Has anyone done this swap? What are the obstacles and upsides of it?
Looking at the gear ratios of the first gen tranny, it looks to be as good as doing a rear end change to a 4:30 diff through the first three gears.

Curtis
'86 GXL autox'r

 

Mazdaspeed7 has done it I believe.

 

I have a Miat box in mine

 

I thought the FB tranny uses a different diameter flwheel, clutch, & pp.

 

yes, I have an 84 GSL tranny in my car. Actually, I have 84 internals in a 90 case. The 90 case has more bracing, and makes the tranny a direct bolt in. While I had it apart, I put the speedo gear from my 90 tranny onto the 84 tranny. You must do this because the 84 had a 3.9 rear, while FC's had a 4.1.

KNO, youre halfway right. Pre-84 FB's used a 215 mm clutch disc/pp. Post 84 used a 225 mm clutch. But the input shaft is the same for ALL smooth case trannies(1st gen and n/a FC). The 215 and 225 mm clutches are not interchangable. To keep it simple, Im only referring to FC's gere. You just use the FC flywheel/clutch/etc because the imput shafts are identical.

Its MUCH better than 4.3 gears. My friend has 4.3 gears, and my car STILL has shorter gears, with the exception of 4th.


90 ratios 84 ratios % shorter
3.475 3.674 6%
2.002 2.217 11%
1.366 1.432 5%
1.000 1.000 0%
.711 .825 16%

 

Makes me wonder if you could get a Miata 6spd in there......
Probably not though

 

Mazdaspeed7: Would you consider doing that internal swap and then selling them?? Id pay a few hundred to have that done. I have a S4 though, but should be about the same thing.

 

Actually Im going to be forced to go with a TII driveline pretty soon, so I will be selling mine. I would consider it, but the price will depend on how much I can get the 12A trannies for. You could also use a GSL-SE tranny which has the same first through 4th gears, but with a .756 5th. It would be better on gas mileage, but I still like the .825 5th.

Right now, 50 mph is at 2500 rpm, 80 at 3800 rpm, and 100 at 5000 rpm. And I dont have to downshift to pass anymore

But if anyone really wants one, I know of at least 2 FB trannies I could get my hands on. If anybody wants one, PM me for details.

 

Bad idea, very bad idea. Stock miatas are breaking those trannies. Theyre pathetically weak. But the 5 speeds are near bulletproof. Bonzaitoyotas tranny is gauranteed to hold 300+ hp.

 

Thanks, Mazdaspeed.

I was looking at Felix Miata's site, it looked to me like you could just bolt in the FB unit, as long as you switched the shifter housing. I understand you were opting in favor of the stronger case, good idea! By Felix's info, all the '84 - '85's including GSL-SE had the same 1st - 4th ratios, even with differing 'diffs'.

So, how do you like the shorter ratios? Does it 'pull' noticebly better? E.T. reduction?

Curtis
'86 GXL autox'r

 

It doesnt just bolt in. The suspension mount is different between FB's and FC's. You either need a custom mount, and switch the shifter housing, and the shaft for the shifter. So you need to pull the tailhousing off anyways. Youre better off just putting the FC tailhousing on the FB tranny. That way it will bolt right in, and the shifter will be in the right spot. Youll also need the speedo gear off an FC, and the shaft for the shifter. So you MUst pull the tranny apart to make it work, period.

My car pulls MUCH better with the shorter ratios. 5th gear is amazing.

The only difference between the GSL-SE and 12A FB trannies is the 5th gear. Its .825 on the 12A one, and .756 on the GSL-SE.

 

i would want a tranny with better sychros.....as that is the weak point in ALL rx7!

 

Thats why you cryo-treat

You wanna play, you gotta pay!

 

you give CREATINE to your tranny

 

Yep and magical synthetic fluids!

 

Theres a DSM shop that cryo-treats anything! They can even treat a whole rotating assembly if you want. Im not sure how this would work with aluminum, but hey im sure its great for a tranny.

 

You want to find a shop that does computer controlled cryo-gas heat cycling.

 

Heres an excel spreadsheet I made about the differences in gear ratios and such. But keep in mine everything is based upon my 225 50 ZR 15 tires, which are very slightly smaller than stock.

 

Didnt know anyone would actually want a less durable transmission.

 

Well, I havent broke it yet......

Ill also go ahead and say it doesnt shift as smooth as the FC trannies. But I can live with that for now, the gear ratios are AWESOME.