And if so how do you like it? Does it shift good?

Can Miata tranny take 10lbs of boosted 13b? Or will the torque eat it up?

 

I was wondering the same thing a while back, especially switching for a 6-Speed

 

Well, I used to own a miata, so I can answer, I suppose. The miata is in my opinion one of the finnest shifting trannys in existence. Compared to my S5 tranny, it's just worlds above it. As far as handling the power, a lot of miata guys that are serious about making power move to an RX7 tranny. www.miata.net is a good place to look for more info regarding that.

Edit: the Mazdaspeed tranny might hold up better, but that's not the kind of thing you can find in a junkyard, ya know.

 

The Miata tranny is the same internally for all intents and purposes as the NA RX-7. The shifter is a bit different but there's no reason a RX-7 tranny with new shifter bushings shouldn't be as tight. The Miata shifter is much shorter and makes for shorter throws but the actuation mechanism is practically identical.

As far as mating one to a rotary...the input shafts are different. The Miata shaft is a bit longer and needs to be cut down to work. Also the tail housing is different since it it built to accomodate the PPF of the Miata. The tail housings can be easily swapped but the trans will have to be completely torn down to remove the input shaft to have it machined.

Power limit? My turbo Miata made 260whp and the first trans lasted about 6 months. I sold it before I could break another. At 10psi on a stock 13B you'll be on the ragged edge of reliability and it won't be any where near as strong as an R turbo box. Again, they are internally identical to the M NA tranny (with the exception of closer ratios). So if you wouldn't feel comfortable with an NA tranny behind you're turbo motor forget the Miata trans.

 

you can also use the rx7 input shaft, since you have the case apart its an easy swap

 

I stand corrected. I suppose it could be the short throw shifter the previous owner installed, but my GTUs shifting feels.... vague....

 

Not with the Miata internals. The gears will mesh when you're assembling the gearbox like it's gonna work but it won't. The tooth count on the input and counter gears of the Miata and RX-7 are different. The OD of the RX-7 input gear is slightly larger than the OD of the Miata gear. When you assemble the gears it looks like it will work until you try to put the front cover on and the shafts won't fit in the bearings. I've tried...

 

Roger Mandeville used to offer the conversion of 5 speed Miata gearset into RX-7 trans cases for $1200 for the complete finished transmission.

 

Thank's C. Ludwig, that makes sense.

Tom, I got 2 guys on the west coast that'll do the miata box for much less but thanks for the info. I'm gonna just have one of my 626 boxes put into a 79 Rx7 case as that will bolt up to REPU just fine. And save me the $220 of purchasing the Miata box from them.

When it's time to turbo, I'll get a T][ box.

I am talking to people who are putting Rx8 6 speeds behind 12a PP GT-2 cars and so far it is holding up very well and the extra gear works great at the local track. And they are getting brand new ones directly from Mazda so they shift like a beaut and were made for 238fwhp instead of what, 125 for the Miata box which ate 3rd gears all the time.

They might buy 10 @ discount! But still too pricey for me.

 

Brad,
Who might these GT folks be?? The 6-speed RX-8 box (essentially a Miata box) might just work decently behind a 12APP, but I'm still doubtful of their reliability. Good experiment though...

-Bern

 

They seem to like the 8 boxes and they shift better than miata. The 8 box is built for more power, are NEW and they have not had a failure yet. You can ask our friends @ BRP for more info

 

don't get the miata tranny if your lookin for it to hold power.....trust me i own one and have seen many others with this problem.....thats why we look to swap in the turbo II tranny (not a direct swap).....when the whp and tourqe gets around 250 things start to break....it's a very nice tranny but weak also.....this has plauged turbo and supercharged miata drivers forever

 

I think there's a bigger picture here in regards to reliability - if you're road racing, it's one thing; if you're drop-clutch drag launching, it's another - and you have to factor how well the car is hooking up. If you put a lot of power through the mazda smooth case tranny, and hook up a heavy car like an FC, it will eventually break.

I have friends with boosted miatas, who autox / solo I without problem. Light car with big power, & the rear wheels become the 'weak link' - go up in smoke - saving the tranny.

I have a 12A FB tranny in my FC, and so far no problems with it!

 

Will do... should've looked into it yesterday!

-Bern

 

NEW and YET. To me that means that no one has run the transmission ragged for 3 or 4 seasons of racing and found out that it chews up X part the way that Miatas eat 3rd gear, or whatever part gets eaten on a 7 transmission.

I'm not knocking anyone, especially since I'm learning a LOT from you guys, just offering a word of caution. Sometimes its better to run something that you know will break at a certain place after a certain time/condition rather than find out that a newer more expensive piece just had a defect that hadn't been discovered.

 

Their GT2 cars were eating Miata 3rd gear at least once a season, sometimes more. So far they have done a LOT of testing and some races on Rx8 box and it still works fine. Agreed on possibility of defects showing up later.

I dropped of my 626 box yesterday to get put into a rx7 case for my REPU.

Good to C U yesterday Bern. Always a pleasure to chat. Wish Wayne did better but dem's the breaks. Lots of attrition in EP. What happened to Bob Neal's rear quarter panel?

 

Bob Neal: flat tire that took out his quarter panel.... out from 1st place

Wayne G: flat tire that took out us out for a lap or two, to change... out from 2nd place

Eric Estrada (RX-3) out with flat tire...

it seems like there were some sharp rocks thrown onto the track by some ones off....

I guess it's kind-of wait-&see on the RX-8 box.

-Bern