Converting the Budget Spec Miata to ST6


I spoke about this before in Trying Not to Finish Last at NCM. Although the budget Spec Miata is a blast to drive, it will never be competitive when the expensive equipment comes out to play. That is why I have decided to move the car to Super Touring 6 (ST6).

The move to ST6 will open up the rule book and allow me to build the Spec Miata to a power-to-weight limit that will closer mirror the front-running Spec Miata field.

Identifying the Budget Spec Miata’s Shortcomings

My Spec Miata is a 1.6l NA chassis that is allowed a curb weight of 2275lbs with the driver. The lightest I have been able to get to is 2335 with the driver and there is very little if any weight I can legally remove from the car. Making curb weight, or lack of hitting the minimum, is issue number 1. The result of extra weight pushes the power-to-weight in the wrong direction.

Issue two is the car’s lack of power and torque. On the dyno at NCM, the NA ran 118hp. Due to the dyno operators experiencing a tach signal issue, I had to back into the torque numbers. The torque was estimated to be 98 ft-lbs.

Combining these two issues, we can compare the NA to the expensive equipment. Namely the 25,000 plus dollar NB Spec Miata premium builds. These cars can dyno as high as 130hp with similar torque, with a competition weight of 2400 lbs.

With these figures, the power-to-weight difference is huge.

CarWeightHorsepowerTorquePower/Weight HPPower/Weight TQ
Budget SM23351189819.7823.82
NB SM240013012818.4618.75

The problem jumps out immediately when looking at the power to weight. And running the numbers with the minimum weight of 2275 still lands at 19.27 for hp and 23.21 for torque. A world away from where I need to be to run at the front.

There are a few solutions here:

  • Drop money into a new engine and move the power-to-weight in the right direction. The cost would probably be more than I paid for the car and most likely wouldn’t get me there. Besides, the current engine runs well with good compression.
  • Abandon the NA for a top-tier NB build. The big drawback is a cost of 25k plus.
  • Forget the SM rules and build the car to Super Touring 6. Allowing me to achieve a 19 to 1 power-to-weight by almost any means I see fit.

The solution here is to have fun without spending a ton of money … yet. As the title states, ST6 here we come.

The ST6 Build Plan (Part 1: Weight Reduction)

I haven’t been able to really play with the rule book since my drift racing days so I’m pretty excited about this one. Step one, getting the power-to-weight down to 19-1 through weight reduction. On power alone, I am allowed a curb weight of 2250 which means I have 85 lbs to remove. The upside, weight reduction is a free mod.

I performed this step in two phases. Step one was gutting the headlights, removing the blower motor from under the dash, and deleting the charcoal canister as well as the IACV (Idle Air Control Valve) and a throttle body coolant delete. I will walk through my zero cost IACV and Throttle Body Coolant delete in a future article. (insert here) The result is around a 40lbs loss, moving the curb weight to 2295.

Here’s the carnage from round 1 of weight reduction on the NA Spec Miata to move it to ST6. Blower motor, IACV and TB Recirc delete. Charcoal canister is in here somewhere and the pop-up headlights are gutted. Also got rid of the zero-purpose front lip.

Weight Reduction Part 2 (Goodbye Dash and …)

Although 40 lbs is a good start, it’s not even the halfway mark. Part 2 of the weight reduction consisted of removing the entire dash, a bunch of wiring, the tach, any ancillaries devoted to the AC, miscellaneous and unnecessary shielding etc… Pic below, but the result is another 30 lbs and inching closer to the 85 lbs needed.

Round 2: Full dash removal as a bunch of misc. wiring and random shielding. (30.6 lbs total)

Next Steps

As of now, the current curb weight is around 2265. Below the Spec Miata limit but well within the ST6 power-to-weight yielding 19.1 lbs per horsepower. Moving forward, I plan to sort the chassis by upgrading the suspension and bushings. But as a teaser, I ran the car at Barber Motorsports Park in July, and it was 2 seconds faster than it was the previous year in the Spec Miata trim. I’m optimistic that there is a lot of time left in the car with changes I have in mind. Stay tuned for more updates on the ST6 journey.

By Chris Simmons

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