yup I am an idiot, the transmission isn't the issue the differential is and the transmission I put in won't go into 5th, not a huge deal but kind of annoying. 6th is ideal for around town with the short rear end.
Time to drop the diff and investigate. no rally this weekend.
How did you figure out it was the diff? I have an SA RX-7 and would've gone through the same diagnosis you did and also probably got another transmission.
Rotated it in gear and could get a clunk out of the case. having the wheels still on made it easier to roll and with the exhaust leak I couldn't really hear but the vibration was still prevalent. I had ran through the gears without the driveshaft attached in the newly installed transmission, no noise no leaks just a missing 5th gear or bent 5/6th rod. not a huge deal mostly useless gear anyways but then started hooking things up and spinning the diff on its own without the axles wasn't loaded enough to replicate it but after having the rear end attached and rolling the wheels the clunk was present, it even had a not so smooth engagement from the pinion that was subtle which made me think the LSD broke some spider gears or something funky so I pulled the differential and found this:
needed to take the cover off to find the pieces, they did not come out in the initial drain.
well I can't get any fun ring and pinion ratios in time for the October event so we will play a game of minor fabrication + parts bin swap, lets play the game!
what do all these have in common?
I'll give you a hint, its 7 inches
In reply to fidelity101 :
So, bigger than average? 
Rear diff is my guess.
In reply to thewheelman :
harrrr
also
ayeee.
the differential is common in some aspects but the FC rx7 suffer from long pinion disease and that makes swaping stuff harder so I have a 4.777 ring and pinion on my shelf from an s2000 that is interchangeable with the mazda axle so I can use that ring an pinion with my rx7/miata OS giken diff, use the FC rx7 housing (which i think is interchangeable with a miata) and still use the same axles and stub shafts that I currently run.
Quick sanity check at rockauto shows that they all use the same pinion bearing and differential case bearings, and all refer to them as "mazda axle" so thats pretty neat.
Only fabrication needed is a front diff mount and a new driveshaft, which I think I can tackle within the next 3-4 weeks versus waiting around for a ring an pinion so rare you would think they were made out of gold...
The Miata pumpkin definitely fits in the FC differential uterus, the FC part is used in 7" equipped Miatas to replace the Miata one that breaks at the engineered in weakpoint. FC do not have a PPF so they don't need the housing to break to allow the drivetrain to move back in a hard frontal collision. They are dimensionally identical except for the mount bushing inner sleeve, you have to use the bushing that fits your chassis.
They all use the same bearings and stuff. I have mixed and matched all sorts of Miata/Kia/FB/SA stuff over the years. Evan's Miata has an FC diff in a Miata case.
The Sportage/B-series front diff housing looks like it might be easier to make a mount to fit the FC crossmember. Alternatively the Miata piece might be useable with a chunk of PPF used to make a diff mount off of.
I talked about the short nose into FC swap idea with j9fd3s on The Forum, we figure the biggest hurdle is that the subframe might need to be notched to clear the pinion flange and after that, access to the bolts to hold it together will be a pain. So no 10 minute trans swaps in the service park.
The old solomiata site has been archived, there are driveshaft lengths available. I know the driveshaft for the 7" uses the FC pinion flange and the one for the 6" uses the S3 FB/Kia pinion flange, and the 6" driveshaft is 4" shorter than FB and a couple inches longer than 7" Miata. I have a sneaking feeling that an FB driveshaft would be what you need.
Not sure how much this helps since you need a Turbo II sized front yoke, now that I think of it
notching the subframe won't be that bad if necessary and either way I can have it in a way where I can swap between a short pinion or long pinion diff easily. I'll retain my old driveshaft just in case but the the FB could work if I swap the front yoke but either way I'm having a visit to the local driveshaft shop. I wanna see if I can fit the 3" diameter driveshaft which is what I'll need for a custom shaft likely. the current one is 2.5" and has plenty of clearance.
I did find that solomiata page, oodles of information on there.
Sportage carrier may be easier but miata parts are everywhere, waiting on a friend to return home from ohio as he may have one for me on the cheap (free) and then go from there.
the sportage carrier doesn't look like it will bolt into the FC/Miata cover:
It does. It is a direct fit into FB rearends, they're all the same bolt pattern. At least, all the bolt holes are there.
which years sportage?
looking at it closer maybe not all the bolt holes are used on the KIA version
Whichever ones were longitudinal, it has been so long that I have forgotten. Certainly I can't find $100 4.78 dropouts anymore.
In reply to fidelity101 :
The longitudinal engined ones are the first generation vehicles.....1994-2002.
friend managed to help me out and gave me 2 diffs for experimentation purposes. Time to break out the welder!
Following with interest. I started to really look closely at a Ford 8.8 IRS swap when I came to realization that the same amount of work would be required to use a Miata assembly. There may be more 8.8 ratios but I really don't need the strength nor want the extra weight. Not to mention no need to fiddle around with the axles with a Miata setup.
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:
a bit closer here:
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...
RX-8 trans, Miata diff, makes me wonder about using a PPF.... 
5.12s would rock! Never need 1st with that, and then you are left with basically the same gearing as the Mazda Comp 5 speed, which is what the Group B cars had.
you would need a newer gen miata (06+?) which wont use the rear diff cover like the old style it would be more rx8 style with the top mounts, you could shorten or lengthen a PPF potentially but if you use the 94-05 miata diff with an rx8 trans the ppf mount is on the wrong side.
yes! 1st gear will be used once or maybe even launch in 2nd lol.
and with an s2 transmission:
basically the same
anyways back to the miata diff in question, I THINK it may be possible to install it without trimming the subframe if you lower the subframe and get the nose in first then lift it up, i was trying to install the diff without touching the subframe.
stock rear end with rx8 gearbox:
stock rear end with rx7 5 speed gearbox:
rear end change is huge on these cars, completely changed the car for the better., especially with the OS Giken LSD unit.
I always removed/installed the differential and subframe together. It just seemed easier with the RX7 setup. I didn't even think about that not being a requirement with the Miata differential due to the shorter pinion. Looks like the only things that need solutions are a front mount for the differential and a custom driveshaft.
I saw that listing for the 5.12 rear end as well. That's a little too short for me as I'm now regularly hitting 130+ mi/hr at some tracks. Even with the S2 RX8 transmission that'd have me shifting into overdrive. Hell, even a 4.78 rear end would have me shifting into 5th at some of the higher-speed tracks like VIR. I need something like a 4.44:1 and step down to a 245/450R17 tire. That would put the top of 4th right at 140 mi/hr at 8500 RPM.
with the lift in the garage its alot easier to just unbolt the forward subframe bolts and drop it down slightly from the mounting studs and then the front diff mount is just an extension away, super easy with my transjack. on the floor I would probably lower the whole thing but with the 2 post in the garage I just hang the subframe from the shocks.
5.12s is a sweet spot for me, the 4.89s I had were really nice but 3rd would still bog coming out of corners or uphill in some instances. really just want to be driving between 3rd and 4th on stage when possible. top speed of 100mph on dirt is plenty fine the car can barely hit that anyways.
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?
I think this is why it has not been done or documented before.
I thought some lateral offset was okay or even desireable but I'd need to revisit that. Is the Miata differential really offset that much more than the RX7 or is it the short pinion length making it look worse than it really is? I know the RX7 differential is offset to the passenger side and the whole engine-transmission assembly is angled so maybe the angle isn't as bad as it looks.
fidelity101 said: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:
a bit closer here:
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...
I'm guessing that is Vincent. He is building a Mercedes powered fc for champcar.
Offset shouldn't matter as long as they are parallel within 1.5 degrees or something like tgat.
In reply to infernosg :
It is a few inches away, ~2-3" but the image does make it weird.
In reply to wvumtnbkr :
correct! Not sure about what angle it would run at but I would guess closer to 5* if I had to ballpark it.
driveline is in, just need to slap wheels and the exhaust on and we are ready to rock. eager to try the 5.12s and a few pounds saved with the aluminum driveshaft.
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