Last year, I converted the 1.6-liter NA Spec Miata to run in the Super Touring 6 class with NASA. This power-to-weight ratio class allowed me to shed the weight of the underpowered racecar aggressively. The result was a 2230-lb racecar with the driver, a reduction of 45 lbs. from the Spec Miata trim, and a more impressive 100 lbs. reduction from where the car started.
I felt the difference immediately as the car went from mid-pack to nipping at the heels of the front. This resulted in two consecutive regional championships in ST6 and a couple of track records. However, I am still encountering some heavy hitters who are outpowering me on longer tracks with better equipment. So, I have devised a plan to further improve the power-to-weight ratio through additional weight reduction.
Finding a New Dyno
Let’s look at where we started, where we are, and where we are going.
To start, I found a new DynoJet dynamometer ‘in town’ as I wanted to start this process with verifiable numbers. The last two times I have dyno’d was on the ‘wonky’ dyno in the Great Lakes region. Yielding 118hp and 98 ftlbs one year and 120hp and 420 ftlbs the next. Yeah, their stuff is messed up, at least on the torque side.
This time, I went to Multiple Viscosity Motorsports in Gadsden, AL. A little over an hour from the house. Once again, the dyno had trouble getting a consistent rpm signal from the car, but the owner, Johnathan, could manually calibrate the rpm. The result was a much more believable 117hp and 114 ftlbs. Manual calibration means the torque may have a slight variation. But Super Touring is classed based on horsepower, so the variance is unimportant. What is important is that I finally have a correct dyno sheet to calculate this build.
Power to Weight Revisited
The chart below shows the initial conversion to ST6, when the car started at 2335 lbs.
| Car | Weight | Horsepower | Torque | Power/Weight HP | Power/Weight TQ |
|---|---|---|---|---|---|
| Budget SM Start | 2335 | 117 | 114 | 19.95 | 20.48 |
| NB SM | 2400 | 130 | 128 | 18.46 | 18.75 |
| Current ST6 Trim | 2230 | 117 | 114 | 19.05 | 19.56 |
As you can see, the initial conversion to ST6 significantly improved the power-to-weight ratio. I was able to reduce the weight to 2230 lbs by removing the dash, fans, compressors, and IACV and replacing the battery with the 3 lb NOCO unit. The wiring harness and doors were also significantly trimmed.
Digging Into the Rules
Super Touring 6 has a base, minimum power-to-weight of 19 to 1. However, some modifiers allow you to shift this power-to-weight based on certain criteria. Using the NASA ST calculator, I can go a few ways with this build. For the Miata at 2230 lbs., I take a -.5 penalty weight and a -.5 penalty for double a-arm suspension. Allowances are +1.6 for Toyos, +.4 for BTM Aero, and +.3 for mechanical throttle. With these considerations at 2230 lbs, the maximum power I can run is 125hp.
Option 1: Add 8 hp and, hopefully, torque to the existing 1.6l engine. This is a tall task as this motor makes great power for a 1.6.
Option 2: Swap the 1.6l engine with the 1.8l NB engine and run a restrictor plate to cap hp at 125. This is probably the intelligent route, as it would add a proportional amount of torque with the extra displacement.
Option 3: At the current power, I can run the weight as low as 2100 lbs and stay right at the limit of the rules, drastically improving the power-to-weight ratio.
| Weight | Horsepower | Torque | Power/Weight HP | Power/Weight TQ | |
|---|---|---|---|---|---|
| Option 1 | 2230 | 125 | 122 | 17.8 | 18.5 |
| Option 2 | 2230 | 125 | 125 | 17.8 | 17.8 |
| Option 3 | 2100 | 117 | 114 | 17.9 | 18.4 |
I find it difficult to replace a perfectly running motor, especially one posting these kinds of numbers out of a 1.6. Knowing that additional gains are going to be near impossible without opening up the motor, it’s time to find another 130 lbs to remove from the car.
Extreme Weight Reduction Plan
I already run the car without a dash and have removed any ‘low hanging fruit’ from the car. This time around, I need to get creative. The easy avenues I have identified are to sell the factory hardtop and purchase a fiberglass unit. The fiberglass one weighs 15lbs compared to the almost 50lbs of the factory one. I will also cut and remove the trunk floor as well as trim the rear bumper and trunk. I also have a nasty habit of overfilling the fuel tank and running with all 10 gallons. In reality, almost every race is 5 gallons or less, so an easy 30lbs. from just proper fuel management.
Depending on where that leaves me, a Lexan windshield will most likely get installed, and then it will be a matter of finding a pound or two from various places. I am also going to focus on removing a few pounds from the driver.
What’s Next?
All in, I think another 130 lbs off the ST6 Miata’s race weight is doable with some serious effort. I plan to execute the plan before the November event at Barber Motorsports Park, as I have plenty of baseline laps to compare to. I will add pictures below as I make these changes to the car for reference.
