Low Speed Wings, Part 1

I live near two very different tracks, Watkins Glen International and Pineview Run. WGI is the fastest track in North America, and Pineview might be the slowest. When I run racing simulations in OptimumLap, Watkins Glen is one of the few tracks where drag matters. On any other track I can increase drag quite a bit and it’s not a big factor. Conversely, at Pineview Run, drag is almost inconsequential, and the same would be true on an autocross course.

I’ll run a quick simulation and show you what I mean. Here’s a Miata at three local tracks, Watkins Glen International (WGI), Pineview Run (PV), and New York Safety Track (NYST) using drag coefficients of .45 and .495 (110%).

WGI .45WGI .495PV .45PV .495NYST .45NYST .495
Lap time140.58141.3473.8573.8998.7699
Max speed120.61117.8875.2274.96106.8105.49
Effect of adding 10% drag.

At WGI, adding 10% drag is a difference is .76 seconds and almost 3 mph top speed. That’s the difference between winning and losing. Conversely, at Pineview, it’s only .04 seconds and not worth talking about. I put NYST in just to show what an average track is like, and here the added drag is only a quarter of a second.

So when optimizing for low speed aerodynamics, let’s ignore drag and go for as much downforce as possible.

Aspect Ratio

Wings make most of their downforce in the middle of the wing; the further you move from the center, the less downforce the wing makes. This is because the ends of the wing create vortexes that destroy lift.

Wingtip Vortices
Image from Flight Literacy

Therefore, for a given area, a longer wing makes more lift than a shorter wing, simply because the ends are further from the center. You may have heard of a high aspect ratio wing; that’s a wing that is very long and thin, such as on a sailplane, or glider. Theoretically, you might think that would be a good wing on a racecar.

Glider (sailplane) - Wikipedia
Thanks Wikipedia for the image.

Well, not necessarily. For one, racecar wings (and most airplanes these days) use endplates, which keep the upper and lower pressure zones from creating the lift-destroying vortexes. So you don’t need to have a really long wing to avoid losing lift. Just use a big enough end plate.

Another reason a high aspect ratio wing would be difficult for cars is construction. Building a really long wing with a narrow chord is going to be a noodle. The stiffness of a wing is the thickness cubed multiplied by the chord, and so a high aspect ratio wing would be fragile or at least very flexible.

Another reason a high-aspect ratio wing isn’t ideal is because the mass is high up and away from center. Mass centralization is important for handling, and a high-aspect ratio wing is the antithesis of that.

Finally, probably the most significant reason a high-aspect ratio wing won’t work well at low speed is it would generate low Reynolds numbers. What?

Reynolds Number

I’m not going to go into a deep explanation of what a Reynolds (Re) number is, but I think of it as a resistance number. At low speed, air doesn’t have much resistance; you can put your hand out the window as if it were a wing, and at 5 mph change the angle of attack freely. Do that at 60 mph and you’ll feel a lot of drag. At that speed, air seems to have the same resistance as moving your hand through water.

Likewise, a smaller hand will have less resistance out the window than a larger hand. So you can probably tell that the Re number depends on the chord of the wing (the size of your hand), and the density of air (which is basically the speed you’re moving).

Let’s see the relationship of chord and speed on the Re number. I’ll take three wings: a high-aspect ratio 4″, an in between 6″ (eBay single wing), and a more standard 9″ wing (9 Lives Racing is about that), and see what the Re numbers are at 62 mph (the average minimum corner speed at Mid-Ohio, and also 100 km/h for the metric people).

Wing chordMPHReynolds Number
4″ (.102m)62 187k
6″ (.152m)62280k
9″ (.23m)62420k
Reynold’s numbers with different chords

In the table above, you can see that the 4″ wing has a Re number of just under 200k. The 9″ wing has a Re approaching 500k. OK, so how is this significant?

Look at the image below, which shows a Selig S1223 wing at three different Re numbers. The gold line represents a Re of 1 million, the purple line is 500k, and the green line is 200k. The lines show the coefficient of lift divided by the coefficient of drag (Cl/Cd), which you can simply think of as how efficient the wing is at making lift. The higher the Re number, the more efficient this wing is. At 200k, the wing just isn’t working that well. At 500k, it’s about 130% better, and at 1 million, it’s better still.

Reynold’s number of 200k (green) vs 500k (purple) vs 1 million (gold).

All of this means that for a wing to be useful at low speed, it needs a really large chord. For argument sake, I’m calling this 16″. This drives up the Reynold’s number to where the wing is more efficient. In addition, flow separation is delayed at higher Reynolds numbers, which means you can use a low-aspect ratio wing at a higher angle of attack. (And if you grok what I’m saying about the Reynolds number, you understand that you can also use more wing angle at higher speed.)

Wing Shape

The Airfoil Tools website has a pretty good search function, you can look for wings of different profiles and sort on factors like thickness, camber, and even Reynolds number. I started by searching for wings that had the highest lift/drag ratio (Cl/Cd) at 500k Re, using this search. That returned a lot of results, some of them rather strange. Next I looked at the Cl vs Alpha graph, and many of them peak around 1.5.

But what I’m specifically looking for are high lift wings. I don’t want a wing that’s the most efficient at low Re, I want one that has the highest lift at low Re. So I changed the search parameters to use a minimum thickness or 10% and minimum camber of 8% (both values that should drive up lift) to find wings that have a Cl of 2:1 or greater. (Note that single wings have peak downforce at about 12 degrees, but double wings and low Re numbers can use a thicker wings for more angle.)

That search returned four pages of results, and after culling those, the ones I’ll use for this investigation are the Church Hollinger Ch10, Eppler 420, and Selig S1223. (There are other potentially good ones I didn’t look into, such as FX 74-Cl5-140 MOD, FX 72-MS-150B AIRFOIL, GOE 525, etc.)

Airfoil Tools allows you to compare wings by clicking Add to comparison. So let’s do that for each wing. You get a view like so, with the corresponding data.

Wings compared

Turbulence

Wings work best when they can have laminar flow. Conversely, when the air is turbulent, it increases drag and decreases lift. Airfoil Tools allows you to simulate turbulence using the Ncrit value. An Ncrit of 9 is like a wind tunnel, or about what you’d experience on a race track, on a still day, with no other cars in front of you. In other words, pretty unrealistic.

To simulate what your wing is doing behind your car’s canopy, during a race, possibly in the wake of another car, you need to use a Ncrit value of 5 or less. Unfortunately, Airfoil Tools doesn’t give you all the Ncrit values, and depending on which wing you choose, 5 might be as low as you can go.

Head to Head on Reynolds

I’ll use these three wings and then compare the various graphs at a Ncrit value of 5, starting with Re 200k, then 500k, and finally 1 million. In the graphs below, Cl is lift, Cd is drag, and Alpha is the angle of attack.

Re 200k

A Reynolds number of 200k is absurdly low, and for our theoretical wing of 16″ chord (.41 m) represents a speed of less than 20 mph. Still, this is useful to look at because most people don’t have a 16″ wing, and so this represents how these wing shapes would perform at a more standard 9″ chord and around 30 mph.

At 200k, the S1223 wing is clearly ahead of the CH10 and E420 in both Cl and Cl/Cd, which are the most important factors. (Sorry about the colors, I can’t choose them.)

Re 500k

A 16″ wing at 41 mph would have a Re of about 500k. Pineview Run’s average minimum cornering speed is around 43 mph, so this is a close approximation for that track, or for an autocross.

The S1223 wing is winning on Cl and Cl/Cd. This is a good wing for low speed applications. The E420 is starting to diverge from the CH10, showing a bit more promise at higher wing angle. Where the CH10 wing shines is in lower drag, but we don’t care about that for this application.

Re 1 Million

At this point the 16″ wing is traveling at 87 mph, which is not what I’d call low-speed cornering, but it’s worth looking to see how the wings perform at higher Reynolds.

Conclusions

  • S1223 looks like the low-speed wing of choice. It has the highest lift, but also the highest drag by a fair amount. This might be a good all-purpose wing on a powerful car that can overcome drag, but not one of my underpowered Miatas.
  • CH10 looks like it would be a great low-drag wing for a momentum car. It’s very efficient and seems tolerant of wing angle. This looks like the best wing for a “set it and forget it” mentality. The 9 Lives Racing wing profile is a lot like the CH10, but 9LR has more camber. Camber is good, it makes more downforce. This is one of those situations where I look at the 9 Lives Racing wing and say to myself, why the fuck am I doing anything with wings?
  • E420 splits the difference and seems like a great choice for an all-purpose wing. It’s thicker than the other wings, and would probably be a better choice for a double wing setup, where that can be exploited for more wing angle and downforce.

What Next?

In Part 2 of this series I’ll build one of these wings, and in Part 3 I’ll test it at a low-speed venue. But while the S1223 is a clear winner, I’m not exactly sure how I can build it strong enough, it’s just so damn thin at the end.

S1223 the right way up

The S1223 also comes in a Richard T. LaSalle modified version, which is slightly thicker and has more camber. This wing generates even higher lift and can be used at 15 degrees angle of attack, but it does so with more drag. This would definitely be a low-speed application, especially on a Miata.

S1223 vs S1223-RTL at 500k Re

Big Wing Tests in 2021

Last weekend I went through the MSF Level 2 course taught by Hooked on Driving. It’s an amazing program, I learned a bunch, and now I can right-seat coach for a lot of HPDE organizations. That means more track time, and more aero testing!

The first thing I want to test is different wings at different tracks. For low speed, I’ll go to Pineview Run, where I have a membership and can go whenever I want. For high speed, I’ll go to Watkins Glen, which is 20 miles from me. Finally, for medium-speed tracks, there’s NYST and PittRace, which I’ll go to a couple times each.

All the wings need to use the same mounts, and I’ll make sure they are all optimized for angle. Then I’ll record min speeds through a few important corners, max speeds on straights, and of course the overall lap time. The wing contenders are the following:

  • 9 Lives Racing – I have their 60″ wing. Most Miata people order 64″ (body width on NA/NB), but I was going to eventually mount mine from the end plates and so it’s a bit shorter. With welded mounts and end plates it weighs 13.8 lbs, and has an area of 546 square inches.
  • APR GTC-200 – This is a 59″ wing in a 3D shape, carbon fiber, very light, just 5.2 lbs with end plates and all. I modified the wing slightly to fill it with lightweight expanding foam and I made new end plates because the old ones were seized inside. The 3D shape is wider in the middle than the sides, and has an area of about 442 sq in.
  • Areyourshop – This wing is sold under numerous names on eBay, Amazon, etc. It’s a single 53″ extruded aluminum wing with adjustable brackets underneath. I reshaped the bottom and set the mounts at 41.5″. I bought mine for $60, but they’ve gone up in price since. I threw out the mounts and end plates and made my own. It weighs 5.8 lbs with end plates and a small Gurney flap, and measures 297 sq in.
  • Mophon Double – Another MIC wing, this one I’ve had for a while, also sold under a variety of names. I’ve done a couple blog posts on this one already, it makes downforce, but too much drag. I’m trying to reduce drag this time and left off the top Gurney flap. I made new end plates and re-set the upper wing angle to 35 degrees. It weighs 8 lbs and has about 424 inches of total area.

All four contenders. End plates from street signs, $1 each.

There’s one final contender, but I’ll only use it at Pineview because it’s going to have so much drag it won’t be funny. Well, maybe a little funny. This is a triple wing made up of the two made in China wings.

The main wing is the single MIC wing, and the other wings mount via the end plate. First I drew out how to mount them on a scrap piece of plastic, which was a great way to get the angles.

Mocking up the wing angles

Then I transferred these holes to aluminum (street signs) and cut out a shape that felt pleasing to the eye. I put most of the end plate area low and forward, because that’s where the low pressure zone should be. I have a digital manometer and may verify this later, but I’m not sure how much it really matters. This is mostly a conversation piece, anyway.

Three wings is three times as ridiculous!

All assembled this wing weighed exactly the same as the 9 Lives Racing wing, 13.8 lbs, with a total wing area over 720 inches. Like I said, this will be for Pineview and autocross speeds only, I have no illusions that this would work on a big track.

What am I doing for front aero? You’ll just have to stay tuned. 2021 is going to be a fun year of aero testing!

Theoretical Best Laps (and what we can learn from each other)

Last fall, Sahir brought his Miata to Pineview and allowed Josh and I to drive it. Sahir had just won the C4 class of the Pineview Challenge Cup the week before and was on pace. Josh is a two-time Challenge Cup overall winner and one of the fastest people I know. I haven’t won any championships, but I’m a self-proclaimed Miata and Pineview specialist, and I go fast when those two things are combined. So we all know the track really well, but our driving styles differ. I wanted to see by how much, and what we could learn from each other. 

I imported all the laps in Race Studio and threw out the first lap of each session. For some reason, the first laps create fast sector times that are impossible. Then I created a track map with seven sectors. Formula 1 uses three sectors, MotoGP uses four. I wanted to get more granular, and see where each driver was doing their best. This put the track divisions on the straights between T2-T3, T5-6, T7-T8, T10-T11, T11-T12, and T12-T13. See the shitty image below. 

I then used the Split Report feature to see how we did in each split. If you click the History tab on each sector, this gives you a histogram chart that makes it easier. A long green bar is a slow sector. A short red bar means the fastest sector time. Check it out. (Yellow bars are rolling laps, ignore for now.)

  • Run 3 is Sahir. He does six hot laps (laps 2-7) with a best lap of 1:18.537. He sets the fastest time in the sixth sector, the Blind Hairpin. 
  • Run 4 is Josh. You’ll notice he only puts in two laps with a best lap of 1:18.873. In an unfamiliar car, in two laps, he almost matches Sahir’s time (and only .1 seconds off Sahir’s PV Cup wining lap). Josh kills it in Sector 1. 
  • Run 5 is me. I do seven hot laps with a best lap of 1:17.674. I set the fastest time in the remaining sectors. Enh, I’m fast in Miatas, and on this track in particular. 

As good as any of us are individually, if you put all of our best sectors together, we’d do a 1:17.192! That’s half a second faster than I went, and about 1.5 seconds faster than Sahir and Josh. OK then, what can we learn from each other? 

In the following speed traces, Josh is black, Sahir is blue, and I’m red. I’m using the two fastest laps from each person. First let’s look at Josh, he’s magic in Sector 1. Notice on the bottom graph (time-distance) that one black squiggle that’s below all of the other colored lines. He has a higher minimum speed in T2, and puts some time in his pocket.

Next is sectors 2-5, and I’m fastest at three points in particular, circled in orange: In T4, my line allows me to get to the throttle earlier; In T7, I use a straighter approach that allows me to brake deeper and harder on the entry; In the Knuckle, I take a deep double-apex line that keeps my speed longer. Interestingly, we’re all very even in the Uphill Esses (notice that all the lines in the bottom graph are basically horizontal from 1700′-2400′). Nobody is winning that part. 

Sahir is fastest in Sector 6 by .01 seconds. The time-distance graph has a dip at 3500′ which corresponds to his speed advantage at that point. He’s simply braking later than us chickens, and that’s understandable because the Blind Hairpin has claimed a couple cars on the berm. 

In the final sector I gain a little time by braking later (notice the height of the red lines at 4100′), and I get another boost of speed right between Turns 14 and 15 at 4500′. (Ignore where the cursor is.)

None of us are professional drivers, and we can all improve by simply looking at what each other is doing. But this is really only possible by using data. It’s a bit late to make a New Year’s resolution, but if you aren’t using data, make a resolution to do that now instead of next year. I’ll be your accountability partner!

205 vs 225 vs 245 on a Miata

This blog post is going to sound like a pissy rant. Fuck it, I’m due one.

Social media has created a dangerous situation where opinion is more important than fact. It’s almost like whoever yells the loudest and most often wins. And this brings me to a tired topic on Trackable Miatas. What’s faster, a 205, 225, or 245?

It’s a circular topic. At some point someone will chime in that it depends on power. Someone else will say you have to put a 205 on a 8″. That only lasts until someone says 205 is better on a 9″. Then another will say that 205s are proven to be faster than 225s or 245s on California tracks. As if tracks are different there? I’ve raced at Laguna Seca, Willow Springs, Thunderhill, and Sears Pt and they are no different than WGI, PittRace, NJMP or other east coast tracks I’ve raced at.

Next someone who is sick of being tread on will say, where’s the data? And all those people fall back on “it’s a proven fact,” without showing any. Honestly, I had to leave Trackable Miatas for a while because it’s the same people shouting the same nonsense without a shred of data to back it up.

It really is a bunch of hooey, and I’m just getting in a groove. First off, the notion that 205 tires are faster is flawed by the very fact that 205 tires aren’t all the same width. A 195 RS4 is wider than a 205 RE71R. A 205 Hoosier is wider than a 225 NT01. What 205 tire are you talking about? And then there’s the whole wheel width variable, as if offset, weight, tire pressure etc, weren’t factors to consider. And then some tracks are high speed and some are more about cornering.

I really have to question if any of the people that claim 205s are faster have done back-to-back testing using the same compound tires of different widths? Have they swapped wheel widths (same weight, same offset) on the same tire? If they have, they aren’t sharing the data.

Well, I can understand that, I haven’t either. But I’ll open the door just a tiny bit and shed some light on this.

Last Memorial Day four of us got together in Miatas and tested tires. Me, Alec Fitzgerald, Alyssa Merrill and Davey Thai got together in four Miatas and tested 10 sets of tires. We divided up the tires so that there would be some crossover.

  • Alyssa tested Hoosier A7s and R7s in 205, Maxxis RC1s in 245 and Hankook Z214 C51 in 2o5
  • I tested 225 RS4s on 8″ and 9″, and 245s on 9″, RE71Rs, and both Hoosiers
  • Alec tested RC1s, Z214, R7, and 245 RS4s.
  • Davey tested 205 Conti ECS, VR1, RE71R, 225 RS4s on 8″

We did this as scientifically as we could, recording air, track and tire temperatures, set optimal pressures, and ran multiple redundant sessions to gather data. I’m not going to share the lap times and data with you because information like that isn’t free. But I’ll give you this chart: same driver, same corner over 200′ of distance at 20′ intervals, two runs averaged, 225 RS4 on 8″ and 9″, 245 on 9″, and some other tires mixed in.

Average cornering Gs over 200′ of a corner.

Tuning a 1.6 Miata

Years ago there was a site called solomiata.com, and it was the best resource for people who wanted to tune their 1.6 Miatas. Later the author added information for 1.8s. The site was down, it was back up, it was down, and now it lives over here.

#57 Roswell Mazda Miata. Turner Field-Atlanta Aug 20, 2000 Photo by John Swain

The Solomiata recipe for cheap horsepower was:

  1. Advance the timing 4 degrees, for about + 2 hp.
  2. A cat-back exhaust, for about 4-5 hp.
  3. Replace the AFM with a larger flapper valve from a RX7. The standard Miata flowmeter is too small to flow at high RPM, and so this added 5 hp above 6000 RPM.
  4. Aftermarket header for another 3-4 hp.
  5. Add a programmable ECU, larger injectors, and an adjustable fuel pressure regulator.
  6. Shave the head .010″ for about 4 hp.

All of that would get you a maximum of 115 hp at the wheels. I think a more realistic estimate is 108-112 hp based on most stock 1.6 Miatas dynoing around 92 hp stock, and our cars being a lot older at this point.

The modern Solomiata formula

That was then, what do people do now? Swap in a VVT 1.8. A bone stock NB2 motor will put out similar horsepower as the Solomiata formula, and way more torque. I drove Napp Motorsports’ VVT swap back to back with my well modified NA6. My motor has only 5 hp less, but the torque difference was astounding, like it was a completely different car. It was the lightbulb moment where I was like “I made a huge fuggin mistake.”

However, if you aren’t ready to take the motor swap plunge, here’s the modern Solomiata formula for a 1.6 Miata.

  1. Standalone ECU. I got a Megasquirt PNP2, which is about $800, but these days you can buy a Speedy EFI unit for half that. It even comes with a variable TPS (the 1.6 Miata has an on/off throttle position sensor). A standalone ECU combines steps 1, 3, and 5 from the Solomiata formula.
  2. Cold-air intake. If you want to keep the stock airbox, get a cowl intake (Randall or DIY a snorkel). If not, find someone to 3D print you an intake.
  3. Cat-back exhaust. There are many to choose from. Optionally replace the catalytic converter with a high-flow cat for another 1-2 HP. Your OEM cat is worth over $100 at any scrap yard, so that will defray the cost.

My car with only a MS PNP2, cone filter, and cat-back exhaust pulled 106 hp on a Dynojet. I hadn’t decked the head at that point, but if I had, I’d be right in the Solomiata ballpark.

All of these mods can be done cheaper and easier than a motor swap, with about the same result. You won’t have shit for torque, but you’re used to that, right? You might have noticed I didn’t add a header to this list. The reason being, that’s specifically a 1.6 part and won’t work on a 1.8. You can do all of the mods above and still come to your senses and do a 1.8 VVT motor swap.

Up to this point you won’t need to dyno tune your car, either. You can use the ECU’s base map, as its programmed for mild bolt ons. But if you modify the car further, you’ll need to tune the ECU. And that’s where things get difficult. Not the tuning itself so much, but because the 1.6 head doesn’t flow well. Quality control on these heads wasn’t great, and there’s a lot of core shift between different heads, and the port geometry could be improved (and later was).

You might think that the shorter stroke means you could go after high RPMs, but if you do that, then the oil comes out of the hydraulic lash adjusters. So there’s a low ceiling on how far you can tune a naturally aspirated 1.6, both bottom end, and top end.

This is a smart place to stop modifying your normally aspirated 1.6 and look for a junkyard NB2. Unless you need to replace a head gasket or remove the head for some other reason, this is a smart place to stop reading.

I don’t care, I’m tuning a 1.6

If you’re going to modify a naturally aspirated 1.6 Miata any further than this, then you’re already beyond reason. If logic worked, it would have worked already. But misery enjoys company, so thanks for joining the club.

OK, so let’s do this. The next thing you need to do is pull the head. An older engine may need a valve job, and you can get a lot of things done at the same time.

  1. Number all the valves and their locations and remove them (socket and hammer trick.) Clean up the ports with a Dremel tool. It’s free HP for a bit of your time. Remove casting flash, smooth any hard edges around the plunge cut, and blend the web between the ports. Polish the exhaust side, but leave the intake side a bit rougher.
  2. Take the head to a machinist and have them measure the valves and springs. Make sure to order new OEM valve guide seals. You might consider +1mm intake valves, this will add about 4 hp and torque. In any case, have them do a valve job.
  3. Have the machinist deck the head .040″ to bump up compression by one point. I’ve heard of people safely taking off more than that, but on Premium pump gas, you really don’t want to go much further. This will also retard cam timing a few degrees, which also helps. FWIW, there’s no bigger bang for the buck than decking the head, it cost me all of $50. If you need to replace a head gasket at any time, just deck the head and start using premium gas. I’d do this even on a bone stock NA Miata.
  4. Next is cams. The cheap way is an exhintake cam, modifying a MX-3 cam and putting that on the intake side. This gives about 8 hp when properly tuned. Or you can get a Kelford cam and double it. I went with a 203-B cam, which is about max for the street. A larger cam will have a rough idle, and you’ll need to replace valve springs, retainers, etc.
  5. With all that work into the 1.6 head, put on an aftermarket header. I have a Racing Beat in my race car and a Raceland in my street car. One of the welds failed on the Raceland, and the header was replaced for free. However, it was a hassle, and when you look at the difference in quality between Racing Beat and Raceland… well, you get what you pay for.
  6. Tuning is a must at this point. Reprogramming the ECU can range from free (your laptop) to $600 or more (dyno operator).
Getting the 1.6 tuned by Rick Gifford

This is pretty much where my car sits right now, with 129 hp on a Land and Sea dyno (which reads like a Mustang). This equates to about 145 hp and 122 ft-lbs on a Dynojet. An NB with all the bolt ons will have slightly less horsepower and more torque, and overall similar performance.

Where next, NA6?

OMFG you’re still reading? The smart people left the room a while ago. They swapped in a NB2, Honda K motor, Ecotec, or used forced induction.

Speaking of forced induction, the Miata 1.6 engine was originally designed for a turbo. Back then people would complain about turbo lag, but modern turbos with standalone engine management offer instant throttle response, fat torque, and a top end rush. My teammate’s turbo NA6 turbo is in my garage right now, so I know what a good turbo feels like.

But I don’t want a turbo. I can’t answer that logically. Somehow I’m keen to get a supercharger, even if it’s not as good. And then part of me just wants to see how far I can push the normally aspirated 1.6 envelope. I guess I’m a glutton for punishment and disappointment.

So let’s say we continue this normally aspirated experiment, just as a thought exercise. Where do we go next?

  • Intake manifold. The NA6 intake manifold is a single cast piece, so you can’t pull it apart and polish the runners or add volume with a plenum spacer. There are some things that can be done here. None of it is money well spent, but that ship sailed a long time ago.
    • Extrude hone. This is basically forcing liquid sand through the manifold, which polishes the places you can’t reach. I would expect at most a 5% gain in power for $500.
    • Manifold spacer. A piece of phenolic machined to the dimensions of the intake manifold runners would help lower intake temperature and provide longer runners. I’d probably have to space things out in the engine bay, and install longer studs in the engine. And there’s the issue of the injectors now being further from the port which might not be good for atomization.
    • Skunk2. The Skunk2 intake manifold is available only for the 1.8 block, and adds 5-10% more power on the top end. Could this be adapted to a 1.6 with a manifold spacer? Cylinders 1 and 4 would need the ports angled 6mm in, and the injectors would have to be welded into the manifold (they are in the head on the 1.8).
    • Log manifold. These are primarily for boosted cars, but might unlock some N.A power. The one from RZ Crew is beautiful, if nothing else. The intake runners don’t wrap around the bottom, and might be shorter than stock, which wouldn’t help.
  • NB6 head – In the USA, the 1.6 was last sold in 1993, but in Europe and Japan, they continued to sell the smaller motor. When the 1.8 went from NA to NB, so did the 1.6. The NB6 got the better port geometry, solid lifters, and I believe there’s a square-top NB6 intake manifold as well. I have the cam for the NA6, which has a different profile for the HLAs, so I’m probably not going down this route. But if someone were starting from scratch, this is a better starting point for 1.6 insanity.
  • Ram air. I’ve actually built one of these airboxes, and it worked out to 1% power gain hp at 100 mph. At less than 100 mph, and at partial throttle openings, there was no change in manifold pressure. But, pinned WFO doing the ton, I watched 4″ of water drop (intake manifold pressure change). This was later verified on a dyno with a leaf blower, adding 1 hp. My intake has gone back and forth, and is currently not a ram intake, but I might go back to it again if I go to Watkins Glen regularly and desperately need 1 hp.
  • ITBs. Some people have used Toyota AE101 intakes, other have used a more plug and play version from Jenvey. There’s a long thread on Club Roadster where a guy threw everything at a 1.6 including Jenvey ITBs and got 158 HP IIRC. My research says this is worth about 10% more power, and that’s after a lot of dyno tuning.
  • Displacement – There’s a cheap stroker kit, it’s called a 1.8 swap. So what options are there for boring the block? Most of the aftermarket big-bore pistons are low compression, meant for boost. On the other side are 12:1 pistons that would now be 13:1 on a decked the head. Yikes! Somewhere in the middle is a unicorn big-bore standard compression piston with my name on it.
  • The little things. I haven’t fooled around with cam timing yet. I might need new injectors. People say coil-on-plugs will do something more than nothing. Mathematically, the throttle body flows enough, but maybe boring it out would help.

In reality, it’s unlikely I’ll do any of these. My money is better spent on anything else. Just the same, stay tuned, I might venture further down the path of disappointment.

First Timer Building a Track Miata

A friend and his dad are just getting into track driving, and building up their NA8 Miata for that purpose. It got me to thinking, if I was in that situation, knowing what I know now… what would I do, and in what order?

My wife next to my first Miata, California 2012. Also the first hardtop I built.

Phase 1: Track Ready

The first thing is a car that’s track-legal, and safe. It’s also never too early to start collecting data.

  • 4-point Rollbar – Most tracks and HPDE organizations require this.
  • Brakes – StopTech 309 pads and high temp brake fluid. The StopTechs don’t have a lot of bite, but are great on the street, and handle track temps OK. Their big selling point is price, sometimes I find them for less than $40.
  • Tow straps – Baby teeth are fine, but if you removed them, you need something to attach a tow hook to, front and rear.
  • Data – If you’re just getting started, a phone app is fine. You’ll want to add a 10 hz bluetooth antenna eventually, or better yet, get an Aim Solo or similar device made for motorsports.

Phase 2: Mechanical Grip

Drive the car like that for a few events. Resist the urge to put on sticky tires; All-season tires and stock suspension are learning aids. But once you can slide the car through every corner, it’s time to get more mechanical grip. This is a big step, and requires several things at once.

  • Tires – A Miata on sticky tires is what momentum driving is all about. I like Hankook RS4s for their predictability and durability, but they aren’t super sticky. At the other end of the spectrum are take-off Hoosiers and Toyos from Spec Miata racers, which is an economical way to go fast. And there are a lot of 100-200 TW tires in between, the tradeoff is always between grip and longevity.
  • Wheels – The stock wheels will hold you back. A 15×8 +35 wheel and 205 tire will fit with no modifications. If you roll the fenders, you can use 15x9s and run 225s, which is the current go-fast formula.
  • Hubs – Sticky tires break stock hubs. The fronts are usually the ones to go, but my race car broke at the rear. I have BroFab hubs on my street car and Miatahubs on my race car. There are other options, and some people simply throw out the OEM hubs every year.
  • Suspension – Shocks with stiffer springs and NB top hats. Coilovers so you can corner balance. Stiffer front sway bar and a bracket so you don’t tear the mount. If your car has a lot of miles, it might need new suspension bushings. Ugh.
  • Alignment – You’ll probably need extended lower ball joints to get enough front camber, otherwise you’re going to wear the tires out. Get a proper track alignment.
  • Seat and belts – At this point you’re going to be thrown side to side more, and a race seat is nice. You might also want a 5-7 point harness, and with that comes the requirement of a Hans device.

Phase 3: Hurt Machine

Bolt-on power and areo are next on the list.

  • The usual suspects – A cold-air intake, header, and cat-back exhaust will each unlock about 5% power, which is still doable on the stock ECU. You might bump the timing a few degrees and do other minor tuning tricks if you haven’t already.
  • Maintenance items – Some performance gains can be had if you’re replacing parts. If you need a clutch or throwout bearing, then do a lightweight flywheel at the same time. A 10-lb reduction in flywheel is worth about 7 hp in 1st gear, 3 hp in 2nd gear, and 1.5 hp in 3rd gear. If you need to pull the head for any reason, deck it .040″. That’ll cost about $60 and you’ll have to use Premium gas, but it’s the best bang for the buck. A high-flow catalytic converter will get 1-2 hp, but you can sell your OEM cat for almost the same price.
  • Front aero – Airdam, undertray, ducted radiator, and hood vents. You have to do all of these at the same time because they are related. Brake ducts are optional, but easier to do that now than later.
  • Rear aero – For a car that does more street than track, I like the looks of a spoiler. It needs to be at least 4″ high, preferably 7-8″. For a dedicated track car, use a 9 Lives Racing wing and add a splitter to the undertray.
  • Misc aero – Fender vents, side skirts, flat bottom, diffuser, etc., are all worthwhile. Just say no to vortex generators.

At this point the car is the Trackable Miata build many people aspire to, and it’s a damn fast car. The engine is still basically stock, and you can beat on it all day. You can also beat on average drivers in Porsches, BMWs, etc. This is a machine that can hurt a lot of feelings.

But if you want to run with a well-driven Porsche, Corvette, or whatever, you need more power. This is the point where you decide if you’re going to NA tune the engine on a standalone, swap the engine, or go forced induction. Those decisions are like diets, religion, and politics: you don’t bring them up in pleasant company. And so I’ll leave my opinion out of it, except to say that anyone tuning a normally-aspirated 1.6 deserves their beatings.

Where Drag Comes From on a Miata

The total coefficient of drag on my racing Miata (hard top, airdam, splitter, wing) was measured precisely at Cd .48. Where does that drag come from, and how can you reduce it?

The following table shows estimates of where that drag comes from. You’ll notice a splitter reduces drag, but every other feature adds to it. This data is largely from Katz, but I’ve supplemented this with some Miata values and racing parts (airdam, splitter, 9LR wing) and included a value for the windows being open (we are racing, after all).

FeatureDrag
Splitter-.01
Mirrors.02
Rear wheels + suspension.023
Engine bay.024
Front wheels + suspension.025
Wing.03
Cooling drag.048
Windows open.05
Rear surfaces.085
Underbody and chassis.085
Bodywork.1
Total drag coefficient.480
Parts of the car that contribute to drag, and how much.

There are lots of ways to reduce drag. One way is to use a fastback, which reduces the drag in three places: bodywork, rear surfaces, and open windows (my fastback is narrower and considerably less air goes in the windows).

My fastback reduced drag by .07 to a Cd of .41. What does a 15% reduction in drag do for lap times? I’ll run a quick simulation in OptimumLap (2375 lbs, 125 hp, 1.15 grip).

CdWGINYSTPineview
.482:20.551:39.401:15.38
.412:19.321:39.011:15.32
Miata lap times changing only the drag value.

Right away you can see that drag reduction at a course like Pineview (or autocross) is nearly worthless. At a medium-speed track like New York Safety Track, it amounts to less than half a second. But at a high speed track like Watkins Glen, drag reduction is worth 1.2 seconds! So the value of drag reduction depends entirely on the venue.

For endurance racing, drag reduction is almost always worthwhile, but you need to make large gains. A 15% reduction in drag works out to only 2% less fuel usage at Watkins Glen. But if you’re right on the cusp of a 1:55 stint time, that’s huge.

Every once in a while someone talks to me about reducing the drag from their wing, or choosing one single-element wing over another because it’s more efficient. If you look at the table above, my 9 Lives Racing wing only adds .03 drag. Even if you could reduce your wing drag by 20% (doubtful), that would be a .1 second per lap at WGI and .03 seconds at NYST. Seriously, there are bigger fish to fry!

When setting a course for drag reduction, start at the bottom of the table, where the biggest items are, and work your way up.

Wing Angle, Efficiency, and Downforce

If you have a wing, your next question might be, is it better to run less angle and have a more efficient wing, or more angle for downforce? Assuming you can set the car up to handle equally in each situation, which would be fastest?

Let’s run another simulation. I’ll use my data for a 9 Lives Racing wing and use three values: 0-degrees AOA representing the most efficient setting, 10 degrees with a 1/2″ Gurney flap representing the most downforce and drag, and 5 degrees no Gurney to split the difference. I’ll use the same car and the same three race tracks.

SetupWGINYSTPineview
Zero angle of attack, L/D 14:1, Cd .4612:21.221:39.901:15.66
5 degree, L/D 13:1, Cd .482:20.551:39.401:15.38
10 degree, 1/2″ Gurney, L/D 8.5:1, Cd .522:20.761:39.331:15.27
Wing angle and lap times

What’s interesting to note here is that the most efficient setting (zero degrees angle of attack and no gurney flap) is the slowest at all tracks. The setting with the most downforce (10 deg AOA, 1/2″ Gurney) is fastest at Pineview and NYST, but not at WGI.

The best wing angle for your car is obviously track dependent, but if I was going to set it and forget it, I’d run the wing at 5 degrees and add a 1/4″ Gurney flap. I’m not going to run that simulation for you, I have to keep some speed secrets for myself.

Calculating Wing Downforce

How much downforce does your wing make? Mathematically, you can calculate downforce using:
downforce = 1/2p * A * Cl * V^2.
If that means anything to you, then you don’t need this website. If that calculation scares the shit out of you, read on.

You can break this formula into four parts:

  • 1/2p – I’ll simplify this as a constant value of .00119. That’s all you need to know about 1/2p. (OK, someone actually asked about this, it’s air density, which changes due to elevation, temperature, and humidity. If you want to calculate the downforce at sea level, on a cold and wet day, versus a hot and sunny day in the mountains, go ahead. Or just accept my static value and move ahead.)
  • A – This is the area of the wing in square feet. If you have a 64″ 9 Lives Racing wing, then your area is about 4 square feet (64″ x 9.1″ / 144 ).
  • Cl – This is Coefficient of Lift. It’s a tricky value because it changes with different shapes of wings, how fast you’re going, the wing angle, and turbulence. For the time being, you can use a value of 1.0. I’ll explain later why this is a reasonable value.
  • V^2 – Velocity in feet per second, squared. For this you need to know that 1 mph = 1.46667 feet per second.

OK, so let’s figure out about how much downforce a 4 square-foot wing produces at different speeds. Three things will remain constant, the 1/2p value, the wing area A, and the coefficient of lift, Cl. Multiply all those factors together and you get a value of .00528836. Now all we need is to multiply .00528836 by the velocity in feet per second, squared. I’ve also added the Reynolds number at that speed for later discussion.

VelocityFPS ^2DownforceRe
40 mph3441.816.5 lbs270k
60 mph7744.037.2 lbs405k
80 mph13767.266.2 lbs540k
100 mph21512.1103.5 lbs675k
120 mph30976.0149.1 lbs810k
Downforce at various speeds, 9LR 64″ wing, Cl 1.0

How Much Added Grip?

If we assume that tire grip is linear, we can calculate the amount of grip you gain at different speeds. Downforce helps lighter cars more than heavy cars, because the percentage gain is greater. I’ll use the same wing and speeds to illustrate this.

Speed2400 lb car3000 lb car
40.76%.60%
601.70%1.36%
803.03%2.43%
1004.74%3.79%
1206.82%5.46%
Grip increase at different speeds using the same wing.

Coefficient of Lift

In the above calculations, I chose a Cl value of 1.0. In reality, a good single-element wing in free-stream air, at high velocity (high Reynolds number), can create 50% more downforce than that (Cl 1.5). Meaning that if we can place the wing in non-turbulent air, and drive faster (creating a larger Reynolds number), the wing will be at peak performance.

In the real world, you can’t get to peak performance. There is always some degree of turbulence, whether from your roofline, the wake of other cars, crosswinds, etc. Turbulence destroys lift. I saw this firsthand when I changed only the roofline shape on my car and back-to-back tested them: no roof was a 250% loss in rear downforce. A fastback was a 130% gain in rear downforce. Turbulence is a HUGE factor in generating lift. When I swapped roofs, there was probably a degree or two of change in the angle of air moving over the roof, but that wasn’t going to change things on this order of magnitude.

So this is a very long-winded way of saying that a coefficient of lift of 1.0 is fine for rough calculations on a hardtop Miata.

In another post on low speed wings, I go into a bit about how wing chord and speed affect the Reynolds number, and that generally wings perform better at high Re numbers (faster speed or larger chord). Conversely, low speeds and narrow wings generate lower Re numbers. You could use that to fudge the calculation slightly. For example, at a Reynolds number of say 300k or less, use a coefficient of lift of 0.7, and for a Re of over 1 million, use 1.2.

As it pertains to roofline shape, use the following modifiers:

  • If you have an open top, divide the wing’s downforce by 2.5.
  • With a choptop (or a hardtop with the window removed), cut it in half.
  • With an OEM hard top (also probably applies to a convertible with the roof up), use the figures in the table as is.
  • With a fastback, multiply by 1.25.

If you want to nerd out on it, you can experiment with wing shapes, angles, turbulence, and lift using the NACA wing calculator (change the NCrit value) and see for yourself. If you want real values, you don’t do calculations, you hire a professional to measure the real-world differences on your car.

Light Makes Right? Wrong.

Every racing series strives to have fair competition between dissimilar cars. The NASA Super Touring series, Gridlife, Pineview Challenge Cup, and many others use the weight of the car divided by horsepower as the primary balancing factor. Any why not? Mathematically, that seems like a good way to create parity between different cars. Well, if they were all racing in a vacuum.

In the real world, air has resistance, and overcoming resistance requires power. Drag increases at the square of the speed, so there’s about twice as much drag at 100 mph as there is at 70 mph.

Let’s take a look at two cars with identical shapes. They both have a frontal area of 20 square feet and a coefficient of drag of .48 (typical race car with windows open). The cars differ in weight, one is 1800 lbs the other is 3000 lbs.

If I input those numbers into the RSR calculator, I find out that it takes about 81 hp for the lightweight car to reach 100 mph, and about 85 hp for the heavyweight car. So weight is not a large factor here. For both cars, 60 hp is lost to drag.

Now I’ll increase the speed to 120 mph. The lightweight car needs 137 hp to reach that speed, while the heavy car needs 143 hp. For both cars, over 110 horsepower is lost to drag. If these cars were in the same class based on 12.5 lbs/hp (GLTC), the light car would have 7 hp remaining at 120 mph. The heavy car would have 97! Because these cars have the same power to weight ratio, they both accelerate at the same rate. But once they reach 120 mph, one car can barely maintain that speed, while other has power to spare.

In other words, at a faster track, you want power at the expense of weight. To illustrate that, let’s see what happens in OptimumLap when we add power, but keep the power-to-weight ratio the same. In the following table, all cars are at 20 lbs/hp. I’ll run simulations on three very different tracks, Watkins Glen, New York Safety Track, and Pineview Run. (Conveniently all within 100 miles of me.)

LbsHpWGINYSTPV
180090150.10104.9678.55
2000100149.14104.6278.47
2200110148.36104.3578.41
2400120147.53104.0278.32
2600130147.00103.8478.28
2800140146.54103.6878.25
3000150146.00103.4778.19
All cars at 20 lbs/hp, Cl -0.3, Cd .45, Cg 1.1

As you can see, the heaviest car is the fastest at every track. It’s a small difference at Pineview, and a significant 1.5 seconds at Safety Track, but a whopping 4 seconds at Watkins Glen. The simulator shows the difference in top speed is 12 mph at WGI. Wow. A difference of 4 seconds and 12 mph with cars that are supposed to be equal!

But don’t lighter cars corner faster? Not really Friction (grip) depends on weight, and the more weight, the more friction. Don’t lighter cars stop faster? Not really. A lighter car still has less grip because of less weight. The simulator factors all this in so we don’t need to.

But it’s worth noting that OptimumLap is a single-point-mass calculator. It can’t factor in elevation changes, track camber, or the fact that your car has four tires. When cornering, the outside tires are loaded more, and on a heavier car, even more. At some point you get diminishing grip from the outside tires, and so lighter cars do grip and change direction better than heavier cars. I just can’t simulate that in OL.

Another way lightweight cars are handicapped is by having to run skinnier tires. In many series, a lightweight car might be limited to a 205-width tire, while heavier cars could use 225, 245, or whatever. And what’s the logic behind that? I’m not sure. As you can see, the lightweight cars are already at a power disadvantage (anywhere outside of a vacuum), and limiting their tire width only makes it worse.

In the NASA system, not only are lighter cars penalized with narrower tires, but as cars get lighter, they take additional penalties to HP. So if your car weighs 2450 lbs, it takes a .4 lbs/hp penalty. If it weighs 2250 lbs, that’s a .5 penalty. And so on. This is completely the opposite of how it works in the simulation, because lighter cars need more power to be competitive.

Ideally, there should be a reverse corrective factor that balances the lbs/hp ratio for cars of any weight, so that lighter cars get a bit more power, and heavier cars less. Let’s take a look at what that factor could be. Ideally, you’d like to see the 1800-lb car, the 2400-lb car, and the 3000-lb car in the same second at all tracks, not 4 seconds apart.

If I do a corrective factor to figure lbs/hp like this:
Lbs/(HP+((2400-Lbs)*0.016))
Then I get simulated lap times like this:

LbsHpAdj HPLbs/HpWGINYSTPV
18009099.617.65148.32104.0278.21
2000100106.418.52148.00104.0178.25
2200110113.219.30147.74104.0178.29
2400120120.020.00147.53104.0278.32
2600130126.820.63147.52104.1478.39
2800140133.621.21147.45104.1978.45
3000150140.421.74147.38104.2578.50

The heaviest car still wins at Watkins Glen, but it’s only a 1 second difference, not 4 seconds. At Pineview, the lightest car wins, but only by 3/10ths. And at New York Safety Track, it’s mostly a draw until the cars get heavier than 2400 lbs, but then the greatest difference is only 2/10ths instead of 1.5 seconds.

Naturally the formula could be adjusted slightly, by using a different median weight (2600 lbs instead of 2400 lbs, for example), or changing the hp factor from .016 to a lower or higher number. In any case, for a series that used lbs/hp series for classing, they can make it more fair by using such an adjustment. Of course they won’t, but data supports that they should. I mean, ideally, they’d have a different adjustment for every track….

I wrote the rules for the Pineview Challenge Cup series, and those rules also use lbs/hp as the basis. Pineview is a slower track (72 mph top speed in this simulation), so there’s much less in the deltas. One of the benefits of this series is the thin rule book, so I’m not tempted to complicate things. But if we ran our series at any other track, I would make an adjustment like this.

The reality of it is, if you race in a series that uses power-to-weight ratio as a balancing factor, you’re better off with a heavier car and more power. This often gets you into a wider tire size, as well. Adding lightness is adding slowness.