Dak

So I think I've determined it's my actual differential that's my problem. Here's a thread for reference https://www.rx7club.com/2nd-generati...ormal-1140889/
So on the surface it seems easy peasy buy a used Miata torsen unit, switch the ring gear, and be done, and this was my original plan. The problem is I have aspirations to go turbo one day with a final power goal of 250 to 300rwhp. From what I've gathered on here that's beyond what the n/a stuff can take. Now granted since I don't do a lot of drag launches I may be alright. So I thought now would be a good time to just switch to a turbo rear end. Problem is I have a GTUs with the 4.3 rear gear. The only way to keep it if I switch is to buy the 4.375 gear set from Mazdatrix for $826.Finances don't currently support that. What pisses me off is the auto Rx8 IIRC had a 4.3 rear and is a 8 inch ring gear and complete diffs can be bought for under $200 on Ebay, but it's a short pinion so I can't use it. The other thing that pisses me off is FD and Rx8 torsen units are a dime a dozen on Ebay and cheaper than the Miata units. Also complete Rx8 LSD rears with the 4.4 gearing can be bought for less than the Miata units, but alas the long pinion hoses me yet again. At this point I'm about ready to kiss the 4.3 goodbye in trade for a complete turbo rear end, re-drill my driveshaft flange and be done. Sorry for the rant but I had to vent. The limitations of the long pinion gear is really pissing me off!

On another note I'm real tempted to buy a Rx8 pumpkin and try to fab up what it would take to put it in the FC chassis. Sort of homebrew something similar to the Ronin 8.8 kit. Anybody know if TII half shafts bolt to Rx8 output flanges or if the TII stub axles will work?

Dak

Well after browsing Ebay, the forum, google and some initial brainstorming on how to do it I think I'm gonna drop the hammer on a Rx8 rear end. I will still lose the 4.3 but it will be replaced by a 4.44 which is probably a plus while I'm still n/a, and as mentioned before the auto Rx8 rears that are 4.3 are pretty cheap. I have a welder and can halfway use it, so why not. Looks like finding S4 TII stub shafts may be the hardest part to source. I already know where to get parts to build a custom driveshaft from a thread on here I have saved.

j9fd3s

iTrader: (3)

i think you could fab up a bracket between the FC diff mount and the Rx8 powerplant frame mounts fairly easily

Dak

I hope so but I haven't put too much thought into it yet as it seems the easier of the mounting points. I've been mostly thinking about how to mount it to the body. I'm hoping the factory mount is narrower than the ears on the FC diff and I can fab some brackets that let me use the factory Rx8 mount. I have a diff and mount ordered so I'll know more once they're in my possession and I can get some measurements.

On another note I'm guessing it's the '88 TII stub shafts that I'll need to connect the axles since the Rx8's axle was a complete assembly rather than having a separate stub shaft? Luckily Mazdatrix still has them if I can't find a used set.Though they're not cheap.

djSL

iTrader: (5)

I'd be interested in seeing how you accomplish this. I'd like affordable rear gearing for my peripheral port but like you, am screwed by the FC pinion length. Word to the wise, if you pick up a later gen Rx8 diff, they come with a 4.77 IIRC.

Dak

Yeah, I recall seeing that. They don't seem to be as common on Ebay as the earlier ones. A lot of 4.3's out of autos on there, but they're open diffs mostly and I didn't fully trust that when they said locking and auto that they really were. They're so cheap for the non LSD though. Like under $200 including shipping. I'd be like getting a gear set and then a rebate from the scrapyard for the housing.The one I have coming is supposed to be 4.44.
I think I will try to do a build thread in the build section once I get going and am sure I will be able to do it.

djSL

iTrader: (5)

True, they are much more expensive too.

Please do! I'd like to see someone do this.

j9fd3s

iTrader: (3)

https://www.rx7club.com/build-thread...1123568/page3/

djSL

iTrader: (5)

o0o0o0oo0o0. I like how the Rx8 transmission was adapted. I didn't realize the tail housings were compatible.

Dak

Cool. I had thought someone had already done this but my search fu was weak when I looked. Now that you posted it I remember seeing this. Which is probably why in the back of my mind I thought it had been done. I really like his top mount but I don't have a mill so may have to take a different approach. I might can get access to one so we'll see. This is so cool. Makes me excited that I might just pull this off.

djSL

iTrader: (5)

It's not explained in that thread, but I'd like to know what axles were used.

j9fd3s

iTrader: (3)

you could ask, Higgi is cool. i suspect the Rx8, although shouldn't that be too long? Higgi is resourceful and thrifty, so it is probably something easy.

also axles are dumb, you could probably take the Rx8 inner, and FC outer, and then just use the right length shaft, and it would work

Dak

Maybe I got lost somewhere but isn't the Rx8 axle all one assembly? The shaft that goes into the diff isn't separate like on the Rx7. Are you just saying take an Rx8 assembly and swap to the FC outer because the spline count is different between the hubs? If so is there enough play in the assembly to allow it to be popped in and out of the diff once installed in the car?

I'm guessing based on the above he used his TII stub shafts which actually stick out of the diff too far for the oil seals to work because he said the Rx8 housing is narrower than the FC housing. Thus he made spacers to relocate the oil seals. Based on all this I'm thinking he then used his stock TII half shafts. May take a PM to Higgi to sort it out.

Would be nice if what I think you're suggesting would work though as I'd like to get out of having to make those spacers.

Dak

Ok. Here's an update and probably my last post on this thread as I hope do a build thread once I get moving. I received my TII stub shafts today. Bought the only ones on Ebay I could find. Was nervous because they were in Australia but even after shipping they were half the cost of new ones from Mazdatrix. I'm not gonna post the pictures here. I'll save them for the other thread. They pop right in but just like in Higgi's thread they are too long and I will need to relocate the oil seals as he did. As for mounting: The stock Rx8 mount is a bust. Too complicated IMO to make it work so Plan A is my original thought of fabbing something up that uses the bolts on the top of the RX8 housing and the stock mounting points on the FC chassis. Plan B is machine the stock FC housing like Higgi did. Bought FC Mazda comp diff mounts cause they're half the price of stock ones and have metal on the way. Metal should be here this week so hopefully next weekend the fabrication begins. Wish me luck.

P.S. As for cost. It's gonna significantly be more than the Miata Torsen mostly because I have to buy some TII parts I don't have because I'm starting with a n/a. I hope to sell my 4.3 gearset to offset some of that cost as I should no longer need it. Will still be less than a TII rear and gears from Mazdatrix. One would save $350 by starting with a turbo car because you won't have to buy half shafts and axle stub shafts( $500 if your stub shafts come from Mazdatrix).

hIGGI

iTrader: (7)

stock TII (euro) halfshafts and stub shafts were used in my build, spacers for oil seals are by far easiest part from whole conversion.....
i have all kinds of fc/fd/rx8 halfshafts and there was not simple mix parts option so i ruled it out....
there might be option of running rx8 halfshafts but then u need VW T4 wheel bearing, pressed into FC spindle and use rx8 rear wheel hub (possibly modified) and then check the fitment of the oem calipers/rotors...

Dak

Thanks for the info.I was about to PM you. I noticed a spacer on your pinion flange in your build thread. Was that so you could use a driveshaft you had? I was planning on a custom driveshaft so is that something I will still need? Also does the Rx8 use the same bolt pattern on the pinion flange as the TII? Since I'm n/a I don't have a turbo driveshaft. I keep meaning to call Mazdatrix but seem to only remember before or after they are closed. Thanks.

hIGGI

iTrader: (7)

I have custom driveshaft and there is no spacer in my build. Its just modified flange (i think its modified TII one, which was closest in lenght and i have plenty of TII ones).

Dak

I discovered the Rx8 uses the same bolt pattern as the N/A driveshaft flange. Also build thread is up.
https://www.rx7club.com/build-thread.../#post12403274