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Kirkham Motorsports

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  #1 (permalink)  
Old 06-09-2018, 01:59 PM
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Default Which differential for a scratchbuild?

In order to put the rear cross-member where it goes in the frame, I evidently need to decide on a differential.

As far as I know, there are the following choices:

1. Reproduction Salisbury (But where do you find these?)
2. Jag XJ6 https://www.ebay.com/itm/JAGUAR-XJS-...8AAOSwOd1azoV4
3. 90s Ford T-Bird, Lincoln, Cobra https://www.ebay.com/itm/Lincoln-Mar...-/302752932206
4. BMW E36 https://www.ebay.com/itm/BMW-E36-3-7...-/232794518308

Anyone have experience with any of these and have suggestions/sources?

Right now I am leaning toward the XJ6 diff.

Thanks!
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Old 06-09-2018, 02:35 PM
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I would go with either the T-bird or a Mustang Cobra rear. You might also want to look at a C4 Corvette rear. They are reasonably bulletproof and all aluminum. You would probably have to have the axles narrowed, but there are places that will do it for a reasonable cost.
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Old 06-09-2018, 03:31 PM
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Originally Posted by joyridin' View Post
I would go with either the T-bird or a Mustang Cobra rear. You might also want to look at a C4 Corvette rear. They are reasonably bulletproof and all aluminum. You would probably have to have the axles narrowed, but there are places that will do it for a reasonable cost.
Either of those options are available, but would complicate the mounting, of course.

I found this picture of how the original were mounted in this thread Salisbury and Jaguar differentials

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Old 06-09-2018, 03:41 PM
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BTW, the XJ6 and C4 Corvette internals are structurally identical. They are both Dana 44s. Only the C4 rear requires redesign and re-engineering to fit into the space of a 289 leaf spring rear suspension. It "appears" as if the C4's mounting points are exactly where the leaf needs to go.
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Old 06-09-2018, 03:46 PM
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Of those, the Jaguar is the closet to the original 4HU differential. There are companies selling brackets that allow the Jag unit to be used with original mounts like would be in an original chassis.
The ultimate would be an aluminum 4HU, available from a couple different manufacturers.
Kirkham makes and sells one, that's what I have. I think they're around $5000 or were a few years ago. It has the correct mounting bosses in the nose of the case. Gerry Hawkridge in England also has them. You can contact him at Hawkcars.co.uk.

Larry
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Old 06-09-2018, 04:05 PM
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Of those, the Jaguar is the closet to the original 4HU differential. There are companies selling brackets that allow the Jag unit to be used with original mounts like would be in an original chassis.
The ultimate would be an aluminum 4HU, available from a couple different manufacturers.
Kirkham makes and sells one, that's what I have. I think they're around $5000 or were a few years ago. It has the correct mounting bosses in the nose of the case. Gerry Hawkridge in England also has them. You can contact him at Hawkcars.co.uk.

Larry
Like this one?

https://www.ebay.com/i/112333973510?chn=ps

And since I am fabricating the frame myself, I could make it fit.

Drew
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Old 06-09-2018, 04:25 PM
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I think so, yes. There are a few different 4HU diffs used in the Jaguar though. The issue with them all though is no front bolt bosses in the nose for mounting like the Cobra 4HU has. You can see them in the photo you posted above. Will the mounts you weld in be strong enough is the question.
I always felt like a mount could be fabricated that wrapped around the nose of the Jaguar differential and be bolted to the frame but I don't know of anyone who's done that. It's basically round and I would think it could be done. Side mounts could be fabricated that bolt to the existing side plates, so that's not that big of a deal.
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Old 06-09-2018, 04:32 PM
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So I am wondering. If you need the nose bolts, why cannot you use the mid-90s XJ rear diffs? They have four nose bolts and the two casing bolts. While I don't know if they are located exactly in the right place, since I am fabricating the mounts anyway, what makes them not a candidate?

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Old 06-09-2018, 04:39 PM
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What about the 2015 Mustang IRS? Mounting system is straight forward.

Factory Five is using these. Good selection of gears, plenty strong (stronger than the old Super Coupe version), reasonably compact. It uses CVs for output and CV input but Factory Five adapts to a flange yoke & u-joint.

Cheap plug, I have an extra that is available.

Jim
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Old 06-09-2018, 04:55 PM
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I guess I am concerned about how the leaf spring suspension will fit with one of those.

Drew
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Old 06-09-2018, 04:59 PM
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I suppose that mid 90's Jag could be used but I don't know anything about them. I'm always looking to copy what was original, so beyond that, I can't advise very well.

I did use a 2003 aluminum Mustang Cobra 8.8" in my Hurricane. Nice differential and strong. Just change the rear cover to one that's a lot stronger if you go that route. The production cover is the weak point on that one.
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Old 06-09-2018, 06:24 PM
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Quote:
Originally Posted by LMH View Post
I suppose that mid 90's Jag could be used but I don't know anything about them. I'm always looking to copy what was original, so beyond that, I can't advise very well.

I did use a 2003 aluminum Mustang Cobra 8.8" in my Hurricane. Nice differential and strong. Just change the rear cover to one that's a lot stronger if you go that route. The production cover is the weak point on that one.
Larry
I'm thinking based on affordability and my half-butt accuracy requirements, the XJ6 diff is the way to go. Now to find the bracket you speak of.

I talked to David Kirkham tonight, and their differentials are now $6k new, and $5k rebuilt, and come with diff only.

Drew
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Old 06-09-2018, 06:34 PM
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I founded a picture of a side by side adapted Jag unit next to a Kirkham unit.



It was in this thread.

Differential unit
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Old 06-09-2018, 08:08 PM
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Yeah, I remember that thread. Those are the brackets I was referring to. It allows the Jag 4HU to be bolted into an original or original style frame. Seems to be a nice set up.
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Old 06-09-2018, 08:52 PM
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I installed an 8.8 Tbird IRS in and original style frame. It was a bit of work, but it fit and was affordable and I was able to adapt the Ford uprights with my scratch built control arms to make it all work. Let me know if you need any pictures and I'll see what I can do. Much cheaper than the Jag IRS pumpkin and a much, much less than the Kirkham offerings and original style uprights.
If you are building a 289 leaf spring chassis and original uprights, this can also be adapted to that unit easily.

Bob
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Old 06-09-2018, 09:19 PM
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The pictured Jag diff top adapter mount in post #13 will not work in an original specification 289 leaf spring chassis, it is too tall and will hit the frame. However, that adapter mount will work in an original specification 427 coil spring chassis.
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Old 06-09-2018, 09:41 PM
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The Dana 44's are very nice units and have served well for several decades, increasing power levels not withstanding. There is a relatively good supply of aftermarket parts available for them and by and large I have not heard of people who had any significant design, fabrication or installation issues with them. If I were going to use one I would definitely use one of the Kirkham units, that are easily the very best and nicest Dana 44's available anywhere today.

The other very good choice as several have already said is the 8.8 inch Ford IRS center section as used in several of their passenger cars. I would recommend you look for an 8.8 IRS housing out of an Explorer rather than a passenger car. The Explorer 8.8 IRS housings are by far the most robust of all the aluminum 8.8 IRS housings Ford has ever produced. Be sure you get a 2007 through 2010 housing. Those are the ones with the massive front ears for locating the nose of the housing.

When ACE23 was struggling with the same decision alternatives you currently are I posted up some pics and suggestions for him on the site here. This is a link to the post => Ford 8.8 IRS Suggestions.

You might find something in there that could be helpful as you go about noodling out the best alternatives for your build.

BTW congrats on the decision to do a scratch build. It is the best way to get your Cobra exactly they way you always wanted and thought they should be — lots of work but also lots of satisfaction and immense pride in the finished product that is YOUR Cobra.


Ed
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Last edited by eschaider; 06-09-2018 at 09:46 PM.. Reason: Spelling & Grammar
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Old 06-09-2018, 09:44 PM
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Yeah, that's true! The brackets I was thinking about bolt onto the side, more like the original does. It doesn't use that top bracket.

Larry
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Old 06-10-2018, 11:06 AM
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Quote:
Originally Posted by Three Peaks View Post
I installed an 8.8 Tbird IRS in and original style frame. It was a bit of work, but it fit and was affordable and I was able to adapt the Ford uprights with my scratch built control arms to make it all work. Let me know if you need any pictures and I'll see what I can do. Much cheaper than the Jag IRS pumpkin and a much, much less than the Kirkham offerings and original style uprights.
If you are building a 289 leaf spring chassis and original uprights, this can also be adapted to that unit easily.

Bob
That is good to know. My wife and I are adding up the costs of this build and would like a more affordable option.

I would very much like pictures of your setup. It will help with the decision, for sure.

Quote:
Originally Posted by Tom Kirkham View Post
The pictured Jag diff top adapter mount in post #13 will not work in an original specification 289 leaf spring chassis, it is too tall and will hit the frame. However, that adapter mount will work in an original specification 427 coil spring chassis.
Also very good to know.

Quote:
Originally Posted by eschaider View Post
The Dana 44's are very nice units and have served well for several decades, increasing power levels not withstanding. There is a relatively good supply of aftermarket parts available for them and by and large I have not heard of people who had any significant design, fabrication or installation issues with them. If I were going to use one I would definitely use one of the Kirkham units, that are easily the very best and nicest Dana 44's available anywhere today.

The other very good choice as several have already said is the 8.8 inch Ford IRS center section as used in several of their passenger cars. I would recommend you look for an 8.8 IRS housing out of an Explorer rather than a passenger car. The Explorer 8.8 IRS housings are by far the most robust of all the aluminum 8.8 IRS housings Ford has ever produced. Be sure you get a 2007 through 2010 housing. Those are the ones with the massive front ears for locating the nose of the housing.

When ACE23 was struggling with the same decision alternatives you currently are I posted up some pics and suggestions for him on the site here. This is a link to the post => Ford 8.8 IRS Suggestions.

You might find something in there that could be helpful as you go about noodling out the best alternatives for your build.

BTW congrats on the decision to do a scratch build. It is the best way to get your Cobra exactly they way you always wanted and thought they should be — lots of work but also lots of satisfaction and immense pride in the finished product that is YOUR Cobra.


Ed
I can literally buy a half dozen of those today, and even better, it appears as if they come in 3.73, which was similar to the original competition cars 3.77 final drive ratio.

Thanks for the congratulations. It's been a very long time coming, and I've finally refined my goals in the build.

1. I want to learn about the processes and gather the tools necessary to scratch build a car
2. I want to end up with a 289 Cobra that not only replicates the look, but also replicates the "feel" of an original competition car
3. I want a "piddle around" business in my retirement where I build a car that interests me, play hard with it and then sell it to finance my next project. And I'm not in the position to become Jay Leno and actually buy and keep all those exotic cars I've always wanted.

All of this has been very helpful. Thank you very much!

Drew

Last edited by 120mm; 06-10-2018 at 11:08 AM..
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Old 06-10-2018, 11:15 AM
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Aaaand..... Look what fell in my lap today!

https://www.ebay.com/itm/113051509277

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