225/45 Setup "Slow Track" Question
 

Hi Guys,

This spring I'm hoping to get my NASA race license and take my 1999 Spec Miata into NASA TT6, eventually ST6 and then eventually SM. Starting slow cause SM is so competitive/hardcore, it is a bit scary to jump right in. TT/ST6 point system gives some room for easily reversible mods on a Spec Miata, so my goal is to remove restrictor plate and probably end up at 120-125whp at 2,400lbs comp weight. With this, I could also upgrade from 205/50 Toyo RR on 7" wheels to 225/45 Toyo RR on 9".

I did a bunch of reading here and Emilio (and others) have stated that if running under 140whp, 225 may actually be slower that 205 on on faster tracks, but will almost always be quicker on slower tracks. My question is: what are some examples of Slow tracks where a 120whp NB running 225 RR tires would be quicker than 205 RR? I run all my events in CO region (5,000 feet elevation, so until I dyno the car, 120whp is just a guess).

My usual tracks are High Plains Raceway and Pueblo Motorsports Park. Both seem on the slower side -- 2nd and 3rd gear everywhere except for 1 straight, but High Plains has two big uphill sweepers that can bog SM cars a bit. In SM form my highest speeds on the straights have been around 105mph. Given this, are 225 a no-brainer?

Thanks!

In the realm of TTE/TT6 config when referring to size they're typically talking about R7s. Hoosiers run really wide, a 205 R7 actually measures out to 226mm on a 15x8.5(the golden setup for TT6). Then the 225 R7s are pretty close to 240mm. So if you're speaking about Toyo RRs only, you'll be quicker on the 225s vs the 205s especially considering the wider wheel as well.

Thanks flier. The TT/ST NASA points system definitely seems to strongly favor Hoosiers, but I don't care about winning enough to burn that kind of cash.

I suggest you do some more research. If your little heart is set on SM, then I would just set it up for SM now and just use your time in TT to dial in your setup.

There isn't a lot of crossover between TT/ST6 and Spec Miata. Meaning the ideal TT/ST6 setup isn't a spec miata, and optimizing for TT6 won't have any SM legal mods. So you would need to change everything over: ECU, intake/exhaust, all other engine mods to recover a crapload of power, coilovers, wheels/tires, sway bars, etc.
And the elevation is going to be a bigger issue if you want to be competitive. The ideal setup for the RR is the 225/45 at 2450lb comp weight. Assuming you don't take dumb penalties like ELBJ, aftermarket brakes, etc. you can run 138whp max avg. Which if you add a couple HP of margin for dyno variation would be about 135whp max avg, or an estimated 15hp down on your current guess. That's a good few seconds per lap just on power. You're going to need to probably build the engine to make the ~140whp peak power at altitude, likely e85, and multiple tunes since if you ever run at lower altitude, you're going to be over your power limit.

I guess it depends how competitive your region is and how competitive you plan/hope/expect to be. If you don't want to dump money into the engine to find the power, then the RR is definitely not going to be your fastest option. I'd suggest the R7 and ditch as much weight as possible to hit your W:P limit.

He's not going to have the power required to maximize a 225 R7, so that would just be wasted aero drag and points.

Thanks, but all I was hoping to learn was: what constitutes a "slow enough" track such that a <140whp Miata would be faster on 225/9 vs 205/7.

Yeah agreed, read my post again? I'm referring to the 225 RR


So in regards to a "225" vs a "245" I can't think of a real-world track where the much larger tire would net a faster lap time. And again for a 205 RR vs a 225 RR the 225 RR is going to be quicker in nearly all tracks except maybe something like Daytona or other high-speed rovals? Also, you don't have to run the 205 on a 7" wheel if you're worried about rolling resistance really. A 205 RR will perform better on an 8" wide wheel as you'll have more grip overall with the wider wheel

Thanks flier. This is helpful. High Plains and Pueblo in CO are nowhere near as fast as Daytona and Rovals -- 90% of the lap is 2nd and 3rd gear.

I think I'll go with 225 on 9" and see how that works out. Good suggestion on 205s on 8" ... this can be my backup plan in 225s end up being too much on my car.

One more consideration is tire expense. With cheap 200TW tires, I've found it difficult to avoid wearing out shoulders prematurely in 225/9". The 205/8" combination gives me better life. Interestingly, the RRs wear evenly in both sizes. I attribute that to sidewall stiffness. You also get more options and cheaper prices with 205s. If what you are trying to accomplish is max seat time prior to going SM, then I would run 205.

That is weird. 205 on 8 should have about the same stretch / sidewall support as 225 on 9.

I understand this has gone well beyond your original question, but here's another point.
Racing in SM you are going to want at least 8 rims, with 12 + a spare being a more comfortable choice. That is not cheap.
The value of spending a bunch on larger rims you can't use in SM is a factor you should consider.

Sounds like you want to develop your driving and get seat time. Remove the restrictor plate, and go out and drive the otherwise SM car.
Get a year under your belt then join the tail end of SM, you will learn fast there.
Save the big rim money and use it for occasional buys of Hoosiers. Run RR the rest of the time.

That said, engineeered2win gave good advice on how to build a TT car.