.......use the stock RX7 5 speed. If not, what do you use and why???

 

A lot use the stock box as it is required by the rules. I use the stock box with Miata gears because the ratios are closer.

 

I wouldn't if I didn't have to, but at the time the class rules required it. Now that it's PT, I could swap and take a points hit, but I'm not racing and I don't care

 

I have not had a problem longevity wise with a stock box. You can't be Ricky Joe speed shifter. I rebuilt one that went thru 2 seasons and 2 12 hours without a problem.

If you want better ratios
Rear wheel drive 626 gear set converted
Miata gear set converted
PBS
G-force(?)

 

I'm not yet in class and will be doing Time Trials, most likely. I have one hot swap unit available. Both have 5th gear mod from the 929 to make it a pulling gear. My first unit lasted over 100k miles, with 40k of those at 400 rwph on the street. The second unit has seen 4 days of HPDE 1/2, will see another 2 days of HPDE3 next month.

 

I run the stock box with Miata gears 1-4 and the truck 5th. As Mark said above, much less of an rpm drop off between shifts. In SCCA production racing you take a weight penalty moving from a stock box, to alternate gears, then to a dog box. I think it's 5%. I'm at 2153# with alternate gears.

 

I did up the attached chart awhile back comparing Miata and FC gears showing the drops between shifts. My ratios were pulled from Mazdatrix site.

While just researching the truck 5th gear, I came across conflicting ratios for the Miata, not sure which ones are correct now.

You also need to take into account rear end gears. I run a 5.12

 

I noticed that the GCR (for ITS) listed 0.76 5th as a legal gear, along with the 0.70 and 0.71. I always thought that the 0.76 was from the GTUs but I looked back through the 2nd Gen FAQ and it lists the 89&90 GTUs as having the 0.70 5th (scans of the original brochures). Were those just misprints and the GTUs did come with the 0.76 final gear?

Also, where would be a good place to source a 0.76 5th? Can you swap in a one from a GSL-SE tranny?

Thanks,

-b

 

The GTUs had a .756 5th gear. Get it from MazdaSpeed or I have a 2nd Gen Box w/ the .756 that was built with almost all new parts (receipt for $1500 or so from MER), put 6 weekends on it then had it refreshed and it's been sitting ever since. I don't need this box and one day will get around to officially putting it up for sale. PM if interested.

 

When looking at OD ratios you need to consider what the input ratio of the trans it's used in is. The Miata input ratio i shorter than the RX-7 NA for example. So using the same OD gear set in a Miata will give it a shorter output ratio than with the RX-7 gearset.

 

I've run the stock TII 5 speed in my 400 rwhp FC. I'm on gearbox #3. The stock boxes are ok, but I've dropped 3rd gear twice now. I do push the shifts a bit when I'm in hard traffic, but mostly I'm pretty easy on it. with the slicks and the beating they don't stand up for long. I'm hoping to get my Geforce T5 in this year before the Indy support race.

-Trent

 

Hey guys,

Is the truck 5th gear a drop in or is machining required?

 

Do you run a sprung clutch disc or a solid one?

 

The clutch is a Tilton dual disk 7 1/4" setup with two solid disks. I agree with where you are going, that the shock is what is ending the gearboxes, but there aren't a ton of options in light weight, high horsepower clutch assemblies. My compadre who runs a sprung ACT (I think) setup also took out 3rd gear last year during a time attack event. Same spot I did mine. Good shift, into the boost and bang, sudden loss of all forward drive. Add to that bad sounds from the gearbox. Quick check, 4th gear, yes, 2nd, yes, third, hello, third? I think the stock gears just aren't made to handle hours of road racing upshifts, downshifts and 400 rwhp with slicks. You put a strong clutch in the mix and the stock gearbox becomes the weak link.

-Trent