I need shorter gears on my rx7. i have heard about 12a trannies. is there any other way. or does anyone know if thers a company that sells a tranny with shorter grears.

 

I think changing the rear diff ratio is a little bit easier than attempting to install a 1st gen tranny in your 2nd gen. It will accomplish the same goal. Every gear will be shorter and your top speed will be reduced. But you can accelerate faster.

 

There's a lot more options for a shorter 5th than there are options for installing a shorter rear end gear. It'll be cheaper too. Beyond the tiny difference of the 4.3:1 of the GTUs, there's the 4.88 and 5.12 that are custom made from 1st gen gears, and a full diff setup will run you about $2500.

I've got a 1st gen tranny in my FC, it's got S4 housings front and back, but I believe that you can use the 1st gen ones, but they're not perfect, I think the tranny mount doesn't line up, but I think the bellhousing part is not a big deal though. It's got a shorter 1st through 3rd, giving it wider gearing (slightly) and a shorter 5th.

You can also modify a Miata input shaft to allow that to be installed into an FC tranny housing to allow you to use that. It's got a taller 1st through 3rd and shorter 5th, giving it closer gearing.

Some Miata guys also just install an FC 5th into their trannies to give them the taller ratio without changing the rest. You could do it the other way around.

There's also 3 different ratios available just within the different NA FC gearboxes.

 

A used trans is way cheaper than different gears *for an FC*.

A 12A trans will have slightly shorter 1-2-3 (3.6 vs. 3.4 for 1st, rest correspond) and then 5th is going to be much shorter.

It is fairly simple to swap a 13B FC (N/A) trans into an FB. You need to pull the FC tailhousing off, remove the giant damper weight, and install a FB tailhousing. Going the other way should be even easier because you don't *need* the damper weight, it's just there for NVH. If you don't mind having to adapt the trans mount and modify the floor slightly or swap the shift towers, you don't even need to swap tailhousings to put a FB trans in an FC.

 

thanks for all the advise. does anyone now of a how to build on the 12a tranny in a fc. im sure i can figure it out but i would like to see how some other people have done it.
'Thanks'

 

If you like the 12a gear ratios I would go with a transmission from an 86-93 B2200 1st through 4th are the same gear ratios, 5th can be swapped out to about whatever you want. The newer truck transmissions have some slight design improvements over the older 12a. Most notably slightly larger input and output shafts and a reinforced center bearing support plate. Using the 12a 5th in it works out to a .835 ratio instead of the .825 but you could always but a 5th in from a miata, S4, S5, or something to get that a little lower if you don't want to completely kill you're top speed in 5th.

http://www.mazdatruckin.com/B2200/Transmission.html
http://www.mazdatruckin.com/TrannyInfo.html

Edit: Some guys on my forum have been screaming for a writeup on doing full tranny swaps, I already have pictures and the process is the same between most of the transmissions so I'll work on that and post a link.

 

Bump, about half way done the page, would be finished by now but I've been in the hospital with a bad stomach virus.

...liquid diet for the next couple days... yum.

 

With 3.90 gears and 23" tires, 7,000 in 4th is good for 120mph. Divide by .835 and you get 143mph. What top end problem?

 

Lol, sorry I'm used to thinking with about a 4,000 rpm limit the trucks beat themselves to death if you run them much higher than that on a regular basis.

 

No problem.

I'm used to never going over 80mph because the driveshaft harmonics over that point are hell on the transmission.

 

Lol, at least you guys aren't using a 2 piece 5 plane drive shaft that is 5 ft long I bet if you cut the truck in half, sectioned it, and installed an RX-7 drive shaft you would gain 10 hp at the wheels

 

Ok, just about finished with the page, have problems locating a few pictures that ran away from me