oops:
https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png
bye bye fun 4.857 ratio rear end. too many burnouts on woodward dreamcruise...

time to do some custom work and fit a miata diff in there with a new driveshaft.

on the other news I got rid of the drift knuckles (for now) and went to a 1.5 way steering quickener but I managed to beef up the inner/outer tie rod ends with caravan units. only needed to trim the inner rod about 3/4" and it worked out great, while I was in the steering area I replaced the rack bushings with polyurethane ones as the rack was moving under heavy load due to 37 year old rubber being destroyed.

so fun fact, I got started down this and I had to trim a lot of my skid plate mount excess framing for the front pickup point but a stock car won't have this issue, they only need a small section of the subframe notched:

https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png



a bit closer here:

https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png

it almost clears with some hammering but not quite, it interferes with the lip of the subframe.

You only need about 5-7mm trim there which can easily be welded back up, will finish this and mock up for driveshaft. Like the IRS swap because this goes over the subframe you will need to change the u-joints to a serviceable style, the c3 corvette yoke works (apparently) and that way you can disconnect the u-joint from the joke end and drop the shaft, otherwise installation of the shaft will be pretty difficult. It doesn't look like you will need to notch the subframe to clear the driveshaft but that still may be option. Either way it can be setup where I can swap between a miata diff and rx7 diff without interfering with eachother. the ronin speedworks instructions have good detail in this clearancing needed but that is for their 1000hp rating where the 3" driveshaft isn't enough.



In parallel the driveshaft shop is giving me a quote for an 8.8 IRS swap axles as their other off the shelf solution is 800whp rated and way overkill, it switches out the hub so you need to do wheel bearings and then the hub may have the wrong hubcentric ring for the wheels as apart of the hub so this whole area is a big waste. Basically helping them with a budget version for us normal folks who want to use FC hubs so it can be bolt in.



however all of this is now a moot point because I have just found a set of 5.12s for the non turbo rx7 diff, somehow one just popped up for sale at the right time. I will finish mocking up the miata diff as its pretty close just to get closure as the 4.78 option is pretty good too for any future plan B or C needs lets say...
https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png

dug into a bit deeper with the mockup, seems feasible but one big flaw. the pinion is way off center. I set the pinion angle to 90* and see how it lined up, a simple chop of the subframe would work but how offset can a diff be from the transmission output shaft?

https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png

https://s3.us-east-2.amazonaws.com/p...ge_mmthumb.png

I think this is why it has not been done or documented before but I have seen people on this forum swap in newer miata diff in and just get spacers for the axles.

why not supra IRS diff?

Hmm.... I know that solid axle RX-7 axles are equal in length. I also know that the pinion is slightly offset to the right in the tunnel because the flange hits the right side of the tunnel. (which is why I offset my 9" setup to the left of center) I used to know that the pinion was coincidentally on-center with the differential, but now I am not so sure.

I don't know about FC but in SA/FB the drivetrain is not only angled to the side but it is also cockeyed in the chassis. The engine axis points to the left, I think so the exhaust manifold can clear a RHD steering box, but at the same time the interior of the car is LHD biased, there is a lot more room on the left side than the right. Kind of like an old Dodge van but not as extreme. So the engine and trans sit at a couple degrees of angle that you can never really eliminate from the front U joint, and the mass of the engine, battery, and all of the steering/brake stuff make LHD cars have pretty significant LF corner weight

Should be fine. Send it.

probably more work to look into but stumbling upon the correct ring and pinion was the easy button here, just spendy but not as spendy as 8.8.

besides, there are way more ratios out there for cheap for the 8.8 than the supra diff, in the future a miata diff swap could be feasible with a light notch and a new driveshaft. Fabricating a front mount wouldn't be that hard and there is plenty of room for it

the 7" diff though isn't much better for durability.

It is fine at less extreme ratios. Since the ring gear is a fixed size, the pinion has to get smaller when you go to steeper gears, and then things get weak.
3.909 gears? Strong enough, or at least not the fuse in the system.
4.875? HMMM....

Really short 9" pinions look like worm gears

Im debating between a 4x108 ford 8.8 or a volvo 1030 for the rally car. I

15 rallies and never shattered anything diff related, driving like a dick doing burnouts during woodward dreamcruise is what ticked this last pinion. I don't have any torque so I can break anything. most subuaru diffs are less than 7" anyways. If I do upgrade with power I will just skip the miata swap and go right to 8.8 IRS. I dont always like to engineer everything, so much of the car is what it is because nothing was off the shelf, this is as close to off the shelf as possible just expensive unfortunately.

Like Pete said the pinion becomes the weak point as that gets smaller with a faster ratio.

looking at it the 5.12 vs 4.857 its the ring gear, its a 39 tooth ring gear (compared to 41) and both have an 8 tooth pinion.

Ah but the gear pitch changes. That 39 and 41 tooth ring gear have the same OD, the two 8 tooth pinions will be different diameter. The difference is the pitch.

Stick 'em side by side and the difference will be clear.

I ran into the same issue when I did my Rx-8 rear end swap. I got worried at first that I measured something wrong somewhere. The I went and looked at my parts car since it's subframe and rear end were still bolted together but on the ground. I then discovered the stock rear is offset as well. I'll see if I can't get a picture of it tomorrow in the daylight. It's just the flange for the driveshaft on a FC sits in front of the subframe so there's no issue. Like someone else mentioned I think the FC engine and transmission sits a slight angle. Anyway my flange sit almost exactly like yours. For some reason I didn't mention it in my build thread. I only mentioned that I had to remove some of my subframe and partially box it back in but this was part of the reason. Here is a picture taken during the boxing back in process.
https://cimg0.ibsrv.net/gimg/www.rx7...be2a1cea44.jpg

If you do go cutting up your subframe, I'd try to add as much metal back as possible. On my 8.8" swap, Had to cut and clearance the subframe to fit a 1350 u-joint. After doing so, I've cracked the patched metal twice! The last time, i took a bunch of 1/8" bar stock and 3/8" steel rod and welded it to the underside and seam welds of the subframe to try and add some strength back. Granted, my car is on sticky tires and makes 600wtq lol. But I'm also not jumping it...

I saw that in the ronin instructions, I think an easier way would have been just cut a large diameter tube to weld in there in and hammer to shape. One of these days I may just tube the rear subframe if it comes to it. especially if I DCT swap it in the future - you always give me bad ideas.

The Miata swap is a good budget plan B, the local driveshaft shop charges 425 out the door for a 3" aluminum driveshaft.

Just got the 5.12s with LSD setup and now with some free time I can slap the case back together and get it reinstalled for the next rallycross and rally.

I was digging around some of my spare parts (I have a whole S2 rx8 parts car aka my former daily driver) so I was able to test a few things, using the 4.777 diff out of that was an idea too like your swap. The rx7 axles fit into the rx8 hubs. mazda is so lazy

When I welded up a 26 spline 7" rear, I used the intermediate shaft from a Protege. Apparently the "G" transaxle uses the same diff as an FC/NA/NB except with a collar to accept a speedometer gear, so technically you could put a clutch type limited slip in a Ford ZX2 with a Miata diff and some light machining work on a lathe.

The Group A 323 GTX had 5 lug hubs courtesy of the rear of FCs as well, apparently.

Spline hobs and rolling equipment are expensive, and Mazda does all that stuff in house, easiest to reuse as much as possible.



I'm deeply interested in said $425 driveshaft if it includes weird import yokes and flanges...

This is the driveshaft 3'' Aluminum Driveshaft - Up to 500 HP - Precision Shaft Technologies (pstds.com)I had made from PST. Of course it was cheaper in 2020. I supplied them with the u-joints and yokes/flanges which I Think I got here Northern Drivetrain, LLC.
The flange and yoke take a 7620 U-joint and the driveshaft took a 1310. I used a conversion joint. I think it was this one Neapco 2-3100. Sent them the u-joints and yoke/ flange and it all came back assembled. Here is the thread that explained how to do it and has all the needed part numbers.
How to make your own aluminum or carbon driveshaft - RX7Club.com - Mazda RX7 Forum

i guess the spline machine is like a thread machine, they are big and expensive, so one you buy it, you're making that thread/spline

the fun example is my dad worked for a summer (1969 or 1970) at the Westinghouse facility here in Sunnyvale, and they bought a giant lathe table, and it makes the gaskets for the missile tubes on a submarine. fast forward to now, 54 years later, and they still make missile tube gaskets, cause that is what you do with a giant surface grinder.... its Northrop Grumman now, they make submarines there, apparently its big enough that they can run the drivetrain up to full power, which is kind of weird in a suburban neighborhood....

speaking of lazy, why not make a short shaft that bolts to the miata drive flange, and then ends in an FC drive flange?
something like this, but less plumby
https://cimg9.ibsrv.net/gimg/www.rx7...7b18e0d7b8.jpg

plan B would be to have a 2 piece driveshaft and piece 1, bolts to miata diff, and center bearing is somewhere around the front of the subframe

so, i have a 9", and a Mazda driveshaft, and connecting the two is a kinda rare pinion FLANGE from a mid 70s Thunderbird, since the other 99.99% of 9" rears had yokes that took straps or U bolts. the flange allowed me to have its ID turned to take the Mazda "small square" ('83-85, 6" Miata, others) driveshaft.
https://hosting.photobucket.com/albu...503_182727.jpg

Had been chasing weird vibes a lot. Played a lot with pinion angles. Swapped driveshafts with various spares. Finally thought to measure runout. .007", or .0035" off center.

I set the high spot down, jacked the car up by the pinion flange, loosened the nut and reefed it down again. Runout went to .002".
Vibrations almost entirely gone.

So, if you make an extender spool, make sure it has absolutely positively zero runout...

I think it's slim to none that the DCT would be usable in a rally car. The pan in mine sits below the frame rails. My car isn't slammed and I try to be conscious of debris on the road, so it's yet to make contact. The OEM pan is also plastic and billet pans are $500+. I bought one from HPR in Norway. Hopefully I won't put it to the test lol.

https://cimg2.ibsrv.net/gimg/www.rx7...038f6b2a7b.png
https://cimg5.ibsrv.net/gimg/www.rx7...f5c946fdb1.png

.

Shaftmasters just south of Detroit, it was new flange and new yoke. spicer U-joints and a 3" tube. 425 out the door

that coupler would work too, I don't know about vibrations with that setup though, maybe none? but it is a cheap solution!!

military is like aerospace when it comes to PPAP, that machine serial number is PPAP'd to that part, it would cost too much money to re-validate a deviation so you have some ancient machine spitting out old parts until the end of time.

and in regards to the DCT, the m5 one is way out of the question, I don't know about the M3 one or maybe even a standard 3 series with a DCT. the M5 gearing isn't good when you have 170-200hp.