 1

Sonny at the office.

The prep and testing is done. Practice and qualifying sorted. Car and team are as ready as we’re going to get. Just one final detail is our driver order.

AER requires five pit stops over the course of the day, and in a typical 9-hour day, this means each driver gets about 90 minutes of racing. The race was shortened to 8.5 hours, presumably because it’s late in the season, so that was a bit less track time for everyone. But we still needed to do six stints with four drivers, so we split it up so that Pat and I would drive twice on Saturday, and Evan and Sonny would drive twice on Sunday.

I decided that Sonny should start the race because he’s got the most experience, and track conditions were treacherously damp and cold. As we watched the race cars pull on track for the first time, I noticed frost on the stairs up to the viewing area, it was that cold. Sonny going first also meant he could be coach and crew chief for the team for the rest of the day.

Start your engines (and go backwards)

We began the race in 30th position, with all but one of the Class 3 cars ahead of us, and we slipped backwards immediately. Our momentum machine does not do well when blocked by traffic, and by the 5th lap, all the Class 3 cars were long gone in the distance, and there were nine Class 2 cars in front of us as well.

Sonny getting swallowed up on the race start.

Think about that for a minute. We qualified in Class 3, and a handful of laps later there were nine “slower” cars in front of us. That is not Sonny driving badly, that’s a lot of Class 2 teams not qualifying at their race pace. When the flag dropped, seemingly everyone could get another second out of their car. What a curious situation that is <cough>sandbag</cough>. Kidding aside, this is the problem of a momentum car, if you get blocked in one corner, it ruins the entire lap.

We wanted Sonny to stay out until the tank was dry, which would be about an hour and forty minutes, but he pitted around the 85-minute mark and found us a bit unprepared. Apparently he came together with a BMW in T6 (the right turn after Madness) and was afraid the contact might have damaged something.

Houston, we have contact.

So he rightly brought the car in right away. In fact there was damage, but we wouldn’t know the extent of it until later.

Nevertheless, Sonny did 48 fast laps in difficult conditions, and that brought us up to 29th position. But the long pit stop meant I started the race in 39th position. I lost another spot early on, but then settled into a rhythm and took some positions back. I set my fastest time of the weekend in that first stint, a 1:47.53, which matched my race-pace expectations from Friday. I pitted after 48 laps with us in P31.

Pat was next, and the driver change meant he started in P35. But he quickly went to work, and 46 laps later we were in 23rd position overall. He set his fastest time of the weekend about half way through his stint, a 1:45.373, and did some proper w2w racing. Somewhere around this time we also got reclassed to Class 2, which was appropriate. We then made the least of that opportunity by taking an unplanned stop….

Pat ran out of brakes. In fact, he could hardly get the car stopped going through the RFID gate! We fueled the car in the pits, but then immediately took the car to the garage to change brake pads.

Listen to the brakes as Pat tries to stop at the RFID gate.

If you back up that video 20 minutes, there’s some good w2w race action, including some oversteer saves probably due to the broken sway bar mount (but more on that later).

A short break for brakes

The fact that we wore out the brakes is totally my fault, because I should have put new pads on before the race. I’ve been using Porterfield R4E pads in front (Mu .46), and they have lasted for so long I thought they would last forever! I guess not. In all, I think I got 30-36 hours out of those R4Es, and I would consider buying them again, but being a lot more cognizant of pad life toward the end. On the plus side, they modulated well, and wore evenly. I certainly got my money’s worth.

I brought backup pads, StopTech Sports (Mu .40), which I’ve used many times in endurance racing. They have four redeeming qualities: cheap, last exactly one weekend, aren’t grabby (I’ve had problems with people flat-spotting my tires), and can handle a lot of heat. But StopTech Sports aren’t “race pads” in the sense that nobody takes them seriously, except for my brother and I.

My reservation with StopTechs were that I had never used them with such a fast team, and brake wear was obviously an issue. I also didn’t know where to set the brake balance vs the Porterfields, so I gambled and turned the prop valve two full turns to the front (it was full rear before this). We sent Evan back on track with the new brakes, and I went over to the Summit Racing truck to order new pads, which we’d swap out in the morning.

Summit didn’t have a lot of options for brake pads in stock: PowerStop (no), Centric (fuck no) and Hawk. Of the Hawk pads they could get me tomorrow, it was HP+, Blue, and DTC60. I ordered the DTC60. So as to not confuse them, I told them the car was a 2004 Miata, because the 2003-2005 all used the larger Sport sized calipers, which is what’s on the race car.

Back to racing

We lost 20 minutes changing brake pads and rotors, so Evan started the race back in 35th place. But Evan is our fastest driver, and he put in 56 laps that got us back to 23rd place. His 1:44.501 lap was the fastest of the weekend.

Evan with a triple pass (no audio).

Then I got back in the car for 27 laps and didn’t find my groove for a while. But I finally got settled and moved us from P25 to P23, and eventually set a time only 2/10ths off my previous stint.

Pat was our final driver, and started in P24. In the video below, his pass on a Porsche GT4 Clubsport ($165,000, 385 hp) is a good example of how to drive a momentum car, and pass cars that have three times the power.

Pat passes a Porsche GT4 Clubsport.

At the end of his stint we were P19 overall, and 5th in class. That’s pretty good considering a couple slow pit stops and a longer pause to change brakes. If we didn’t stop for brake pads we would have placed 15th overall and 2nd in class.

Overnight work

We got the car on stands, and the first thing we noticed was that the front left wheel was bent. This happened in the very first stint, when Sonny came together with a BMW. We got lucky that the tire held air the whole day, because it went flat later when on the trailer. The left rear was also pretty hashed from contact, and I’ll throw it out with the front.

The wheels are 15×9 Konig Helix, which have spokes that are proud of the wheel. Nobody should wheel-to-wheel race a car with spokes like this, and that’s on me for making a purchasing mistake. I’d bought these from Goodwin Racing on sale for $109 shipped, and I think the price outweighed my good sense. I will replace all of my wheels at this point, and move these to a HPDE car.

Another problem we found was a broken sway bar bracket. This is a weak point on a Miata, and there’s a simple cure – a piece of aluminum that braces the bar. Well, I didn’t have that and we had to go looking for another part in the pits. A generous team let us pull one off their car, and we got it fixed late that evening.

In the morning we’d get new brakes pads from Summit, put on wheels with four brand new RS4 tires, and win this thing. Or so we thought.

Mid Ohio, Part 2: Practice and Qualifying

Is this a racing team or the Comet Skateboards team?

American Endurance Racing (AER) is a top-notch racing series. This was my 24th endurance race, but only my second with AER. The memory of my first AER event at Watkins Glen is tainted by me crashing out in T6, and I had forgotten how professional this series is. For example, checkin is much easier, because your personal gear is your personal responsibility, and so they don’t verify all your stickers and labels and whatnot. If you treat people like adults, they act like adults.

Each driver and the car have RFID stickers, and with that they know exactly who is in the car at all times. On your AER profile page, that data is saved for you and the entire team. The pit stops are timed via RFID, as well. All very high tech and classy. You also get regular text messages throughout the event, telling you important details.

The cars are more expensive and faster than you’ll see in Champcar, Lemons, or Lucky Dog. Stacker rigs are not unusual, and so our open trailer stood out as an oddity. We were definitely the budget team against a lot of expensive BMWs, Porsche Caymans, and even an Audi R8.

Sahlen’s R8 sets the class.

AER events start on Friday, with an HPDE point-by session until noon. Non-race cars are allowed as long as they pass a quick tech, and so it’s not unusual to see street cars, including rental cars, on the track together with the race cars. This is a good opportunity to learn the track with a coach in the right seat. It’s not a good opportunity to learn the limits of a street car. One person in a rental car experienced stock brake fade and wadded their car. It did not look drivable afterwards.

Friday practice. Let’s get this show on the road!

We each took one run in the car. Sonny did the first run since he’s very familiar with the track (ahem, TT6 track record), and we wanted feedback on how the car handled. He said it was pretty good right away, and that our car should be capable of a 1:45, maybe a 1:44. He said this would be a decent NASA ST6 time. And when you consider we’re using the chicane, which adds a couple seconds, and we’re on RS4 tires, that’s not bad at all. In my previous blog post I wrote that OptimumLap predicted a 1:43.5, so that’s pretty close. But that would be a perfect lap, and we only just got here.

Pat has been to Mid-Ohio a couple times before, so while he knows the track, he had to re-familiarize himself with my Miata. Pat usually races a 265 whp E36 M3, and so I would imagine it takes a bit of time to get on the momentum line. Evan is a Miata specialist and knows how to get the spurs into a slow car, but had never been to the track before. Nevertheless, they both posted initial times that were similar to each other, and not too far off Sonny. Impressive.

I take longer to get adjusted to a new track, which is mostly braking too early, a slow entry speed, and other fear-based survival tactics. I did a shaky and uncertain 1:58. Thankfully we didn’t have the Aim Solo turned on, I really don’t want to see the data.

Coaching moment

We didn’t have time to look at the Aim data over lunch, but we watched video from Sonny driving the car, and that was illuminating. He explained where he was braking, turning in, shifting, etc. The big surprise to me is that it’s all 3rd and 4th gear, and I was shifting twice into 2nd. This is partly because I put in a couple hours sim time in Assetto Corsa, and was driving the stock NA car the same as my race car.

Sonny also carried much more entry speed, barely braking for T1, the chicane, T9, and T11. I chewed on this while chewing my sandwich, and prepared to go out again.

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Worst. Picture. Ever.

Practice and qualifying

The afternoon session is race cars only, in racing conditions. It’s still practice, but it’s also qualifying. AER classes you solely based on your team’s lap times, and there’s a secret threshold (maybe not so secret) for each class division.

I got in the car first, and was immediately more confident, knocking off over six seconds and did a 1:51.7s. Strewth, six seconds. That’s the difference between me trying to learn a track on my own, and literally 10 minutes of Sonny explaining the track to me. I need a coach at every race.

We each got two 20-minute practice runs each, and in my second run I did a 1:48. I had 8/10ths in hand on the next lap, and then felt the car sputter as I sipped the last off an empty tank. I nursed it back to the pits, but it was good to know I’m capable of a middle 47. With more track time throughout the weekend, that felt like a good target for a race pace, as well.

Pat’s times came down a bit and by his second session he was reacquainted with the car and solidly in the 1:45s. He came in with grass on the splitter, evidence that he was feeling comfortable and exploring the space. Maybe a bit too comfortable.

Sonny did a high 1:44 in the afternoon, which gave us an indication of how fast the car could go, and matched his forecast from the first time he sat in the car. Sonny said that he can determine the potential of a car in one lap, and often in just a couple of turns. Not an exaggeration. Fact.

And then Evan surprised everyone by going quicker, managing a 1:44.5. Holy crap this kid is fast. This is his time ever to Mid-Ohio, in his third 20-minute practice session, and he out-qualified our pro driving coach by .35 seconds. This kid had like 42 hours of racing time in his entire life before this event.

Evan Merrill. Two Rs, two Ls. Remember the name.

AER Class 3

So with both Evan and Sonny doing 1:44s, we got put in Class 3. My goal for this car was to be eventually competing in AER Class 3, and here we were! But mostly because we didn’t sandbag qualifying. We were still learning the track, and that required going as fast as we could. Most other teams held some in reserve so they could be classed lower (or classed appropriately, if you wish), and race for a win.

We were classed with these cars in AER Class 3. Seriously?

After seeing the other cars in Class 3, we went to the officials and said how did a 120 hp Miata, the least powerful car on track, get put into Class 3?Obviously it was good driving, but they said that if this turned out to be a mistake, they would re-class us half way through the race tomorrow, and put is in Class 2. Cool.

Pat adjusts tire pressure. Note the green “3” class sticker on the windshield banner. For realz.

But still, we did get AER Class 3 in qualifying, and for me, that feels like a million bucks. When I look over the cars we out-qualified, it goes like this: lots of BMWs (E36 325 S52, three E46 330s, all of the E30s, 525i), all the Miatas except one (two NC, two NA), three Porsche 944s, a Toyota 86 and a VW GTI. Fuck yeah.

Coaching moment

Friday evening we got a chance to look at the Aim data, and Sonny coached us on where we could improve. For me, the biggest problem was slowing the car too much. I always do that, but to a criminal degree in T1, T4, and T11. I also sometimes coast into or through corners, and lost a quarter second by not getting on the gas early enough in Madness.

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Late-night coaching moments require IPAs.

But it’s not like I’m a total noob. There were some corners I did right. In the keyhole and esses (T5-8), I’m as fast as anyone. To understand this, Sonny looked more into the data, and my lateral Gs are identical to the rest of the team pretty much everywhere except T1, meaning I’m cornering just as hard. My problem is I throw away too much time before the corner. This is not new news to me, it’s my particular driving style, and something I will have to correct if I want to go faster.

Aero and setup notes

I saw a lot of great aero this weekend, with most teams running splitters, side skirts, venting, etc. Wings were mostly about roof height and about zero degrees angle of attack. This is much different than a typical Champcar race where you see wing angles set too steeply, wings mounted too low, or otherwise aero done wrong. I only had to threaten to dick-punch one guy, when he said his 190+ hp 2.7 stroker e30 didn’t make enough power to use aero.

There were other Miatas without aero, and they all had motor on us. In fact, we were the slowest car in a straight line. But the extra downforce gave us better cornering and braking, and was clearly felt in the high-speed sections on the track. One Miata with aero and more power was doing 1:42s and was a legit Class 3 car. They were on RE71Rs, though.

I’m still using a 60″ 9LR wing set at about 5 degrees, without a Gurney flap (the flap would add about 15% rear downforce). The front has a typical Supermiata-style airdam, and a splitter of 4-5″ length depending on where you measure it. We didn’t adjust the aero one bit, and the car was very well balanced and easy to drive.

Setup wise, we added two turns up on the left rear shock, which is a Mid-Ohio thing. Otherwise we left the car alone. We set the 225 RS4 tires at 24 psi cold, and they came up to about 30 psi hot, and we’d bleed them down to 30 if they went higher. A pyrometer verified this as the proper setting.

Mid Ohio, Part 1: Test Day at Pineview Run

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!

Evan on bald NT01s in the rain.
Tom in the Neighborhood Trolley, or boat, as it were.

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%.

Sonny doing it at Nationals

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.

Race Cars: Drag and Lift Examples

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).

CarCdClL/DMid Ohio
Standard race Miata, no aero.45.45-1.0104.60
1990 Mazda RX7 GTO (spoiler).51-0.44.86102.76
1990 Mazda RX7 GTO (wing).48-0.531.10102.31
2002 NASCAR.39-0.461.0101.82
Miata, splitter, wing.48-1.012.10101.12
Miata, splitter fastback, wing.41-1.202.93100.11
Audi R8 race car.54-2.604.8298.12
Mazda RX-729P.70-3.805.4397.47

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.

RX7 GTO

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.

Image result for 1990 mazda gto
Early spoiler version, Cd .51, Cl -.44.

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.

Image result for 1990 mazda gto
Later version with a wing. Cd .48, Cl -.53

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?

Image result for 2002 nascar
A 7/8 scale Miata stock car would be dope.

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.