Lets Talk Gearing...
 

I have an issue, I have tall tires and tall gears, I'm running into bogging issues in tight areas because I am not a good enough driver to keep my RPMs up and the speeds are unnecessary.

The vehicle is an FC
I have 26" tall gravel tires (heavy)
4.10 NA rear end
turbo2 S4 trans
streetport s5/s4 mix with side draft.

the application is rallycross/stage rally

1st gear at 8000rpm is about 42mph and 2nd gear is even higher.

I know the 1st gen guys can swap in 4.72s from a kia and I can drop 1000 dollars from mazdatrix and get a 5.12. I am willing to upgrade to a turbo rear end if necessary as since they seem to share bits with the FD and Rx8 as well.

If I could find a GTUs rear end that wouldn't be too bad but a 4.33 isn't enough reduction, an Rx8 R3 has a 4.777 but I'm not confident that the ring and pinion will swap.

I DO however have a S1 Rx8 manual transmission that is "blown" but I've partially disassembled it and have found no damaged gears so it is likely a lockout or destroyed syncro. It would shift into reverse though. However I suck at rebuilding any trans these and lack the tools.

trans bolts up, the t2 input and output shaft are the same and possibly even a t2 starter would work since the flywheel is the same... Only would need slave cylinder from rx8 and fab up bracket to mount to body which doesn't look very challenging. and getting a driveshaft shop near me to shorten or lengthen something is very easy (read Detroit) by my location.

anyone have any good options, input or experience?

I whipped up some charts to give you an idea of what I've been playing around with:

https://imageshack.us/a/img22/2302/currentin.jpg
https://imageshack.us/a/img46/9040/desireable.jpg
https://imageshack.us/a/img703/3945/diffonly.jpg

I want to bring down my top speed in 2nd and 3rd gear for better acceleration, so I can turn the car better with the gas at lower speeds because I will be more in my powerband. I am stuck with tall tires. Granted the engine is getting tired and I think a small PP or half bridge is in the works so that my cure my issue by just having more power.

Your best bet using stock or modified stock parts is a 5.12 and Miata gears in a RX7 case. 4.88 will be about the same price and 4.30 while inexpensive will not be very noticeable. Do the diff first and the tranny 2nd if funds dictate. PM me your email address if you want a much better gear calculator.

Get a bridge port. I'm putting Miata gears in my car this winter so that 1st gear is useful for more than crazy tire spin. Maybe. Miata 3.1 and 4.78s is about the same overall ratio as FB 3.6 and 3.91s.

Really, 2nd gear is also crazy tire spin unless the car is pointed straight or the ground conditions are perfect. I have a solution for that problem too ;) Although the last time out with the car, my co-driver was posting respectable times by leaving it in 2nd even in the slow parts and just using the engine's low-end power to pull it around. Me, I feel icky giving it heavy throttle at 3000 because I think I have too much timing in that range, but it hasn't blown up yet so it much be okay.

Anything would be better than a 4.10 with your engine and tires...

Getting more power by raising the powerband when your gear ratios already don't match your engine is a...bad... idea.

86-95 Ring & Pinions, Bearings, and Seals

FD and RX-8s seem to share diff bits, and turbo FCs, FDs, and RX-8s can use the same aftermarket LSDs (apparently...), but it seems like FC diffs otherwise can't use any of the same parts as FDs or RX8s.

Just spend the $1000 and don't worry about it.

Also, not even Loeb himself could keep the rpms up in your car.

I wouldn't go with a turbo rear end... everything gets more expensive :P

(look at the prices for turbo 4.7s and 5.12s compared to N/A versions on mazdatrix)

I'd just stick some 5.12s in it an call it a day.


putting a 6 speed in my corolla with 4.5s made an incredible difference, the best modification I've done to the car... (also has no low end power)

I used to run the NA trans hybrid, by running a 1st gen 13b trans, you just have to swap the tail shaft section from the 2nd gen onto it and it bolts up, uses regular FC hydraulics, bellhousing, starter and etc. That trans has about slightly quicker gear ratio (across all gears but 4th5/5th) than a 2nd gen NA trans which has a quicker ratio than the turbo trans that I am currently using. I had ran this setup for a while but the transmissions are older and weaker and the clutch itself is smaller, and I did notice much more flywheel hotspots and clutch wear with the smaller unit, also finding syncro kits for that trans was tough.

The FD, FE, FC turbo rear ends I know you can swap the diffs themselves, by going to a torsen out of an FD or rx8 is not an issue, they all share an 8" ring gear but I believe the pinion and carriers are the difference.

In regards to porting, I was thinking of doing a half bridge using 4pt t2 irons and then lengthening the intake manifold runners for better mid range. Ideally the top end is great for crossing the finish line and straight aways but a good mid range makes the car very manageable. Obviously it has no torque because its NA.

When you say gears, specifically what do you mean? ring and pinion? the nest assembly?

I do need to overhaul my S4 diff as it is, the clutches are worn out, after X many miles or maybe just less than a mile the diff becomes more like an open and when I get stuck or high centered because it happens sometimes only 1 wheel spins and you can smell clutches burning from the wheel end that isnt spinning. pretty worn and tired.

He means miata transmission gears. all the internals from NA and NB miata trans will swap into an FC case.



and no, ring and pinions from FDs and RX8s won't work. the FC-T has a long pinion which is unique to only the FC. you can custom lengthen FD/RX8 pinions though which is what mazdatrix does for their 4.77s and 5.12 for turbos (which is why their so expensive.)

ahh okay TRANSMISSION gears, not diff. got it. I will have to compare them to my Rx8 trans gearing (listed in one of the photos)

Yeah I noticed that the ring gear are the same diameter but yes the pinion is the hassle thus mazdatrix outrageous price but I don't know where I can get that done cheaper or better. Better see what my tax refund is going to be...

the miata transmission gears are the budget option, its pretty easy to do it.

the Rx8 gears are even better, road racing there is a gear for every corner, its really close to the MFR race box from the early 80's, but with a granny gear 1st

the FC diffs (turbo and non) both have the long pinion, which makes gear swaps expensive.

Which miata transmission to people look for 5 speed or 6 speeds? certain years. If anything I am trying to make this thread useful for gearing ideas for people. If time and money allowed, I would repair my rx8 trans and run 5.12s with my fresh rebuilt rear end with a lightweight driveshaft.

here is the rx8 trans with 5.12s

These would be perfect gearing for my application

My favorite gear calculator is gearcalc from Quaife's site. It's simple and you can pre-program it with specific gearsets so when you want to play around with ideas you just say, okay Miata 6-speed / 4.10 / Michelin 65cm tire (yeah it has various competition tires built in) and your peak RPM and it spits out charts and graphs.

They never did fix the "Claculate" spelling error too :)

If it's not available anymore, PM me and I'll e-mail you a copy.

I have done far more research on this than I care to admit...and while i dont know the speeds in stage rally the speeds in road racing almost all ammount to the same thing. A 50 mph tight corner up to 125-130 at the end of most straights (At least for a mild non-turbo).

As others have mentioned FC rear diffs require some specialized machine work and the non-turbo diff is different and rapes from different sources (miata, kia, etc) than the turbo diff (RX-8, FD)

For road racing i have found the following: (adapt the info for your application as necessary)

1. The 4.30 is worse in most cases because it makes 2nd too tall to be useful but 3rd not tall enough...by any means.
2. The stock cases almost all have a big gap between 2nd and 3rd gears. This is the reality because the factory engineers wanted the best 0-60 times possible so they engineered 1 and 2 to give them that. But they needed reasonable 3-5 ratios for the highway.

So....the budget solution has been to go with a really tall final drive to get away from use of 2nd gear...so you end up using 3-4-5th gears rather than 2-3-4 (for the most part).

The proper way to compensate for tall tires is rear gear in ANY arena. Off-road, rally, etc...

I found the miata trans gear solution to be a non-starter. It does not eliminate the main problem, the big gap between 2nd and 3rd. While through the rest of the gears it provides a pretty small improvement. Even the RX-8 box still has this 2-3 problem.

So the first thing you need to do is a TALL final drive. In your case you might need to find that balance and may end up with a 4.78 being the right solution. Then if that isnt enough look at a Quaife close ratio set for the miata and have it adapted to the RX-7 or in the case of a turbo trans i think there are some expensive FD gear sets. What you are going to find is at that point you will ($ wise) be heading towards just going with a true race box with whatever custom gear set suits you best...

The other secret is mazdatrix is a middleman in the FC rear pinion machining arena...so you are just paying extra money for the fact that they are the only ones that advertise this option.

FWIW, in theory rallycross speeds never exceed 60mph, so with wheelspin say 70-80mph is an upper limit. (DRSCCA events are generally the slowest ones I encounter, only so much you can do with the sites you have access to AND stay safe)

....? 4.78 isn't a tall final drive. Higher means shorter gear ratios. 4.10 is pretty tall. 3.9 is ridiculously tall. People who do max speed trials often swap to automatic final gears. Especially with 1000 HP Supras and such.

If all else fails, I'm pretty some company in Japan sells close-ratio gear sets for the FC turbo transmission. But it will cost you. And you still might not like the ratios. Usually they only sell certain gears and leave the others alone.

Pretty much all Japanese sports cars stock gear ratios are going to be inappropriate for road racing. The final ratio is always set so that the top gear is quiet at highway speed. My MR-2 only gets to use second and third on most of the circuits I drive on except for places with long straights, and on short straights it's a wash between shifting and bouncing off fourth. And third gear's acceleration is shit (fourth is good by comparison, given the speed).

That said, Integra Type R's have very tight gear ratios.

I drove an Impreza STi with a lightweight flywheel and close ratio gears the other day... it was magical.


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Oh look, a close gear set with three gears (first second and third are all taller, second and third are much closer together)...it's only $2,500. lol. It would probably go pretty well with lower (numerically higher) final gear. I think the page says it's for S4 TII transmissions only. Also, the JDM tranny has a different fifth gear ratio.

There is also a five-gear set including an input shaft, first and second are slightly taller, third is slightly lower, fourth is the same, and fifth is lower, but for some reason it says you reuse the stock fifth gear... weird.

The RX-8 trans is almost exactly the same ratios as the Mazda Competition 5-speed box, with a granny gear attached.

Downsides is that you have to have a Turbo II flywheel/clutch/driveshaft for it to bolt up, and there's no speedometer output.

Well, downsides for most people anyway :)

But how durable is it?

I remember trying to shift at 9000 RPMs at Zoom Zoom Live back in the day, and with a worn out transmission it would NOT cooperate.

The S1 Rx8s had bad syncros and the S2 Rx8s have better gearing and are top loaded, also overall much stronger.

I also think that a T2 starter would bolt up, not 100% sure but I will test that theory another time. And no spedo output is fine by me, I haven't had a spedo in a few years.

i would say the gearing is optimized for mileage, maybe its the same thing. our solution back in the day was the same as yours, instead of 4.88's and using 3-4-5, we took the rev limiter off, and did 2-3-4@9400rpms. it worked fine @thunderhill. once the exhaust was quiet it sounded like a dental drill

the 5 speed Rx8's have a flywheel with a T2 ring gear and NA clutch. why they bothered i don't know, but there is a flywheel out there for you :D

the T2 starter has different "ears", i think the top bolt will bolt up, but the bottom one is different. not hard to get an Rx8 starter though

I guess whenever i have heard people refer to a tall gear set it means a higher number/ratio. To each his own....

Right. 5th and 4th optimized for millage.

1 and 2 optimized for 0-60 for the leaflets and magazine articles...

Well how about 6 speed rx8 transmissions? I don't see myself coming across a 5 speed one here in the states...

I got the 6 speed sitting in my garage and I think I could get the starter to bolt up, but its not a huge ordeal because RX8 starters are easy to come by.

I have a junk rx8 clutch PP and Disc, I will have to measure them and compare to FC t2 or NA setup...

the 6 speeds run a T2 clutch, which is easy

okay was just making sure it wasn't a typo. I didn't know they had 5 speed rx8s. I wonder if they run FD boxes or what...

its kind of an odd combo, but its an FD trans, but with a push type clutch. so maybe more like a T2 trans with a different tail housing

There are several options if you want to run a speedo.... PM me if you need help.... I have a Cable X and sensor for sale!!! if anyone needs it!!! lol