RX7 and Miata diff interchange question
 

I've got a GSL (not SE) limited slip axle, and I can get a '91 Miata dirt cheap. I was considering putting the GSL limited slip in the '91 diff, anyone ever done this? There's lots of info on second gen LSD's in 1.8 Miata, but nothin', and I mean nothin', on the earlier stuff.

TIA.

I might be having a brain fart, but aren't the 84-85 (i.e. "big axle") 1st gen and 86-91 2nd gen NA LSDs identical in size (axle, ring gear)? In fact, I think the clutch-style LSD is the same unit from 84-88, when they went to viscous, and even then are swappable with the later units. Someone correct me if I'm wrong.

Yes, there's lots of stuff on the later diff swap out there. My question is about the earlier (6" ring gear) Miata diff. I'm trying to do something on the cheap with what I have available, and if the GSL diff will go in the early Miata housing with the early 4:30-1 axle ratio, I'll be well on my way. If not, oh well it was a good try. :D

Oh, you are getting a 1.6 Miata diff?! I get it, now -- I was thinking you meant "earlier" wrt RX-7s (i.e. 1st gens vs. 2nd gens); not Miatas (1.6 vs. 1.8). No, the 1.6 stuff is not compatible with anything RX-7. All 1st gens had the same ring and pinion, with the only difference being "big" vs. "small" axles, which only affected LSDs (side gears, at any rate). 2nd gen NAs were the same as 1st gens, except for the long pinion.

well I don't know how helpful, but here's a ton of info on diff interchange..

http://members.aol.com/solomiata/MiataVLSD.html

I don't think the 6" Miata 3rd member will work. I think you have to go to the 7" and it only wiorks for the 84 -85 cars. When you go to the 7" Maita you are stuck with the 4.10 gears, unless you are really lucky and get the 4.30 which is rare.

what your probably going to want to do, is get a 3rd member out of a 94-97 miata with a torsen lsd. If you cant find one with a torsen, buy an open diff. then you can use any 7" rx7 ring gear. Youre also going to need the 94 driveshaft and half shafts. from what i understand, it all bolts right in.

You can bolt the 94-97 Maita 3rd member into a 84-85 Rx7 housing without any other mods.

I think he is buying a 1.6L Miata and simply wants to know if he can upgrade it with an RX-7 LSD; not put a Miata diff in an RX-7. Unfortunately, that is not possible because all the 1st and 2nd gen RX-7 stuff is incompatible with the early Miata stuff. In other words, he is SOL unless he upgrades to 1.8L Miata rear end components or finds another LSD donor (323?).

I know this thread died like 6 years ago but here it is.

I'm looking for a way to get a lower ring and pinion set than a 4.30 into my 2nd Gen RX7. Can I swap the 1.8 Miata parts into the RX7?

I race circle track and already have the '89 LSD with the 4.30 gears and am looking for 4.77 or lower.

Any ideas here would be appreciated

The Aluminum part of the housing is the same, so it will bolt up to the mounting points on the 7.

I suppose then all you'd need is a custom front axle mount and a longer drive shaft.

I've been thinking about this myself, personally, but I dunno what the cost/effort would be over having 4.88s or 5.12s made.

FC racers, due to class restrictions, typically just graft the Miata/FB pinion gear onto the long FC pinion shaft so that it's a bolt in conversion. If I were doing it without the class rules restrictions, I'd use a Miata housing and lengthen the driveshaft and come up with a different front mount.

Maybe this will help.
http://www.reganrotaryracing.com/fc/fcpart17.htm
I've been using a Miata torsen with a 4:30 R&P for a few years without issue.
I'd see no reason why I/you couldn't run a 4:77 or 5:12 with this combo.
Regards,
Crispy

Thanks for the link Crispy, that is some good info. However, I'm hoping to switch housings to the short pinion to eliminate the cost of grafting 2 pinion gears together to make one.

I wish they would have just made some lower ratio ring and pinions for the long pinion design.

Black91 - I agree that is the way to do it. I'm using the GTUs 4.30 LSD and think I can trade it straight across for the 1.8 Miata Torsen diff.

I've read that the aluminum housing part is the same between the Miata and the FC, but the bushings are slightly different. So it should be a more or less bolt in swap. I've got a 1.8 Miata Torsen in my car. The S5 diffs are all viscous, so it's probably not terribly effective. A torsen should be an improvement.