Beyond Spec Miata: Spec Mustang/Iron

By Tom Martin

April 06, 2015

Image: Finish Line Productions

If you are racing (or thinking about racing) a Spec Miata, and you want to try something different, one obvious direction to go is to get a bigger, more powerful spec car. And to compound the obvious, nothing could be more clearly powerful yet affordable than a V-8 powered Mustang.
“The best true spec V-8 series out there.”
Spec Mustang (SCCA) and Spec Iron (NASA) are the places to look if a relatively low-cost Mustang or V-8 spec series sounds appealing. And if your reference point is Spec Miata, you’ll probably understand it. Corey Rueth, NASA Eastern champion in Spec Iron puts it simply: “I love racing in a spec series.” He adds “I was drawn to SI because I’m a Mustang enthusiast.”
Both SMG and SI classes use S197 Mustangs from 2005-2010. Both focus on the 4.6L V-8 version of that car, so even if you’re not a Mustang person, you might enjoy racing these cars because they are V-8 powered. 
Specified Specifications
The SCCA SMG weighs 3475 lb. with driver, while the NASA SI car weighs 3350 lb., again with driver. Besides weight, there are many detail-level differences between the two cars. SMG, for example, runs a Cortex suspension package. SI cars are based around Ford Racing suspension parts with Hanchey Vehicle Technologies dampers and open springs. Both SI and SMG run on 275mm rubber, though NASA specifies Toyo RR and RA1 tires, while SCCA leaves the brand to the driver. SCCA requires a specific wing and a choice of splitters; NASA limits you to OEM bodywork.
In both NASA and SCCA, the rules are written around an OEM engine that isn’t blueprinted or ported. SCCA allows headers, and each group allows a few other external bits to be changed. NASA polices this with measured weight:power (minimum of 11.75:1) and weight:torque  (minimum of 11.25:1) ratios. SCCA, for once, goes down a similar path of limiting cars to 315 wheel horsepower and 325 lb-ft of torque. That’s 11 lb/hp, which seems a little juicier than NASA’s approach but also seems unlikely to happen if you play by the rules since the 4.6L modular V-8 was only rated at 300 crank horsepower. NASA’s limits imply 285 whp maximum (though you can use a restrictor plate to back it down to the required ratio). Rueth says 285 whp or a bit more is achievable.
The primary implication of the rules is that it would be difficult to run a given car in both Spec Iron and Spec Mustang. You may not care because you prefer one group or one group offers all the racing you need. But this bureaucratic shortsightedness with two conceptually very similar cars is bound to limit car counts somewhat at any given event. We should note that SMG cars fit nicely in SCCA T2 as well, so you may end up with a car that can run in two groups per weekend (if it happens that SMG and T2 are in separate run groups) should you choose the SCCA path.
The Cost
Nonetheless, a number of cars have been built. We’ve been to run-of-the-mill NASA events and SCCA events, each with with multiple cars in their respective SI and SMG classes, so car counts are progressing. We think that’s because the cars are pretty attractive especially given that the cost isn’t that much more than with Spec Miata:
.  Donor cars are well under $10k, with lots of examples around in the $7k region.
. The components to build a car will cost around $10k (with many of the components being optional, you also don’t have to add everything from the start)
. Labor and a cage will add anything from $1k to $12k depending on how much work you can do yourself
. The wild cards are a) repairs to the donor vehicle because of age/mileage and b) options you want to add (data, cooling, ergonomics)
. But all-in-all you should be able to have a decent if basic car built for you for $30k
The Draw
Now, this is perhaps $10k more than an SM, assuming you do a new car (of course used Spec Miatas are readily available for less). Let’s look at what you get for your extra money or time:
.  A big step in weight:power. An SM has about 20 lb./hp and SMG/SI is running around 12.
. Bigger rubber, bigger weight and a solid axle, which mean more variability in how the car behaves with power application, bumps and braking.
. A different look and sound package
We’ve spent a lot of time behind the wheel of a Mustang, not exactly in this trim but close enough, and we can say that the car is loads of fun to drive. It is surprisingly similar to how you might imagine an SM with a lot of power and exaggerated weight transfer. For that reason alone, an SI or SMG car may be just the ticket for Spec Miata drivers looking for something new, or Spec Miata image haters. 
+ Beyond Spec Miata