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Hey all, look what I bought this last weekend...

20190602_144218.jpg 20190602_143938.jpg 20190602_143857.jpg 20190602_143830.jpg 20190602_144233.jpg

A little familiar looking huh? Thats a complete IRS out of a Turbo 2 FC RX7 (88/89 era). I was only after the 8 inch LSD diff for a future non Toyota project (shhhh), but the seller wouldn't separate and made me buy the whole thing. But of course, as soon as I saw it, a few things stuck out. Same subframe mounting style as the mk2, 5x114.3 bolt pattern Camber adjustment built in from the factory, improved trailing arm geometry over the mk2s by using a double link inner arm plus an additional stablizing arm, aluminum hubs, the shock pickup is designed to use coilover shocks from the factory, etc. Anyways, don't get your hopes up too much. I busted out the measuring tape as soon as I came home. Here's how they compare...

FC, MK2
Suspension Subframe bolt spacing
115.5cm, 107.5cm

Suspension Subframe bolt hole diameter
22mm, 18mm

Diff mount spacing
59.8cm, 69cm

Rear track, measured at rotor to wheel surface
151.6cm, 143.5cm

Halfshaft shaft diameter (4bolt flanges)
26mm, 26mm

So right there, the extra 7cms for the bolt spacing puts the outer A arm mounts right into our body work, so the clean simple complete IRS upgrade is out the window. Its too bad, the extra track width is minor enough it probably would have just ended up with a touch more then the front track, which would be ideal. Anyways, I won't say its not possible, but if you have to modify the subframe significantly to get it to bolt in, you might as well just adapt a real double wishbone or multi link setup from a car with better parts supply IMO.

But, the diff might have some potential...
 

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20190602_144258.jpg 20190602_144757.jpg 20190602_145726.jpg 20190602_153609.jpg 20190602_145741.jpg

So as you can see, these diffs use axles almost exactly the same dims and size as ours. That's sort of a Minus really since our half shafts are a weak point, but we do have solutions for that and its kind of appealing that our half shafts could pretty much have the holes drilled up one size and it looks like they'd bolt to this diff. Yeah I know you'd still need to get custom axles made since this 8inch diff is notably wider, but still kinda neat. Anyways, I think the diff is in the same boat as the IRS. The mounting points are off by 10cms, which would put the bolt holes right on the ends of the aluminum. This one piece aluminum case\wing ear mount arrangement looks hella strong, but it does complicate swaps. Technically you could cut the mount and tig weld in an extension on each end though. And the sleave inside those diff bushings on just the diff are exactly the same size in both diameter and length for our studs. Thats a mk2 stud slipped in in the pic! Also a mk2 axel for comparison on that one photo. As for mounting at the front, get this, these diffs get all their strength from the back mounts, the just bolt to the front half of the subframe with a little ear tab and 2 bolt holes and an isolator bushing. That long piece at the very front of the diff appears to just be a resonance NVH thing, no holes on it, the 2 bolts further up are the mounting point. If one were to mod one of these diffs to fit, in theory you could get away with just hacking off the cradle from our subframe, then welding on a simple little ear like the Mazda suspension subframe.

Anyways, its just an idea so I thought I'd share some info. I think there are probably better options out there for ease of swapping and availability, but it was pretty cool to see how Mazda was doing things back in the 80s too. Although this is from the end of the 80s when Toyota already had a double wishbone IRS out for several years, seeing all the extra tech put into a trailing arm setup was still kind of neat.
 

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Yeah I'm going to be going to the GX71/MZ12 8 inch G code rear end that is bolt into the subframes (and my driveshaft since I already have the upsized input flange on my 7.5" diff). Will just need custom CV axles, and then a T2 torsen LSD with full 29 spline 3.73 rebuild kit that includes new gears.
 

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Whistles
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This has me eyeing the FC solid axle swap kits.

Very interested in seeing how close the mounting is on the subframe.
 

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Discussion Starter #6
I saw the pic you posted in the other thread, why not just get an oem solid rear if you want a solid axel? Toyota truck axles are pretty easy to get and there are kits for disc brake conversions and such. My buddy with the twin turbo 1uz Celica ran a shortened Toyota truck 8 for awhile with a mk3 supra turbo LSD. Was great till he got greedy and somewhere around 860whp\lbs/ft he broke an axle (and hit a wall).
 

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I have a line on a GX71 8" rear diff that should fit right into the stock mk2 subframes (there might be a tiny bit of modification required). I will also need to:

-Probably shorten the driveshaft, as it looks like the input flange is a little further forward on the GX71 8" rear than it is on the 7.5". Luckily I had already upgraded to the larger bolt pattern input flange from a IFS truck diff on my 7.5 (which matches all the IRS 8" rears like the GX71, MA70 etc.), so the existing rear U joint will bolt right up.

-Definitely need to get custom CV shafts. Plan is similar to what Don L did, using adapter plates on both ends and then a custom shaft in the middle - I'll be contacting the same shop he used to show them there is actually interest with these cars. I'm not sure if the length will also be different, but my GX71 diff has the same 6 bolt stub axles as the mk3 MA70 diff has, so I'll need different adapter plates for that at minimum. Good news is the custom shafts are also 6 bolt, so the inner adapter plates will be easier to make, going from a 6 bolt to 6 bolt instead of 4 bolt to 6 bolt..

-Rebuild the diff with brand new 29 spline 3.73 ring and pinion kit from Driftmotion (includes solid pinion spacer too). This will make the diff crazy strong and comes with new bearings, seals etc. (these are all interchangeable between the GX71/MZ12 8 inch diff and the mk3 diff). Plan right now is to also use a T2 Torsen LSD from either an IS300 diff or a BRZ/FRS/GT86 diff (they are all also G code diffs and mostly the same inside).
 

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Whistles
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You two have great ideas. I missed the first picture with the red arrows. Much more clear now, Sir. Looks like an easy road ahead.

It's great, and I really hope this turns into something popular.


Unfortunately for my rust bucket, any OEM solid housing will not be strong enough to handle the launching I am planning on. I need something I can straight dump the clutch on, while standing on the gas pedal, 5k-6,000 RPM while making 15psi spraying a 150+ shot of Nitrous as soon as the clutch is engaged to the 35-40psi this turbo should see, on bias ply slicks. Solid bushings only.

And do it hundreds of times without a worry in the world. The last thing I want to do is worry about anything in the driveline, period.


Shortened M9 housing, Ford 9" Detroit Locker, 4.11, 35 spline axles. Reuse the brakes from the Raptor kit, and finally get traction control hooked up. Goal is 800's on boost, close to 1,000whp on the bottle, in a H pattern car.

For those unfamiliar with what the medium power (1,000 whp) guys in the MKIV and MK3 world are doing, they are abandoning OEM everything. Those built diffs and components are breaking, and those are both stronger units than the 8 inch offerings. In the racing game, but for street pulls it's different of course.

My goal is to TRY and compete with those guys...

Sorry to hijack your thread. /'splaining
 

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Discussion Starter #10
You mean stuff like this Racerx...
https://www.grannasracing.com/products/grannas-racing-ultimate-irs

Yeah, its crazy territory\money to play in those leagues. Have fun :p

What you are talking about doing is pretty similar to what my buddy with the celica did. After he busted an 8.8 axle and hit a wall, he went all in. His car could do everything before, autox, street, run 10s down the quater etc. I loved it like that, but now its a murican drag missle with a toyota carcass drapped overtop...



He ordered a tubular chassis kit, welded the shell of the ra64 on top, went with a built LS (single turbo I belive) and off the shelf 9 inch rear axle, all American drag racer stuff. He's into the 8s now. Hilariously the car still has a vin number so he can drive it down the street to get gas, but its not even remotely a street car anymore.

Ryan, you should do the subframe mod I suggested too so you can remove your drive shaft without dropping the diff every time.
 

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Discussion Starter #11
Here's a couple more of Steves silly celica...


Rear end setup is a "Fab9". I don't speak drag racer so you'll have to translate yourself lol


 

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Whistles
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That's a lot more work than I will do. For a car making that much power, it makes sense. Tell Steve he is out front! Big difference in 8 second and 9 second quarter mile times.

I already asked Grannas himself to build us an IRS that will hold, complete with hubs. He said he needed ten people to commit. I think there's maybe two of us ready...

I've looked into the Fab9 housing too, and asked my builder. Not required for 1,000hp-1200hp.

Still, I'll have to do a lot more than you will to fit the 8 inch diff.

How's that coming along? Any progress with the subframe mounting?
 

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I don't have the diff yet, it'll be 2-3 weeks to get here. I'll be putting a rebuilt 7.5 in to get me through the summer while I rebuild the 8 inch with 29 spline 3.73 and a T2 Torsen. Then deal with installation and custom CV shafts this winter. Looks like the driveshaft will need to be half an inch shorter as well.
 

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Whistles
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I've only seen pictures of that diff, and that it's unobtainable.

Lucky you. :)
 

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Discussion Starter #16 (Edited)
Well look at Mr. Fancy Pants and his custom 8 inch diff setup, you've been holding out on us Aaron lol.

Very nice. Is that a Toy diff or a Ford? So, are you going to share the custom CNC work and bar? Can people buy this? Obviously it's far from bolt considering the front subframe requires extensive mods, but that's not the end of the world! Spill :D!
 

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It's a IRS Ford 8.8. I built 5 of the front bracket so yes the plan is make 4 more. I have 3 of the stock front crossmembers to use as cores now so I can rotate them out pretty quickly when ready. It's a pretty simple mod, but it needs to be jigged correctly What's holding me up right now the arms. Having a hard time finding a local machine shop that will even give me the time of day. In the end pretty much all the parts are readily available off cars they make now. What you end up with is much stronger drivelines, multiple sway bar options, cheap brakes, many sizes of brakes, and most are much cheaper than the Mk2 equivalent.
 

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Whistles
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Thread contains so much win... :)

That looks great. Looks like aluminum. Do you think it's going to be strong enough to hold up?
 

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It should be be way stronger than needed. It looks great in all the feas I did.
 

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Whistles
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It does look strong around the bushings, I was under the impression cracking was prone with aluminum. I love that sway bar mount.
 
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