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speedbiu
speedbiu Reader
11/7/11 5:16 p.m.

I need to know the good and bad about the drive train.How many HPs will the rearend withstand? Any info. is apprecated.

93EXCivic
93EXCivic SuperDork
11/7/11 5:29 p.m.

Unless you get your hands on a strengthen limited slip diff, not many. And either way the chassis is floppy and the swing axle rear suspension isn't brilliant. So you are going to want to stiffen the hell out of it first thing. The 1500 in a '80s Spitfire isn't great but swapping out the horrible emissions carb and getting to European CR will help a lot. The 1500 is more torquey then a 1296. The hot setup would be a small crank 1296 with an early 1500 head shaved.

Greg Voth
Greg Voth HalfDork
11/7/11 5:37 p.m.

Ive got a bunch of spare parts and am just down the road in Ormond.

irish44j
irish44j Dork
11/7/11 5:40 p.m.

swap in GT6 Mk2/+ suspension/diff and it'll hold a bit more, with bigger brakes and LCA's in the rear. Rotoflex rear > swing axle..

irish44j
irish44j Dork
11/7/11 5:41 p.m.

how much power are you trying to withstand, exactly?

speedbiu
speedbiu Reader
11/8/11 5:02 a.m.

In reply to irish44j:

Somewhere in the naborhood of 475,and its not a V8 or a honda Greg,How can I contact you?

spitfirebill
spitfirebill SuperDork
11/8/11 7:13 a.m.

In short, the standard differential will barely handle the standard horsepower. If you said 475 hp, then I have to ask why a Spitfire? Yea I know its been done, but I'm not sure you end up with a fun car.

MadScientistMatt
MadScientistMatt SuperDork
11/8/11 7:23 a.m.
speedbiu wrote: In reply to irish44j: Somewhere in the naborhood of 475,and its not a V8 or a honda Greg,How can I contact you?

At that point, I'd probably not just skip the stock IRS parts; I'd skip IRS alltogether and go with a 9" Ford, 8 3/4" Mopar, or Dana 60 live axle.

93EXCivic
93EXCivic SuperDork
11/8/11 7:25 a.m.

At that point even a limited slip more then likely won't handle the power. I would either find a Jaguar and get its IRS or go with a good ole stick axle.

Curmudgeon
Curmudgeon SuperDork
11/8/11 7:39 a.m.

The stock Spitfire diff is made of fairy dust and rock candy. But they are much in demand by Lotus Elan owners, so there's a market for it. Spitfires came with a 4:10-1 diff until the 1500, then they came with 3:89's which were the same thing as the GT6. (Some GT6+'s came with 3:27 rears and those are pretty rare.) Due to the 'wasp waist' frame design it's very difficult to do a proper stick axle install. So my suggestion (and I have already done this) is to use a Miata diff and convert the rear to a true full IRS. That can be done by using Miata uprights (that's how I did my car) or for a less expensive method use the GT6 Rotoflex axles. But that won't be as strong.

If you need to handle more HP than a Miata diff will (~300 HP) you can probably get a C3 Corvette diff or maybe one of the late IRS setups from a Explorer as a start point, but on suspension uprights I dunno. The C3 Corvette stuff could be used but the geometry is all weird, same for Jag IRS (and I LIKE Jags). On both of those you need to consider brakes, both the Jag and C3 have inboard brakes and there's not a lot of room back there for that kind of setup. Of course, anything is possible with the proper application of $ and time...

93EXCivic
93EXCivic SuperDork
11/8/11 7:45 a.m.

In reply to Curmudgeon:

I have seen a Jaguar IRS installed in a Spitfire. I will try to find the picture somewhere.

MadScientistMatt
MadScientistMatt SuperDork
11/8/11 9:31 a.m.

I wonder if you could use a Z31 300ZX IRS? It comes on a complete subframe which could make it a bit easier than a Corvette IRS. Probably too wide for the stock Spitfire body, but a good excuse to add some awesome fender flares.

Ian F
Ian F SuperDork
11/8/11 10:30 a.m.

No IRS diff conversion in a Spitfire is common or easy. On the North American Spitfire Squadron (NASS) Yahoo group, the Datsun/Subaru diff (R160 or something?) is often talked about, but as mentioned above, you have to convert to a GT6 style rear suspension with lower control arms as well as convert to CV joints.

One of the main reasons the Datsun/Subie diff is used is because it is compact and will fit into the limited space available in the frame.

Curmudgeon
Curmudgeon SuperDork
11/8/11 11:02 a.m.

In reply to EXCivic: I don't doubt someone has done that one. The unfortunate thing is the Jag IRS geometry is not real good, the driveshaft is the upper control arm and that makes it extremely difficult to get any negative camber gain unless you are willing to have a lot of static negative camber. That camber gain is essential to good handling.

On a stock Spitfire diff, the stub shaft bearings are subjected to a lot of side load. The axle is constantly trying to pull the stub shaft out of the diff, that's why it has the ball bearing and snap ring setup. Just about any IRS diff I have seen is not designed to handle that type of load, making it pretty much essential to have two control arms for the knuckle. At that point, you are just as well off designing an IRS from scratch.

At the '06 Challenge there was a Turbo Coupe powered Spitfire with a TR7 stick axle under it. I saw pics of the frame build; the distance from the frame crossmember to the axle was so short (due to space constraints) that the rear axle would have a LOT of roll steer due to the very short control arms. That's not insurmountable but again it seems easier to build an IRS from scratch.

erohslc
erohslc Reader
11/8/11 11:53 a.m.

I assume that you want to keep the 'sleeper' status as much as possible?

Your diff issue is not 'how much HP', but 'how much torque with wide sticky tires, when driven by a madman'.

Subaru/Nissan diff is probably the easiest to transplant. It's actually possible to fit one into the chassis without any major surgery, using apppropriate adaptors/subframes.

But you will have to design and build the rear suspension to go with it. That's probably a good thing, as you have your choice of rear hub carriers to work with, and stock Spitfire (even GT6) bits are simply not up to the task.

Since there is a seperate chassis, subframes and whatnot for attaching suspension pickups are pretty easy to design and fit. Be prepared to think outside the box for linkages (e.g. Watt's link can also work for fore-aft location)

Figure on coilovers rather than the stock transverse leaf spring. Add a set of custom CV axles to join your choice of hubs to your choice of diff.

The stock front suspension is competent, obviously update the springs and shocks. The front stub axle bolts into the upright, so get a custom pair of stub axles made to fit whatever front hubs you can find that are compatible with the rears. A pair of custom brake caliper adaptors.will allow you to use whatever bigger brakes come with the new hubs.

It's a fair amount of work, but most of it is bolt-on rather than cutting and welding the body and frame. This allows you to easily fix/upgrade/change as you learn more about what works. Maybe even sell kits to others.

Carter

Greg Voth
Greg Voth HalfDork
11/8/11 3:06 p.m.
speedbiu wrote: In reply to irish44j: Somewhere in the naborhood of 475,and its not a V8 or a honda Greg,How can I contact you?

Email is probably best. gregory.k.voth (at) gmail.com

speedbiu
speedbiu Reader
11/8/11 4:20 p.m.

Ok,think motorcycle suspension.and Hayabusa.It will not have the stock running gear.It will weigh somewhere in the 1100-1200lbs.

erohslc
erohslc Reader
11/9/11 8:34 a.m.

Looks like a Subaru diff is in your future then. Use GT6+ front and rear suspension bits, they will mostly bolt-on, but do convert from rotoflex to CV.

Curmudgeon
Curmudgeon SuperDork
11/9/11 9:47 a.m.

I think the GT6 outer axles could withstand a good bit of HP once the Rotoflexes are done away with, but the problem will be allowing for 'plunge'. Now you are looking at a good bit of custom machining and fabricating. Plus you'd be stuck with the rear drum brakes. That would be OK for AX, but for, say, a track day you'd be a hurtin' unit.

The GRM Group 44 GT6 had TR6 rear axles installed in some way. That would fix the 'plunge' thing but not the brakes.

ransom
ransom Dork
11/9/11 10:58 a.m.
Curmudgeon wrote: I think the GT6 outer axles could withstand a good bit of HP once the Rotoflexes are done away with, but the problem will be allowing for 'plunge'.

I've mostly been following along, but I don't want to get lost now...

Is this with respect to changing to CVs? If so, is the issue that there's too much plunge for a CV to handle? All I know about CVs is what I've observed from my 2002 and prior Rabbits; mostly that on a given axle you usually have one that only handles angular displacement, and then one that handles angles and plunge...

I finally just went and found a description of a Rotoflex joint

It's like a Guibo that also does plunge. That's terrifying.

Curmudgeon
Curmudgeon SuperDork
11/9/11 12:25 p.m.

Yeah, sorta spooky isn't it? The only reason it worked in the GT6 was the limited amount of plunge and that was due pretty much to the goofy cheap design of using the leaf spring as the upper control arm.

I <3 GT6's (I had a 1970 GT6+ that I loved and still miss sometimes) but I am honest with myself about their drawbacks.

Curmudgeon
Curmudgeon SuperDork
11/9/11 12:36 p.m.

Something else about Spits and GT6's is the front suspension lower 'trunnion' which was their version of a lower ball joint. The thing is made of bronze and has threads so that it screws onto the bottom of the spindle. These CANNOT have grease in them, only 90wt gear oil. But they have a grease fitting; that means a lot of people greased them. Oops. That makes the threads inside wear out and as I found out first hand they will separate without warning. That happened on the street going to my house right after doing 80MPH on the Interstate.

93EXCivic
93EXCivic SuperDork
11/9/11 12:39 p.m.

In reply to Curmudgeon:

Can you take apart the lower trunnion to check it?

Toyman01
Toyman01 SuperDork
11/9/11 4:07 p.m.

In reply to Curmudgeon:

Reading all this makes me glad you changed the Abomination over to Miata stuff before I bought it.

ransom
ransom Dork
11/9/11 4:17 p.m.

This thread in combination with the "A GT500 weighs how much!?" thread makes me feel much better about the notion that taking an old car and going nuts on functional modernization might be misguided.

It's not screwy; it's the balanced middle path.

I just don't want a 3500-pound car, and I don't want self-disassembling trunnions.

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