mekilljoydammit

OK, I know this is a bit off the beaten path for around here, but bear with me. I'm putting together a plan for a build to an SCCA roadrace class; rules are up to 245 wide (edit: DOT) tires, on wheels a maximum of 8 inches wide. So I know that the kneejerk reaction would be run 15x10s with 245s, but it's not in the cards - are they useless to run on 15x8s, or would I be better off dropping to 225s or even 205s?

Joe Perez

iTrader: (8)

I'm speaking from the Hoosier spec sheets rather than from personal experience, but they spec a wheel width of 8 - 9.5" on the P245/40ZR15 R7, and 7-10" on the 245/580R15 radial slick.

Efini~FC3S

iTrader: (4)

What class? STU? I assume STU since that's the only class I can think of that has a 245mm max with 8" max wheels.

What car and engine are you planning on building?

To answer your question though, 245 tires on an 8" wheel will be faster than a 205 or a 225 on the same 8" wheel. It may not "feel" as good, may even be harder to drive, but it will be faster.

I can almost guarantee you all of fast guys in STU are running 245s on an 8" wheel.


I race in STL, and the rules are similar. 225mm max tires on 7" max width wheel. All of the fast cars on the class are running 225s. For sure, a 205 will fit the 7" wheel "better", but if you want to be at the front of the pack you run the widest tires possible.

Savington

iTrader: (31)

How much power and what sort of front aero package are you allowed to run?

mekilljoydammit

Oh crud, forgot to mention DOT - edited OP. The 245/580R15s would actually make this an easier question as they're designed for 7" wide wheels.

So, I completely agree that the spec sheet says it will fit, but from doing my reading around here, it definitely seems like narrow wheels are well away from optimum for the 245s, but is that enough to make the 225s a better choice?

Efini~FC3S

iTrader: (4)

Again, assuming STU class, front aero allowed is full air dam with 2" splitter (2" past the original bodywork as viewed from above). Minimum "ride height" of the splitter is 3.0".

Engine swaps within manufacturers is allowed, so Mazdaspeed engine and turbo would be allowed, as would rotary, or Mazda V6 of some kind. I would expect between 250-300whp if an engine was built to limit if the rules. More like 200ish if using a N/A BP.

mekilljoydammit

Yeah, SCCA STU - intent is EProd spec 13B, so (depending on development and yeah, I know it's not a turbo) 200-230whp. Front aero is air dam not visible from above and with a minimum ride height of 3", splitter allowed 2" forwards of bodywork as viewed from above with no vertical deviations, and undertray to front axle centerline. Rear wing is the 48" wide APR GTC-200, or something custom with less than 48.25" span and less than 8.5" chord. No widebody but rolling fenders is allowed.

I'm still in the gathering parts stages, but getting things figured out so I can look for deals never hurt anyone.

Savington

iTrader: (31)

At that power level, plan to test both the 225 and 245. It may come down to different tires per track.

Efini~FC3S

iTrader: (4)

What part of the country do you race in? Are you planning to run Majors events or just local regional stuff? What are your goals with this car?

The reason I ask is it's going to be really hard to be competitive in STU with 230hp, even with a 2300lb min weight.

The fast FWD guys have the same power to weight or better, and likely have 250+ at the wheels. And then there's the supercharged Exige that can run at basically the same weight as you. And there are 300hp+ BMWs and Porsches to contend with.

mekilljoydammit

I'm in Wisconsin, closer to Blackhawk than Road America. As for goals... honestly? STU gets me a car that's at the pointy end of the big IT run group in Midwestern Council and towards the pointy end in its run group even at SCCA Majors around here, much less divisionals - the class itself isn't heavily contested but there's a lot of other stuff in the groups. I'm not under the impression I'm building Ye Hot Runoffs Champeen car, because I'm sure as hell not that calibre of driver... but I think I can keep the operating costs pretty reasonable, and I've been building rotaries for Prod for 15 years now so I'm reasonably confident in the power I stated with the parts I have onhand, and possibly more with development. Plus it sorta seems like the boosted stuff has been dialed back a bit since the last runoffs where that Elise was a factor.

*quickedit* Did some poking around - according to a sale ad, the 2014 runoffs winning S2000 was at around 230hp to the wheels and 2600lbs, consensus is that the BMWs are maybe around 270hp and the Elise got another pulley reduction over the winter - from dyno sheets I've seen, that pulley and stock cams is in the 230-240hp range. Again, I'm not expecting to have a world-beater, but I don't think 230hp with 2300lb is automatically dead meat. */quickedit*

Sounds like the idea of 245s on 15x8s at least isn't ludicrous - ah well, if you could do everything without testing, why would there be data systems and such?

Darkar318

I figured I would post in this since i got a hit on this thread when searching for Eprod 13b.

Louis Rivera out of Texas has been running a Miata in STU and is leading in points for the Hoosier Super tour.

His car is a street ported 12A, and I am not sure, but i think it is on 225s, or at least it was when i saw it at Barber.

You can run a stock renesis at 2300lbs, street ported 13B FI or carb at 2300, or 12A street port at 2250.

From what I have researched, E prod 12As make 215HP and E prod 13Bs make 230.

mekilljoydammit

I think the state of the art is a bit past that in terms of power. A few years back Mazdatrix was selling parts from one of their very highly developed engines - they claimed horsepower at the flywheel in the 280 range. Louis Rivera used to do the engines for Tom Thrash (RIP) and he knows his stuff - I know Fastrack quoted 215hp for the 12A and 230hp for the 13B years ago for Thrash, but choke sizes have gone up since then, and it's never been admitted whether that was at the flywheel or wheels.

The irons and housings I have were done by Sunbelt but I've never seen a dyno sheet.

Heck, maybe Rivera will be at Sprints next weekend.

Efini~FC3S

iTrader: (4)

I didn't realize Rivera is running a rotary Miata, that is interesting.

I wouldn't put too much stock into the fact that he is leading the Super Tour nationwide points standings. He's done three Super Tour weekends (more weekends then most of the other STU drivers) and the STU fields he's raced against have been pretty small. Looks like he had three competitors at NOLA, three at COTA, and four at Hallet.

Compare that to STU fields of 10-11 cars at VIR and Mid-Ohio, with multiple national champs in the fields. It would have been interesting to see how he would have stacked up at those races.

If he's planning on taking that car to the runoffs I guess we'll see how a rotary STU Miata fares. I think he'll have an awful hard time competing with the 250+whp Hondas at Indy.


Also, the guy in 2nd place in the Super Tour points standings is a chump and his car is so-so. So again, I wouldn't read too much into those standings.

Darkar318

Mekillyou, Did you ever build an STU car?

STL/STU is something i am interested in, especially with a rotary.

mekilljoydammit

Could be. Tighter tracks might be better though - power/weight could be competitive with as much weight as the K Hondas are socked with. Is that much really doable with the required OEM manifolds, and for which combination exactly? K24 is at something like a minimum weight of 2600 pounds.

Like I said before, it's more that I want to build the combo rather than I think it'll necessarily be a runoffs winning combination, but from sales ads (where people have no real reason to downplay their power anymore) I don't know that the frontrunners are making as much as some people seem to think, or at least not anymore.

I'm kinda playing the long game here. To be blunt, I just got my comp license, so I'm getting seat time in the T4 NC MX-5 my dad and I built a few years ago first. I have a shell, engines (in parts), a couple trans options, a starting point for a rear end, and a bunch of other parts... I figure since I have another car to run in the mean time, might as well take my time and do it right.

Supe

I have a 15x7 wheel on my 944. Compared 205's and 225's. 225's nearing end of life were consistently quicker than the 205's with more life left in them except in November, where I couldn't get enough heat into the tire to bring it up to temp. Turn in didn't even feel any different between the two. I wouldn't hesitate to run the 245 on the 8" wheel, even if its not "optimal".