gtred

I've got a local track that requires a little taller rear gear. (6sp + 4.10 rear gear)

I've been running the standard 23" tall (225x45x15) miata race tires, but find myself having to shift-up briefly at the end of the straights, and I find myself often shifting mid-corner.

Last weekend, I tried a set of 24" tall kumho tires (215-40-17). This worked out great with the shifting. But, I wonder how this would effect the suspension set up? I'm wondering what the trade off is in lowering the car to account for the taller tire, or if this will this just put me down too hard on the bump stops (FMC-SM bumpstops)?

For you folks that travel to multiple tracks: How do you deal with gearing the car for each track?

Savington

iTrader: (31)

They will rub the top of the fender at full bump. Lowering the car will make this worse.

rharris19

iTrader: (41)

N/A, a 6-speed with a 4.1 can be a pretty good combination. If you are turbo, then that is a different story.

If this is your local track that you spend the vast majority of your time on, then I would look into regearing the car to a 3.9. Since going up to a 24" tire solved the shifting issue, this should do it properly.

gtred

Yes, the other tracks in the area SIR, Ridge, ORP, Thunderhill have shorter straights and work out pretty well for shifting. It's just Portland with it's chicane that has me upshifting from 5th-6th for just a second... or bumping off the rev limiter (already raised to 7500 on a stock valvetrain) at the end of both the front and rear straights.

I am wondering if Sav is using the xida's, which allow for more wheel travel into bump than the bilsteins; and that is why a taller tire rubs... ? Well, actually, I'm hoping that is the case.

I lowered the car down to the standard racing ride height yesterday and I'll get a chance to test out a 24" tall tire at the track this weekend. If it rubs, I may just have to compromise ride height a bit and test which combo is faster; 24" and high or 23" and lots of quick shifts.

Is the 3.9 offered as OEM in other miata's? ... it doesn't look too hard to swap out.

Savington

iTrader: (31)

I'd be surprised if they screwed up the Bilsteins that badly. That missing bump travel would be immediately noticeable. If the shock is done correctly, you can't just lift the car half an inch to compensate - it will still find the bumpstop over curbs and such, and you'll still get a bunch of tire rub as a result.

The 23" tall tire and more shifts will almost certainly be faster.

3.9 Torsens were available in 99-02 6-speed cars.

antraasa

I'm happy with my 4.3 on my '99 5-speed on the track. EXCEPT when I'm driving to the track at 4200 rpm at 75 MPH...it hurts to feel the engine straining. I know theoretically a 3.9 will lower the rpm 10%. Is that compatible on '99's with a 5 speed, better for the engine during 2-3 hour drives to the track, and will it hurt track performance? Should I make the change??

Andy

Double O 86

4200 rpm? That's not straining, that's a happy engine singing.

I have the 3.9 with a five-speed, and I go 80 mph at 4000 rpm. It sounds nicer, feels nicer, but the engine doesn't give a shít as long as I don't hit the rev limiter.

Also, this combo is great at Texas World Speedway. I use third in two places, fifth on the big straight, and the rest of the track is handled by fourth.

However, at MSR Cresson, I long for the 4.3, as third gear is too high for the low-speed corners and I never use fifth.

Hopefully, there's something helpful in this post.

gtred

I have a set of 6ul's and 23" tall hoosiers on order. I think I've sourced a 3.9 torsen.

Thanks

antraasa

Thanks DoubleO. I'm going to stay with my 4.3 and 205/50x15 for the track set of wheels/tires, but may do something like 195/55 for DD wheels to get my rpm down a little - cheap/easy way to do (need new DD tires soon anyway). My actual speed is about 2 mph slower than my spedometer, so that should correct that aspect. Anybody see any issues with using 195/55's for DD...?

I do like the 4.3 on the track. I think I'll stick with the current rearend for now...thanks!

By the way, I use 3rd gear on 3/6/10 at TWS. I used to go into 3rd on 9 but then rev limit (or 4th then back into 3rd into 10 was too many gear changes).