In a previous story, we investigated America’s Spec Miata series, and looked into exactly why it has become the most popular club racing series in the US.
This time, we investigate the Spec Miata rule book in detail, and look at what lessons can be taken by someone building a budget track-day MX-5 in the UK.
The reason for basing a build on these existing rules is to save time and money through not reinventing the wheel. The Spec Miata rulebook has been developed and refined over the past ten years to provide the best possible bang-for-buck, so why bother starting from scratch?
Other than the fitment of a full roll cage, the biggest deviation from standard is in the area of suspension. Suspension is the biggest investment in building a Spec Miata-type car, but it’s also where most of the gains over standard lie.
Making it stick
MAZDASPEED Motorsports have developed a kit comprising of Bilstein dampers, Eibach springs, coil-over kits and K-Spec anti-roll bars, and any number of retailers will supply a kit in its entirety. Unless you really know what you’re doing with suspension setup (matching damper valving to spring rates, for example,) installing a kit like this will give you a sweet-handling MX-5 right out of the box. This will allow you to spend more time developing your driving skills, learning tracks and having fun.
Other than the fitment of adjustable links on the antiroll bars, no other suspension modifications are permitted. Ride height and wheel alignment are free, giving competitors the chance to experience the effects of minor setup changes. Properly corner-weighting the car will shave even more tenths, too.
Buying a whole kit like this may be a bit of an expense up front. However, unless you have experience, starting from scratch is always a risk. It can take several track days to properly set a car up, and if you’re not careful you can end up with a car that’s actually slower than it was before you started. And to add insult to injury, it often isn’t any cheaper.
Wheels are limited to 15×7” single piece with a minimum weight of 5.8kg. The control tyre for Spec Miata is the 205/50/15 Toyo Proxes RA-1, but any tyre that has been DVLA approved for road use is eligible in state championships. Semi-slick tyres such as these do represent a significant investment over road tyres, but once they warm up, they are worth every penny. Thankfully, an MX-5 is not heavy nor powerful enough to wear them too quickly.
Spec Miata uses the standard MX-5 brake calipers with the smaller 255mm (front) and 252mm (rear) rotor. The rules permit the removal of the handbrake mechanism, and brake pads and fluid are free.
Making it go
No modifications are permitted to the engine other than the replacement of the air box with a pod-style filter. So long as the factory exhaust manifold remains, the rest of the exhaust system is free. This retains the MX-5’s trademark reliability and will ensure many happy track days before a rebuild is required. Plenty of people even race on old, high-mileage engines.
Gearbox and final drive ratios must remain as the vehicle was supplied, and either the factory Torsen or MAZDASPEED limited slip differentials can be fitted.
No modifications to the chassis are permitted, save for the usual weight saving measures of removing unneeded interior trim and the soft top. A factory hard top can be fitted, and the use of a fixed-back racing seat and five-point harness is mandatory.
The list of modifications that can be made to an MX-5 to improve it for the track is endless, and as you spend more time in your car, you will likely find other modifications to make in the never-ending quest for lower lap times. But as a place to start, a beginner could do far worse than to base a build on the Spec Miata rule book.
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