jmhonnold None
4/24/11 10:46 a.m.

Hi All: I have recently 'inherited' a 1980 Triumph Spitfire with a Miata 1.6 L engine and transmission (Spiata). The diff is stock Triumph. I am thinking about installing a turbo, but have concerns about everything from the transmission back. From what I have read, the diff can't handle much more that the stock Spitfire HP. I am planning on about 160-170 HP which is probably more than the drive train can handle. Has anyone every retrofitted the entire rear end of a Miata in a Spitfire that could offer some guidance? The reason that I am thinking about using a Miata rear assembly is that I have a donor 1994 Miata.

P.S.: I have ordered a copy of the 1996 May/June issue that discusses the Ro-Spit differential install.



huge-O-chavez SuperDork
4/24/11 11:21 a.m.

talk to curmudgeon he did it in a spitty.

triumph5 Dork
4/24/11 12:46 p.m.

IIRC, most of the rear of the roto-spit involved a lot of one-off and machined parts. The stock diff is weak, but there are those on this board who have replaced it with a stronger unit. You are looking at a fair amount of custom fabbing to transfer the entire rear suspension from a Miata. You're best bet is the hybrid the mag wound up with.

erohslc Reader
4/24/11 1:24 p.m.

Depends on what you want to do. It's possible to fit a Nissan R160/180/200 diff:

But the swing axles will certainly have to go, so in addition to the diff, you must also replace the rear suspension. Most straightforward way is to adopt the GT6+ Roto pieces, it's almost a bolt-in (you must add different upper shock mounts and forward radius rod mounts, which are both stock GT6 items, and lower wishbone attachment brackets) but it's strongly advised to replace the Roto halfshafts themselves with CV axles (kits are available).

That's 3 items to deal with.

Might be easier to try to use the entire Mazda rear subframe; diff, uprights, brakes, axles, suspension, etc.:

Depends on how much fabbing and how much buying you feel like doing.


TR8owner Reader
4/24/11 2:17 p.m.

Ditto on the GT6 diff. I'm trying to remember what we used to do when I raced Spitfires way back in the day - If I remember right it the GT6 internals would fit into the Spitfire diff casing. In those days we used to weld up the hub carrier as a cheap alternative to limited slip diff, although you wouldn't bother on the street. Seems to me that was our solution, but maybe somebody else can update me.

Ian F
Ian F SuperDork
4/24/11 4:39 p.m.

While the GT6 gear ratio may be better for racing, the actual differential is apparently weaker than the differential in the 4 cyl cars.

I've been wondering if a E30 diff could be made to fit in the back of Spit w/o too much cutting and chopping with previously mentioned full IRS set-up as well). The main idea here being the ratios are closer to Spit rations and LSD version are fairly common.

Toyman01 SuperDork
4/24/11 5:38 p.m.
huge-O-chavez wrote: talk to curmudgeon he did it in a spitty.

What he said. Did a good job at it too.

Curmudgeon SuperDork
4/24/11 9:02 p.m.

Took for frickin' ever, too.

I made custom lower control arms, shortened the Miata upper arms, cut the 'ears' off of the diff (it was actually a 2nd gen RX7 piece but it's not much different), made a custom top mount plate for the diff and had a set of 2nd gen RX7 axles shortened and resplined. I used motorcycle shocks (Yamaha R1 to be exact).

I'm leery of the R160/180 swap. When that's done in a situation like this, all of the load of the axles pulling 'out' from the diff rests on the spider gear thrust washers and they are not meant to handle that type of load. Triumph used radial ball bearings on the stub axles to get around this. I can see no simple or cheap way to duplicate this on a R160/180.

GT6 stuff can be used for the rear suspension but the upper CA is the transverse leaf spring which leads to some weird camber changes. Not to mention some mounting difficulty where the spring is concerned. I studied and fretted over it for a while, I finally came to the conclusion that trying to fix those two problems was way more work than custom fabbing parts from steel tubing.

From a Tom Clancy book: 'The first A bomb was built by the smartest people in the world, they paved the way. Once the first one was built, it could be duplicated by reasonably adept machinists'.

The Ro-Spit was built when 2nd gen RX7 and Miata stuff was still quite new and not real plentiful in the boneyards. Tim, Steve and the guys did a helluva job of building the first Ro Spit rear axle 'A bomb'. It can be done easier now with boneyard parts.

About Spit/GT6 diff swapping: a GT6 diff of any year will bolt directly into a Spit of any year. Done it several times. In fact, with the exception of the 3:27-1 gear from the 1970 GT6+, GT6 diffs were the same as the later 3:89-1 1500 Spit diffs. There are some bolt pattern differences on the input and stub axle flanges, this is easily fixed. A Spitfire small pattern stub axle goes directly into a GT6 diff with no modifications and vice versa. On the input flange, the locating 'snout' is the same diameter but there are two different bolt circles. I did one by simply slotting the bolt holes in the driveshaft flange, worked like a charm.

The innards are different, GT6 ring gears are thicker than Spitfire meaning the ring gear flange on the carrier is not in the same place. Learned THAT one the hard way. You can swap the R&P and carrier as a set, though.

Those diffs tend to have the carrier break in half right where the spider gear pin goes through. BTDT.

erohslc Reader
4/24/11 9:55 p.m.

I'm with you on concerns with R160/180 diffs in swing axle cars. But Given that these diffs ('pumpkins') were derived from live rear axles, rather than clean sheet IRS designs, it's likely that the early units can handle side loads, as that's what they would need to do in a live axle. My real concern is that they don't have the ability to handle radial loads imposed by the swing axles. For example, assume for the sake of simplicity that the swing axle is 24 inches long from UJ pivot to the center plane of the tire patch, and the axle centerline is 12 inches above road. With a wheel load of 400 lbs, at 1G, the force vector at the UJ pivot point is about 447 lb at 27 degree angle, which resolves to 400 lb axial, 200 lb radial into the stub axle.

Bump and transient loads can spike that figure 2-3X

Failure here will certainly spoil your day.

Go for an IRS, any IRS, to be safe.


spitfirebill SuperDork
4/25/11 7:44 a.m.

An alternative would be putting a Quaife LSD in the Spitfire diff case. I hear that takes much of the tenderness out. They cost around $1,100, but you aren't likely to build a new rear end using Miata parts for much less.

Ian F
Ian F SuperDork
4/25/11 8:36 a.m.

Quaife LSD... plus PRI rifled axles... hell... just add a whole PRI rear conversion kit...

Of course, you would then have more money into the rear end than the purchase price of most Spitfires... and still be saddled with those whimpy little stressed u-joints...

purplepeopleeater Reader
4/25/11 9:21 a.m.

I read something a long time ago about using a Subie diff in a Spit. Memory is foggy but I think TSI was involved

jmhonnold New Reader
4/26/11 10:29 a.m.

Wow, thanks for all of the great input. I think that I will go ahead and use the entire rear section of the Miata. I know I may be biting off a big chunk, but this seems like a solid answer with the strongest results. I'll be referencing the RX7 Club URL that Carter suggested showing pictures of an RX7 rear end implant. I'll give everyone an update as the project progresses. Thanks again for all of the great suggestions.


Curmudgeon SuperDork
4/26/11 7:36 p.m.

Danger: a Spitfire has a 50" rear track. A Miata has a, IIRC, a 57" rear track. It'll look really weird unless you narrow the subframe. The Miata subframe can be narrowed pretty easily (did one for an Opel GT project that withered on the vine) but with a Spitfire there will be a tremendous amount of body shell chopping needed to get it under there. MrJoshua started doing one, he could tell you more.

Also, a 2nd gen RX7 has the 'long snout' diff and boneyard R&P selection is pretty limited, like a 3:89, a 4:10 and IIRC a 4:30. The Miata can use any 1st gen RX7 R&P, or one from a B2600 4x4 or Sportage 4x4 front diff. That gives you 3:89, 4:10, 4:30, 4:44, and 4:77 choices all from the boneyard. There's also a 4:86 and 5:62 available from Mazda Motorsports but they are big $$$$.

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