MG Sports Cars

Thought I'd add a thread on the progress of my Roadster since it's something new and different from the usual conversions and there may be some out there who would like to watch it happen. It also may help me find some of the pieces I need.

Years ago, perhaps as much as 6 or 8, I bought a T-bird IRS and began toying with the notion of adapting it to my car. I had blanks cut out to make new hub bearing carriers which would allow the use of stock length control arms and move the brakes inboard but never got any farther, primarily because I'd not tried to fit the pumpkin into place. Now, in conjunction with the Roadmaster car and using the test mule I have done that and determined that it is the wrong differential for the job. And, although the Jag pumpkin is very heavy it turns out to fit into place exceptionally well. There may be yet another diff that is as good of a fit and lighter in weight but until I find that one, the Jag appears to be the obvious choice.

That doesn't automatically make it the right choice from the brakes out though. The one thing I liked most about the T-bird unit was the upper control arm. It isolates the functions of suspension control from those of power transmission, and also eliminates the loss of a wheel in the event if a u-joint failure and I like that a lot. Plus, from measuring it looks like there may be enough room to fit that in without losing any suspension travel or cutting the bodywork. So while I am adapting the Jag unit to the Roadmaster I am also looking at what can be done to adapt the outer parts from the modified T-bird. Presently the drive axles are the main concern as there may not be a simple way to mix-n-match. The Jag uses u-joints and the ford uses a 6 ball CV joint and a 3 ball spider. The single u-joint is not real compatible with the CV and I haven't yet looked at what it wold take to put a CV at each end.

So maybe the Jag unit is not the best choice for this either. Can anyone make a suggestion for an alternative? It would need to be one that does not have big ears out by the pinion as it can only be 7" wide at that point. It can't have a big mount on the rear cover as that would interfere with the gas tank. If possible it should have an aluminum housing. Plus it needs to have a ring gear larger then 8" in diameter. Then the issue is one of attaching brake rotors to the output stubs, but that may not be too difficult. Anyone?

Jim

I guess an aluminum Jag housing would be too much to ask...

Jim,
Have you considered a Corvette IRS. Not sure if the design lends itself to your application (for instance, the one on my old '65 Sting Ray had transverse leaf springs) and I've lost track of the IRS designs in the newer C5 & 6's, but maybe worth a look.
Graham

Jim, the torque requirements certainly do limit the choices. I'll have to keep an eye open next time I'm wandering through the salvage yard. For what it's worth, I have been doing some preliminary design work for an IRS for my Midget and although I won't require as much torque capability as you I think I might have some ideas that would appeal to you. I have looked at the BMW rear axle and it looks like something you might be able to use, especially out of the 5 or 7 series cars. There's also the axles from the big SUVs from both Japan and Germany to consider. Most of those have V8s and would be close to the torque requirements from your motor. I was thinking about building a cage subframe that would attach to the original spring mounting points and shock mounts so that it would be a 'bolt in' unit. Very similar to the unit being developed by Todd Budde for Rob Ficalora. Might be just what you need.

What about a Subaru WRX rear diff? It's tiny, light and should be able to take some torque given that the kids pump their STi's up to 500hp + and launch hard without failure. There are a ton of HD parts available for it. I've also noticed the IRS in a Ford Escape SUV looks interesting, you can see it quite clearly when you are behind one at a stoplight.

The downfall of the T-bird/Couger when they were introduced was the high weight of the IRS subframes if I recall.

Might want to check-in with Evan Amaya, since he built a Jag IRS for his MGB.

Jim,

I have been experimenting with the Aluminum Lincoln Mk8 pumpkin, it's the same as the T-bird only it's much lighter and limited slip is also included on the 95-98? I think.
With a custom rear cover and sub-frame it will fit.
Plus the rear spindles are aluminum, comes with disc brakes and parts are available at your local parts store/speed store.

Only drawback is the battery boxes need to be modified, relocating the battery to the trunk.

As I mentioned above, I am still in the mock up stage.
Call or email for more detailed info.

Cheers

Pete

Several people have been working on IRS setups over the years and several still are. I have my own ideas about how it should be done and personally I don't think such a complex tubing frame is needed or desired. I do have the considerable advantage of having two upside down cars to work with so I'm not fighting the pumpkin to get it up into place and I can shim and nudge it around all I want to get it into the best position. If anybody wants to stop by with a center housing we can easily see how well it will fit.

The MGB bodyshell has a natural channel between the frame rails above the center of the diff which can be used very effectively to bridge between the shock mounting holes. It's fairly simple to also attach to the hanger strap mounts giving a rear attachment point as well. That gives a very complete upper mount structure but for added strength with the Jag unit a center plate bolted through the floor at the corners will make a nearly indestructable mount. All that is lacking is the lower braces and if you look at the bottom pan of the Jag where those braces connect it is downright tinny. Clearly with minimal bracing the battery boxes will fulfill that need.

The early Corvette diff won't work because it mounts off the rear cover and there isn't room for that. The '84 C4 unit might work but the truss is probably too wide and may be too thick as well and it looks as if it may be an integral part of the rear cover. Plus it will be very close to the upper suspension arm. I may try to get a look at one though, if it will fit it has the advantages of light weight and isolation mounts. I don't know what the C5 and C6 used. I'm a little concerned about the torque handling ability of the WRX. Yes it'll handle 500 hp but will it handle 400 ft. lbs of torque? I've got my doubts and that's in the ballpark of what the blown 340 should be putting out, if not more. Let's not forget also that the WRX is AWD and I just read an article where the record holder was snapping both front and rear axle shafts at the same time. I've not looked at the Escape yet, but considering the V6 is rated at 193 ft.lbs I doubt it's strong enough either. As of now the Jag really does look the best except for the weight and lack of isolation mounts, and I think I've found a way to modify the Ford inner drive housings to bolt to the Jag diff, but I'm missing a small box of parts that has the 3-ball spiders, snap rings and boot clamps and I'll need shorter axle shafts so the picture is not as simple as I would like. Also, T-bird drivers have been complaining about wheel-hop so that is another issue to be resolved. But, nobody said it'd be easy.

Jim

Here's one that could have some potential:
[cgi.ebay.com]
Notice that it has mounting points on the top of the diff at the rear and at the front, and that the side bolts at the front are also in a usable orientation. I don't know if it would be strong enough though.

The Nissan diffs have that rear cover mount, the BMW ones are all a little weird in one way or another and would be difficult to mount, Mercedes could be an option.
And here's another one that is interesting, though again the cover mount is likely to cause trouble:
[cgi.ebay.com]

Interestingly enough, the 8" or 9" ford could be a pretty good choice, because of the banjo type housing which would allow for any type or configuration of mounts, pivot or anchor points to be welded to the housing. The disadvantages are the need for suitable output shafts and seals and the decreased efficiency due to the lower pinion position.

Here's another:
[cgi.ebay.com]
The Lincoln Navigator unit is likely to be a ford 8.8 but would need some help to fit in without cutting the battery boxes.

Incidentally, the later Corvette diffs won't work at all without major redesign since they mate directly to the transmission.

The Mazda RX7 diff might be a good prospect as it has a top mount:
[cgi.ebay.com]

The vast majority are iron housings. If the Navi had come with an aluminum case that one would probably be the odds on favorite, only needing a more compact rear mount and inboard brakes. It seems the Mark unit like the one Pete has is one of very few choices in aluminum. I wonder how difficult it would be to rework the front mounts on that one?

Jim

Hi I have been working on a tr4.I have modified the frame to use a narrowed Miata rear suspension.The diff I am using is an 8.8 from a Lincoln MK8.Its aluminum and it weighs about 57 lbs. the Miata subframe can be narrowed as much as 7 in. if you would like to see how the diff. can be mounted to the subframe go to the Miata.net engine conversion forum.There are v8 Miatas with 400 HP.that use this combo without any problems. most of them use the t-bird 7.5 diff. its iron.but in a light car its more than strong enough. i saw you in terra haute in 06 i also met Dan Masters. I am using his wiring harnss so i can use the EFI and a/c without any problems. Best of luck .

Tim, that's a very interesting statement, it turns out that the T-bird diff, open carrier with no axles, back cover or mount is 68.1 pounds. A posi would add some weight, as would the cover so in reality we're talking about something like 15 lbs difference which isn't a lot in terms of sprung weight. I think that explains the prevalence of iron housings. The problem then is the mounts. Here's what it takes to make one fit into the space available.


As you can see the front mounts become unusable and a stamped rear cover is needed. There is a possible way around this, I hope to know more in a few days and will explain it then.

In the meantime since we have so much interest in the topic maybe I need a few ground rules. I do prototype development work and as a rule if I'm not under contract, which is the situation here, what I do with any information that comes my way is my decision. But it can get complicated. For instance, one of our favored vendors other than Pete is also working on a ford based IRS. He'd rather keep his design to himself until it is ready to release and even afterwards if possible. (It's debatable whether that or an open market approach such as that used by Fast Cars works better in the long run.) The point is, I have to honor his wishes if he asks me not to disclose proprietary information. At this point that hasn't happened but I want to avoid being in the position of exchanging information and then having anyone feel like I gave away their secrets. At the same time I'd like to be free to use any trick I run across, and in the process of doing that I very well may disclose what someone else is working on simply because I found it somewhere else. Don't feel bad about that, it isn't intentional and I didn't steal it from you and if it's out there in the public domain you can't protect it anyway. Should that happen I will usually tell you first but regardless I will try to show where I found it. If I happened to figure out the same or a very similar way of solving a problem, then all I can say is that great minds think alike. So if you have something proprietary just ask me not to share it and mum's the word as long as it doesn't show up somewhere else. The added advantage for you is that once you've disclosed it to me I can't very well come up with it on my own.

Having said that, Pete I do intend to call you the first chance I get. I owe Steve a call too but it's been pretty hectic lately.

Oh, and here's a shot with one of the hub carrier blanks I had cut out.

Jim

I talked to Pete and it turns out we've been working in about the same direction. One find we both made is the Explorer differential cover, which if you look straight at the back of it there is a mounting boss out to each side with a cast in structure that looks sort of like a football.
This cover appears to be shallow enough to fit in the available space. Pete says the Explorer case is aluminum as well and has just one of the same type of ears up by the pinion that the T-bird case has. This may be a good option as only one battery box would be affected. One of the links above is for an auction of an Expedition diff:

The photo shows two important features. First, it is an iron case so it will be roughly 15 lbs heavier, but it is sprung weight. Second note the attachment arm bolted to the case next to the pinion. The dimensions here would be critical, but it appears that this unit may allow the use of a mount which will clear both battery boxes.

There is one other issue. Apparently the Expedition may have used two different units, both the 8.8 and the 9.75, and when I asked the seller did not know which one this is. If anyone has an Expedition or Navigator that they could take a look at maybe we can figure this out.

Jim

My sources tell me that the '03-'04 Expedition used the 8.8" carrier with the 4.6L engine. Next task in this project would be to find one.

Jim

The R200 diff used in the WRX STi is also used in the Infinity Q45 (V8), the 300zx twin turbo and is a popular upgrade for heavily built Nissan drifters and dragsters. It weighs ~75lbs, has limited slip and 4.06 ring and pinion (or 3.60?? depending). See this post for more info:
[forums.hybridz.org]
I wouldn't dismiss this one as too weak so quickly. It may be perfect for a kit in that it is cheap and easily found in boneyards. The dimensions may be more compact than the American units - but I haven't verified this.

Will, that's great info. I have a 3.3 LS in my R200S; hadn't looked yet to see what was out there that would be closer to 3.5 if i decide i want to swap it out later... you just saved me several hours research time!

It's worth looking into. I don't like the rear cover mounts though, there isn't enough room for all that in front of the tank. If there was a cover that looked more like the 8.8 one above maybe it could work though. The other problem is with the side yokes and it's the same problem with the Jag unit, how to convert from a universal stub on the diff to a 6 ball CV stub at the axle. The T-bird uses a 3 ball sliding coupler. I'm not saying any of this is easy, the T-bird has no provision for mounting the brakes inboard and that should be fairly easy with the r200. It might even be possible to plug the T-bird stubs into the dana or r200 case, I really don't know yet and even if it is there's still the small matter of a disc mounting flange that would have to be welded to the coupler body. At this point I'm just trying to find ways to simplify, and believe it or not, the Jag is looking like the best solution. Odds are that system will get done first anyway and in the process I very well might decide it would be best for the roadster as well. A lot of that decision may very well hinge on what I can come up with for a top link. But that discussion is over on the Roadmaster thread.

My new tire came in yesterday, it's a Dunlop Qualifier so it doesn't match the BFG's exactly and it's NOS and hard as the dickens, but it's the right size so I'm grateful to get it. That means the car will go up on jackstands and the springs will get swapped out while I'm sealing the rim halves and changing the tire. The 3 other tires aren't quite as bad so I bought some time by doing this and will be able to drive the car more. I think it'll be nice to get back closer to a stock ride height, even if the cornering does suffer a bit. I've not really had a chance to do anything on the engine or tranny lately as I've got the motorcycle torn down and have been concentrating on getting that back together, but I hope to order parts for the transmission next week. It's a 2004r slushbox but I'm planning a few tricks for that which I hope will make it better suited to the car.

Jim

Jim here is some info on R200 Differential's and ratio's. I'm using the 3.54.1 ratio in my TR6. It was easy to find at local recycle Nissan yards.





I don't know if this helps, how about an irs diff out of a GTO 2004 or 5. On ebay for the whole setup 500.

That's interesting. The two main trouble spots are the gas tank and the battery boxes. This means we need a cover without a mount on it (or perhaps off to the sides like the explorer) and a snout with the mounts located in real tight to the pinion or above it. Looking at that one it appears the diff is bolted solidly to the support, giving the kind of compactness we need. It also has the type of inner CV joint that would allow a spider for a brake rotor to be bolted to the CV body for inboard brakes. It's just possible this could be the missing piece of the puzzle, with more pictures maybe I could tell. But using the T-bird layout for the arms and that center section could be the answer.

Jim

More pic's for you on the 04-5 GTO irs.

The Pic's are to small. I tried to enlarge them but they were all distorded.

I found these pic's on ebay motors . Do a search for ebay store "Cleveland Pick-A-Part" the seller "pick-a-part"

Location OH

You will be able to see them in bigger pic format. His store has 27 pages . At this time it is on page 14. He does have several to look at.

Also there was a 06-7 Dodge Charger IRS to page 22 I think.

Hope this can help,
Calvin

Are people retaining the stock MGB fuel tanks in their V-8 conversions? I had to replace my tank with a 14 gallon fuel cell to extend the range of my car. With the stock tank, I was always looking for a gas station, which gets old fast on a road trip. Here is a picture with the carpet removed. Yes, that's a 2400 watt amplifier - some people never grow up...