Hello fellow Fail Chairot owners, I'm just throwing this thread out there if any of you are interested in a similar project.

My Interest in this all started with RXgirl7's old threadhttps://www.rx7club.com/showthread.p...ighlight=miata . I always thought it would bea sweet thing too do, but at the time that thread was started I was a broke highschool kid.

Recently the trans in my 89' GTUs w/ 146K on the clock has been getting ready to go take a cruise on the failboat. It's screaming bearings have been making me cringe, the countershaft bearing being the worst one. 10,000 miles ago I replaced the fluid with redline, and it didnt help much.
I started off looking around at prices to have the GTUs trans rebuilt with all new synchros and bearings. I was looking at $900+ in my area and about that time that thread was resurrected. I started scratching my head and talking to my neighbor Pele. He Had a miata trans he picked up for $150 and it got me thinking.

I looked around and figured out all you have to do is take the input shaft off of the miata trans and have it machined, then put it back on and put the Miata guts in an RX-7 case/housing. I called up mazdatrix and they said they'd machine the input shaft for $90. I got all pumped. I figured I could get a miata trans, have the input shaft machined, put it back together in my RX-7 case and have a great transmission with longer 1st and 2nd gears for around $500. I found a 01' Miata Trans in florida with 11,000 miles on it and picked it up for $375. My parents picked it up and brougth it to me when the came to visit, so I didn't have to ship it.

I tore into dissasembling it, the housing came apart easy. A few 12mm bolts, some snap rings and I had it down to just the gears in the center case. I started to get hesitant because I expected the input shaft to be easy to come out from what I had read, It wasn't. I took some roll pins out and pulled off the shifter forks and was able to get the center case off. After that I had hard it. Now way I was taking it apart, I had no Idea how to do it.

So now I'm where I'm at now. I called up the closest RX-7 shop out in West VA, RP Performance. I talked to Bret and made plans to bring him both Transmissions and he'd do the work and put it all back together in the RX-7 housing. I took the Trans out of the GTUs and dropped them both off Yesterday. I'll be updating this again after I pick up the finished trans.
In the End I have a feeling this will cost about $800-1,000, but I will have what is pretty much a brand new trans.

All in all I feel like this will be worth it for autocross and haulin' *** making noise which is all I ever drive the car for.

Cliff notes for you Guys:
Don't try to do this if you don't have transmission experience or a ton of patience.
The Input shaft isn't something you get out like a piece of Cake.
NB (98-04)chassis Miata 5-speeds are what you want to get, 6 speed will NOT work in the FC housing.

I probably left some stuff out since i'm tired. I'll answer whatever questions I can.

 

I don't know how strong the Miata trans is, but I have always heard that the T2 trans is very solid even at high power levels. Hopefully you get great results. Be sure to post some before and after impressions.

 

Yeah the T2 trans is a beast. Not only because of the amount of power it can take but also the strength of the synchros and the gears. You can drop the shifter into a lower gear and it'll easily slide in like you rev matched it even if its way off, kinda spoils you. Good luck with the car though bro, I'll have to drop by if I ever visit VA in the Rex!

 

I've read the Miata trans is good for 300+HP. I'm not going for Trans strength anyway, the Car is an N/A GTUs. I'm going for the gearing for autocross. A Longer first, and shorter 2,3,4, and 5. I think the car is going to accelerate like mad for an N/A after I get to my powerband due to the 4.3 rear end.


I've got the stock Turbo trans in my 91 Turbo. It feels pretty good, Better than the N/A, but thats not my purpose here. I think my miata Gearbox is going to be nice and smooth, it looked brand new when I had cracked it open. And Yeah man, let me know when you're going to come back around here, I'd like to see your car. It's gone through a lot since I first drove it!

 

UPDATE!
I picked up the trans last Friday and Pele(The guy who rebuilds FC wiper switches & Headlight switches) helped me get it in today. I love it. First gear is a little bit slow getting started, but after that you wind up and just fly through the gears. The only downside is 80 is around 5,000RPM(since I have the 4.3) now so highway driving sucks. This new trans just feels so much better than the stock 5 speed.
I also switched my trans mounts from the Banzai Racing poly mounts to the Mazda Competition mounts. I like the Mazda Competition mounts better because they are not nearly as rough. I only reccomend the poly mounts for serious racers, they reverberate A LOT of drivetrain vibration. I had the shifter block replaced as well, which I highly reccomend for S5 owners. It's the only way you can replace that last plastic bushing the Mazda doesn't sell. So if you replaced all your shifter bushings and it still feels like soup, especially when in gear, replace the shifter block.
Getting down to cost. My total for this whole project was $906. I paid $375 for the Miata trans w/ 11kmiles on it, $496 to RP Performance(Mazda Race shop at Summit Point WVA.), and $35 for 3 quarts of redline MT-90 fluid.
RP Performance was excellent. I dropped the Trans off on a Wednesday, he had it ready the next Thursday. The extent of their work was disassembling the RX-7 and Miata Transmissions, they had the Miata input shaft sent to their local machinist to be milled, then they reinstalled the Miata input shaft, installed the complete Miata gear unit into the RX-7 case and put it back together with a new shifter block and the new mounts.
Believe me when I say this, It is not as easy as it sounds. If you have no Transmission experience, Have a shop do the work like I did. Save yourself the headache.

I've got a few more things left to do to the car this season, I'm Installing a Taurus fan, and I'm going to finish my power steering delete. This trans was my biggest project this year, and it's worth every dollar.

 

Guess my T2 tranny is on it's last legs then =( You gotta rev match it every downshift or it flat refuses to go in gear. And fifth will crunch if you are off by too many rpms. Any idea on the cost to have the tranny rebuilt or should I just call some shops locally

 

Rebuild is expensive, buy a used one off the forum for $200-$300 and swap it in. A rebuild will run you between $700-$900 depending on the work. Swapping out transmission every year or two would be cheaper.

 

If he has the money to spend to get it rebuilt once, its bulletproof. I say spend the money on a rebuild and not have to worry about your tranny for 4+ years...

Then its just half shafts you have to worry about snapping.

 

I'll tell you right now, I've had them all, worn out tranny, decent used, Rebuilt, and now this almost new one. This one is the best by far. The more I've been driving the car the more and more like it. It's so smooth and the Ratios are much more fun. I found my self trying to upshift from 3rd to 4th today becase I had sped up. I ended up already being in 5th! The car had plenty of power in 5th gear, so much that I felt I was in 3rd.

Extremely low mileage Trans is the way to go!

 

What do you think it would be like with 4.10 gears?

 

A great improvement over stock. It's makes the car feel more powerful, moreso for me because of my 4.3 but it would do the same for you. 80 in 5th will probably be around 4kRPM and your top speed in gear will go down a bit. Fuel economy would be worse, but I say it's worth it. It makes the RX-7 experience that much better. 1st Gear is slow, but 2nd and beyond is fast. I can cruise at 35mph in 5th now without any bucking.
RXgirl7 had 4.1 gears in her car, she's got all the shift point published in her old thread if you search for it.

 

I read through the old thread and eventually found pics of the input shafts. Easy enough. I work in a transmission/machine shop. Frankensteining transmissions together is what we do! Although I feel that ill just use the fc's 5th as it gives a better highway gear. I like to have od type gear. Besides that the gearing fits the bill for the intended purpose of the car. (N/a drift)
Now I just need to find a cheap miata tranny.

 

^

If you go through that thread and look at some of what's involved, the 5th will change and be completely different from the Miata stock ratio and the RX-7 stock ratio...

 

I scanned through it mainly looking for the input shaft differences. Ill reread it. Thanks for the heads up.

 

Sorry to resurrect an old thread, but searching lead me to this.

If you can put the Miata 5spd guts into an FC tran housing, is the opposite possible? Can you put the FC guts inside a Miata tranny, using the FC gears?

And I assume, but can someone confirm...
If the MX-5 6-speed tranny won't work, then the RX8 6-spd tranny won't work either. You can't bolt the RX8-tranny into an FC.

And sadly, you can't bolt the FC 5-speed into an RX8, I guess. Too bad, because the FC 5speed in an RX8, would be lighter and avoid some of that rapid rowing of the 6-speed tranny, making the RX8 a better highway cruiser.

 

Would it be possible to swap gear sets? Like 1-3 from the miata and 4-5 from the fc

 

4th gear isn't a gear per say, as being in 4th locks the input and output shafts together.

I believe the counter shaft is solid, so I don't think you can do that.

 

Oh, thought they would be held by snap rings or sonething like that...

 

From what I have been told you can swap the 5th gear ratios between the S5 n/a, the GTU's and Miata tranys - I didn't ask about the odd ball '86 n/a 5th gear. This may not be the easiest of swaps, but it can be done.

 

I don't believe fourth is locked, it's just a 1:1 ratio. Most five speeds have three shift forks, one for 1-2, a second for 3-4, and a third for fifth. It's why reverse is usually paired with fifth. the blank pairing left by the odd number allows it to do the reverse gear.

Any time one desires a different gear ratio for a specific gear, both the one on the counter shaft and the one on the main shaft will most likely need to be changed. Change the pitch circle on one and the other has to change to keep the mesh. The tooth count can change on a gear but it will now have a different tooth form and diametrical pitch almost always needing a matched pair of gears (same diametrical pitch).

 

Yeah, figured. Im presuming the box is tearable down to the gears, like other Japanese transmission. Im thinking that if the input shaft(miata) is the same diameter as the fc(because the miata one is only machined down instead of reducing diameter), then you could, in theory, swap the gears

 

I promise you with 100% certainty that 4th gear is the input and output shafts being locked together with no power transmitted through the counter shaft.

4th gear is the synchro at the very far right of the image below.

 

The Miata 6 speed won't work because it's a completely different transmission. The RX-8 tans WILL bolt to any '74-up motor, you just have to use a turbo flywheel/clutch/starter. So, in Theory, you could put the FC trans into an RX-8 but you'd have to use a NA clutch/flywheel/starter.
the 5th gears will swap between any of the "M" or smooth boxes (Miata, NA RX-7, RWD 626, some of the B series trucks).

 

you can't?

what do you mean rowing of that 6 speed, do you even drive bro??? i shift 3 times to 60 in my rx8 in everday driving, 1st 2nd 4th 6th... or if i want to go fast. 1st 2nd 3rd 6th.
going from a 6 speed rx8 trans to 5 speed is the dumbist thing i have heard.. today.

 

So....
you say a 5 speed is dumb. And so you prefer having a 6 speed, but only use 4 gears. And for driving every day around town, you only shift 3 times, get to 60 mph, and leave it there. Do YOU drive much? (not for long at that rate)

I was driving a 1986 RX7, IN 1986. What were you doing back then...?

I test drove an RX8 once but didn't like it...guess why...the GEARING.