New Time Trial Rules

Last year’s Pineview Challenge Cup had a unique (and flawed) set of rules, using only tire treadwear to class cars. The “Race” class was any tire under 200 UTQG, the “Track” class was for any 200, and the “Street” class was for 300+. On the plus side, we had no rules lawyering or protests throughout the season, and it was easy to manage three classes. On the downside, the rules didn’t take into account power, weight, tire width, or the fact that there’s a great deal of variation within each tire class. In the end, the racing wasn’t always close.

We need a better set of rules for 2020, and so I took it upon myself to do some research. First, I put all of the lap times from the Pineview races into a spreadsheet, and noticed they fell into four groups. I figured that whatever rules we come up with, they should place people into four groups.

  • Under 1:14 – Just three people were in this group.
  • 1:14-1:17.2 – Fast cars and fast drivers, but only five people in this group.
  • 1:17.8-1:19.7 – Fourteen people, diverse cars and drivers, close racing.
  • 1:20.1+ Six people, some newcomers, a wider spread of times.

I then ran hundreds of simulations in OptimumLap at Pineview Run, using different cars, power/weight ratios, tire grip, and aero. I put all of them into a spreadsheet, and analyzed the data. Let’s take a look at what I did for a Miata with various power and grip values.

MiataHP  (lbs/hp)1.0g 1.1g 1.2g 1.3g 1.4g
Stock NA8 100  (24.5)82.3479.1376.3773.8871.65
Spec Miata125  (19.6)81.2577.9875.1472.6470.46
K-swap200 (12.5)79.6076.1473.2070.6468.42
Turbo, Rotrex265 (9.2)79.3975.6972.5169.7767.42

You can see that tire grip is the most important factor. For example, a Spec Miata on 1.3g tires will beat a K-swapped Miata on 1.2g tires. The lbs/hp ratio is also very important, especially as you add more grip. But on shitty 1.0g tires, power gives diminishing returns, and cars with better than a 12:1 ratio are pretty evenly matched.

Existing time trial rules

With this data on hand, I then reviewed the rules of nine time trial series to see if we could leverage them, and learn how others are doing it. Here’s a quick synopsis.

  • NASA PT/TT – The old NASA rules listed and ranked every car and trim level, and everything you might do to it. It seemed like a fair system, especially how they ranked tire compounds and width. But they abandoned it for a reason.
  • NASA ST/TT – The newer NASA TT rules are much easier, and seem very fair, based mostly on lbs/hp. My main gripes are 1) everyone needs to dyno their car on a Dynjojet. 2) Many common modifications are not allowed. Why not just make them a point value instead of making them illegal? 3) The new tire system favors race tires, not tire choice.
  • SCCA Autocross – 36 classes and a 387-page rule book?! I didn’t wade into this tome; this is the exact opposite of what I want to do.
  • SCCA Time Trials – I’ve got some issues with how cars are classed. In the “Sport” category, all Miatas from 1990-2015 are in the same class. If you know Miatas, you know how stupid that is. If you’re going to do that, you may as well put the BRZ/FRS in the same category as a 1990 Miata. Which they fucking did. I should have stopped there, but kept reading and found that in the “Tuner” category, they put E30s in with S2000s and ND2 Miatas. Ha ha ha ha. Anyway, there are five main categories with subdivisions that make 20 classes. The stock-ish categories are based on a car list, while in the modified categories, cars are classed by displacement divided by weight (as if displacement is a good indicator of power). These are new rules and maybe they need a few seasons to refine them, but the rules look unbalanced and unfinished.
  • EMRA – Kudos to them for making a 12-page rulebook! As such, it’s sparse and open to interpretation. There are ten classes based on a car list. You bump up a category based on modifications, but there’s not a lot of balance to the mods. The only thing they say about tires is that they must be DOT. There’s a world of difference in there, I guess everyone shows up on A7s?
  • Porsche – PCA has a great system of ranking their cars and various points for modifications. It looks very fair and complete. But they have only one brand of car, and they split them into different 28 classes. BTW, if you ever wondered about the comparative performance of different Porsches, look at the point values, it’s great data.
  • Speed SF – The classing system is calculated from a relatively simple worksheet that puts you into one of six classes based on a list of cars and modifications. The classing rules are simple, and because of that, there are going to be inequalities within the classes. Tires are easy: everyone is assumed to be on 100 TW or higher, and you take a slight penalty for using race tires. I like this system for a lot of reasons.
  • COM – Corvette Owners of Massachusetts have the best rules I’ve seen. It’s a bit like the old NASA system, and takes into account tire compound, width, and really anything you want to do to your car. This puts you into one of ten classes. The 98-page rulebook is a tad long, but they have a great spreadsheet calculator that makes classing easy.

After all that research, I’ve come to the following conclusions.

  • Creating a fair classing system is difficult, and nobody does it the same.
    • One starting point is to list and rank every car .
    • Another starting point is pounds per horsepower.
  • Tires are a very important factor.
    • Many rules evaluate each tire individually.
    • Tire width is a factor, and often relates to weight.
  • It would be easy to use another club’s rules, but impossible to manage them. Our time trials are run over 2 hours, and we simply can’t have more than 4 classes.
  • I poke fun at some of the rules above, but I applaud any effort to make racing fair and fun. Every system is full of good ideas, this is just a really difficult problem to solve.

Creating new classing rules

So with all of that knowledge, I went about creating new rules. My guiding principles were:

  1. I need to end up with only four main classes. Ideally these should correlate roughly to the lap time groupings observed in the 2019 season.
  2. The classing rules must be fair, and allow people to run what they brung. It should be easy for people racing in other series to cross over.
  3. The rule book should be short and simple.

Pounds per horsepower

I started with pounds per horsepower, because that’s easier than ranking every car in existence. For people who dyno and weigh their car, getting lbs/hp is easy, but most people won’t do that, so the rules allows using factory values.

lbs/hp = (Factory weight + driver weight) / (Factory HP * .83).

Of course people modify their cars for less weight and more power. So they must declare, on their honor, what their modified weight and power are. This is obviously a place someone can cheat. However, I would make the class worksheets public, so anyone obviously cheating would be publicly shamed, a-la Game of Thrones.

For people racing in the NASA TT series, which is based on lbs/hp, they can use their class minimums. Meaning TT6 is calculated at 18:1, TT5 at 14:1, TT3 12:1, and the faster classes at a 10:1 cap.

For other racing series that don’t use lbs/hp, I’m experimenting with some crossover rules. These would let people bring their race car on their spec tire, and fit into a class. But more on that some other time.

Tire grip factor

I used the tire point values from the old NASA series and COM, and then cross listed with recent tire tests, forums, and various web pages to come up with a hierarchical list of tires sorted by grip value.

PtsUTQGTypeExamples
2400+All seasonAny all-season 400+
3300-390Summer -olderYok S.Drive, DZ102, G-force Comp2
4240-380Summer – betterContinental ECS, Michelin PS4S, Bridgestone Potenza S001, Champiro SX2 (260), Toyo T1R, any 240+ not listed
5180-200200 EnduroAccelera 651, Champiro SX2 (200), Dunlop Z2/Z3, Falken Azenis 615K+, Federal RSR, Hankook RS4, Maxxis VR1, Nankang NS2R, Nitto NT05, Toyo R1R, Yokohama AD08R
6120-200Auto crossBridgestone RE71R, BFG Rival 1.5 S, Federal RSRR, Michelin PS Cup 2, Nexen Sur 4G, Yokohama A052, any 120-200 not listed
6100R-CompNT01, RA1, R888R, RC1
740-80R-comp softNankang AR1, Toyo RR. Pirelli Trofeo R, any 60-100 TW not listed
840RaceBFG R1, Goodyear Eagle RS, Hankook Z214 C51/C71, Hoosier R7, SM7, Kumho V710
1040*Race softBFG R1S, Goodyear Eagle RS AC, Hankook  Z214 C91, Hoosier A7, any 40 TW or less, not listed

I then assigned a point value to each tire, so that I could try out different formulas that would make lbs/hp and tire grip meaningful. I tried a few different ways to balance lbs/hp with different tire grip values, and make that correspond to real and simulated lap times, and simultaneously group people into four classes.

In the end, classing is based on a simple formula: FLOOR(Lbs/Hp/Tire). Or in plain english, the weight of the car with driver, divided by the wheel horsepower, divided by tire points, rounded down to a whole number. This usually results in a number 1 through 4. That’s your class.

Low-powered cars on hard tires may evaluate to more than 4, but they still must run in Class 4. We aren’t making a 5th class, there’s just no way to run that many groups in a two-hour session. For the same reason, there’s no Class Zero. So that makes Class 1 basically a no-limits class. Whatever car, on whatever tire, bring it.

Examples

To see how this works, let’s take a look at a Honda S2000. Weighs 2850 lbs with driver, dynoed at 200 hp = 14.3 lbs/hp. Different tires would put this car in a different class.

  • R7: 14.3 / 8 = 1.83, round down. Class 1
  • RE71R: 14.3 / 6 = 2.38, round down. Class 2 
  • ECS: 14.3 / 4 = 3.75, round down. Class 3
  • S.Drive: 14.3 / 3 = 4.76, round down. Class 4

Let’s take a look at a Spec Miata, which would be at the 20:1 lbs/hp cap.

  • Hoosier A7: 20/10 = 2.0. Class 2
  • Toyo RA1: 20/6 = 3.33, round down. Class 3
  • Dunlop Z3, 20/5 = 4.0. Class 4

Or using a car crossing over from NASA TT4, which has a 12 lbs/hp minimum.

  • Hoosier A7: 12/10 = 1.2, round down Class 1.
  • Toyo RA1: 12/6 = 2, Class 2.
  • Michelin PS4S: 12/4 = 3, Class 3.
  • BFG SC2: 12/3 = 4, Class 4.

So that’s how it works, pretty simple. But is it fair? I ran a bunch of simulations using four different cars that I have data for, and the lap times come out pretty close to how I had them grouped.

One thing I found out was that this formula works best for cars in the middle range. Cars with high lbs/hp ratios, like Miatas were going too fast on sticky tires, and so I put a cap on all cars at 20 lbs hp. If your car is slower than that, you still have to use that value. Likewise, high-power cars at the other end of the spectrum were treated unfairly by the classing formula, and so I put a 10 lbs/hp cap on that. So if your car is 6 lbs/hp, it’s classed at 10. With these mins and maxes set, the simulated laps fall very close to reality.

Caveats and final thoughts

The rules are based on real laps and simulated laps at Pineview Run; I don’t know if this system would be fair at any other track.

I was trying to place people into four groups, and the rules work out for that. If I doubled the number of classes, there would be more equality between cars in the same run group. But it isn’t logistically possible to manage that many classes in a 2-hour evening session.

Here are some final thoughts, in no particular order:

  • This is the first year we’ll try these rules, and we might need a mid-season addendum. This should be pretty easy by changing tire point values, or adding a tire width factor.
  • To keep things simple, I didn’t take tire width into account. But an easy calculation is to take the average tire width, and divide by 245, and multiply that by tire points. This means that every tire under 245 width would get some advantage, and a width over 245 would get some penalty. As an example, a 195 RE71R would be evaluated the same as a 225 RS4. The average tire width could be anything, I’m speculating on 245 because it seems right.
  • I’m not stuck on tire point values, or integers. For example, we might find that Z3s and R1Rs should be evaluated at 5.5 points instead of 5. Or that RC1s are really a 7-point tire, not 6. We need real-world data before making any adjustments, tho.
  • I didn’t take into account modifications to suspension, brakes, aero, or really anything else you might do to a car. All of these are important, but not as significant as lbs/hp and tire grip. Not at Pineview, anyway.

We are still working on the final rule set, which includes things like points per race, and how the season points add up to who gets to put their name on the Pineview Challenge Cup. But I thought people would like to see the classing system ahead of time. The first race isn’t until Saturday May 2nd, so there’s plenty of time to break out your calculator and figure out the optimal tire for the class you want to run in. Or if you’re doing it the other way around, figure out how many pounds to remove or horsepower to add, so you can use your favorite tire.

2020 Aero Tests and Projects

I’ve been thinking about what to build and test in 2020. It’s a long winter here in upstate NY, and I’ll have some time on my hands. I just got my barn floor poured with concrete, and now I have 950 square feet to let my imagination run. Here’s a list of what’s on my mind. Leave a comment if there’s something in particular you’re interested in.

  • Underbody aero – Is it worth doing on a Miata?
    • Flat bottom – I’m very curious about this one, not sure if I’ll do this on my race car, street car, or both. I installed a canon subframe brace in my 1.6 specifically so I could hang the flat bottom off it. So… it’s fairly likely this will happen at some point.
    • Side skirts – The race car lost one side skirt spinning out at Mid Ohio. I’ll make new side skirts, which will be wider, and fit next to the pinch welds, reinforcing the jacking points along the entire car.
    • Diffuser – I made a diffuser out of aluminum street signs (I must have been in a Lemons frame of mind), but I haven’t mounted it yet.
  • Misc aero projects – Some projects are partially done, some are just drawings and ideas.
    • DIY fastback kit – I’ve been thinking about building a fastback kit for a Miata. This would be for race cars only. It would be a bunch of parts that can ship flat, and then you assemble it at home, fiberglass the seams, and paint it. My main problem with creating a product is I don’t want to support pain-in-the-ass customers.
    • 3D wing visualizer – This is a series of pivoting airfoils that I can mount on any car, just like a wing. It shows the shape and angle of air where a wing would be mounted. This would be great for setting wing height and angle, and also figuring out what’s the ideal shape of a 3D wing for just about any car. For science!
    • eBay wing 2.0 – Continue modifications to my cheap eBay wing to reduce drag and increase downforce. I’ve already started on this project because it’s a total fucking waste of time and that’s how I prioritize things.
    • End-plate mounted 9LR wing – This would require building new wing stanchions that mount to the rear quarter panels and come up vertically. Think Ferrari F40.
    • Shooting brake – I didn’t finish the shooting brake top I started earlier this year. Theoretically, it could have less drag than my fastback, and better visibility. And it’s unique looking.
    • NASA TTop – The TT6 rules allow convertibles to change the roofline shape, but the roofline must terminate at the front edge of the trunk. I have some ideas on how to build a roof with those limitations, but they are all sort of weird looking, like a Lotus Europa or Porsche 914. But they would have less drag and would work well in conjunction with a rear wing. From a practical standpoint, the trunk would still be functional.
    • Mazda RX500 top – This would be a custom top for 24 hours of Lemons. It’s almost like a station wagon, and would be simple to build.
    • Shark fin soup – Make a LeMans prototype-style top, complete with shark fin. If it works, race it in AER. If it doesn’t, race it in Lemons.
  • Aero+ tests – To do these properly I need to hire Jeremiah again. Without him, it’ll be just Aim Solo data which is a lot less accurate.
    • Underbody aero – Test different combinations of flat bottom, side skirts, and diffuser.
    • Front end aero – I didn’t get to test a stock front end vs R-package lip vs airdam. I have all the parts. I think we all know which is best, but how much better?
    • Low-speed aero – It’s easy for me to test low speed aero at my country club, Pineview Run. I just need to get my shit together, bring all the parts, and do it.
    • Spoilers by the inch – There’s a neat website called Ballistics by the Inch, where they start with a long gun barrel and progressively cut it shorter and record the resulting velocity. My test would be like that, but progressively a taller spoiler and record lap time. What’s the optimum spoiler height?
    • Tires – Pineview is a great place to test tires. I have some ideas in mind, like using one tire, and trying different widths, to see how much just tire width changes things. For example, I could test RS4s in 195, 225, and 245, on 7.5, 8.5 and 9″ wheels, respectively. I’m not sure how useful that is, but I’ve always been curious about this.

What’s on your mind? Leave a comment.

Pineview’s Future and Black Friday Sale

The other day I was invited up to Pineview Run for a presentation of the club’s three-year plan, and to provide feedback. I’m honored to be included in such things, and I’ll do whatever I can to help secure the future of this club. The future looks bright!

One of the things track enthusiasts will be excited about is Phase 2 of the track, which adds more real estate. It’s much faster, with large radius corners and straights that should get the horsepower cars over 100 mph.

A racing-line (smoothed) version of the track in OptimumLap.

The details aren’t final yet, but my brother Ian made a version of the track in OptimumLap based on initial sketches, and so we’re already running simulations on lap times and top speed. On the existing track, I run around a 1:20 on Yokohama S.Drives (don’t judge!), and on the longer track I’d be looking at a 1:41. Top speed on my 110 hp Miata goes from 67 mph to 87 mph in the simulation. So, yeah, your Corvette, M3, 911 or whatever will do the ton.

I don’t know when Phase 2 will be started, but one of the concerns is surely safety. I brought this up in the meeting, and Todd, shared some statistics. Since opening the track in the fall of 2017, nearly 1500 drivers have completed 50,000 laps, with a perfect safety record. That’s zero injuries and everyone has driven their car home (barring any mechanical failures). Safety will remain a primary concern going forward, especially with the track extension in mind.

Black Friday sale (updated 12/4/2020)

Last year I bought a six-pack of track passes on Black Friday so I could give friends a discounted rate when they come to the track with me. At $150 each, I thought they were a good deal. This year Pineview has an even better deal in the annual pass.

For $149 per month, you get unlimited track time. No, you can’t join for just one month, this is based on a yearly subscription. If that’s too steep for you, there are two other options. For $79 per month, you get unlimited weekend track time. That works out to less $1000, and that’s a lot of weekend track time.

I told my brother about this, and he said that he’d be more interested in a weekday version of the same package. I guess that might appeal to people who have other things to do on the weekend, but have weekdays free. Or maybe they want to “work from home” at the clubhouse, and do track sessions in between meetings and whatnot. Todd added this package, as well.

This is a Black Friday sale, and Todd may not offer this deal every year, so if you’re weighing this vs a gym membership, do some pushups and sit-ups or something. I want to see you at the track.

NEWS FLASH – I hadn’t noticed this before, and maybe Todd added it for Cyber Monday, but the Pineview Challenge Cup is on sale for $140 plus tax. That’s 12 races for $12.60 each. This is the member price, but people who buy the annual pass also get the member price. You will need a transponder, and there are limited number available at Pineview, so I’d add that to your cart if you need one.

Pineview Challenge Cup

Last year’s Pineview Cup Challenge was a scheduling challenge for many, and the feedback was that some people could only do this on the weekend. This year the Pineview Challenge Cup will have twelve events, on Thursday nights and Saturday mornings.

5/2/2020
5/28/2020
6/6/2020
6/11/2020
6/25/2020
7/4/2020
7/23/2020
8/1/2020
8/6/2020
8/20/2020
9/5/2020
9/26/2020 – Challenge Cup final

Like last year, you’ll only need to come to some of the events to be included in the final race, and so this should work out for a lot of people’s schedules. The rules for next year aren’t finalized, and I’ve been working with Josh and Todd on some ideas. I’d like to get a rules steering committee together soon, and come up with a simple classing system that is fair to all. This is obviously going to be a challenge, since every racing series does it differently, and most of them are anything but simple. Stay tuned.

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.
  • 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.