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1/24/2015 8:43 am  #1

Newb questions on changing gears in an XJS differential for a MGB V8

I beleive the unit I have is a 1996 XJS V12 Diff with 2.88 gears. I have a set of 3.08 gears (the proper setup for my MGB V8 given the tranny) and I want to swap them out. My first question is does the carrier have to be changed to use the 3.08 gears? The unit is limited slip BTW ( all of them might be, like I said I am a newb). Second question is what do builders typically do to limit the axle travel? It seems that the axles could potentially drop enough to actually bind the u joints if not controlled (maybe I am overthinking this and it would only be an issue if I was sending the car airborne on a regular basis, but I am curious). And thirdly, I appreciate any advise you can provide on how to successfully work this diff into my build. A shop has already shortened the overall width to fit, but I am wondering what the best way to attach it to the MGB unibody is. Any advice on that will be appreciated.


1/26/2015 11:07 am  #2

Re: Newb questions on changing gears in an XJS differential for a MGB V8

The Jag unit is a Dana 44, 8.5 inch ring gear.  If the 3.08 setup is also for a Dana 44, you'll be in good shape.  A specialized axle shop should be able to do the swap and proper setup for about $300.
The carrier size doesn't change until you get between 3.73 and 4.11.  I don't remember the size exactly, I just know it is between the two.
Bump stops are tyically used to keep the shocks from bottoming out for a fully compressed suspension.  Shock length "should" limit the extended travel.  However, (I think) with some 'vette rears, breakage has been known to happen when power is applied and the rear suspension is fully extended while on a lift.  I think this is a rarity and has something to do with the way power is applied and the limited slip distributes power.
Research the unit Daze placed in his Mustang.  As a uni-body, some fab work was needed but overall this was pretty straight forward.

Good luck with your project!


2/17/2015 2:39 pm  #3

Re: Newb questions on changing gears in an XJS differential for a MGB V8

Copied this from the early IRS threads;

 -  NOTE 04/09/10 It was recently brought to my attention by Sedat Yalcin that the above information only applies to Dana 44 carriers.  If  you are using the original Jaguar carrier you need to be aware of three different carriers.  The 2.88:1 carrier will only work with  2.88:1 ring and pinion.  If you have a 2.88:1 carrier and want a lower ratio you must change the carrier or buy a custom "THICK" gear.  After getting this information I confirmed it by contacting Mike at  CWI and he added that  Jaguar made there break between low and midrange ratio at 3.54:1 and 3.76:1.  The important thing to keep in mind when deciding weather you need the parts set up for high  or low ratios (excluding the 2.88:1)  is not weather the carrier is Jaguar or Dana 44 but weather the ring and pinion are Jaguar or Dana 44.  If the ring and pinion is Jaguar, regardless of weather  the carrier is Jaguar or Dana the break in ratios is between 3.54:1 and 3.76:1.  But in reverse if the ring and pinion is Dana, regardless of weather  the carrier is Jaguar or Dana the break in ratios is between 3.73:1 and 3.92:1.


Nothing is impossible, some just cost more!

2/18/2015 3:17 pm  #4

Re: Newb questions on changing gears in an XJS differential for a MGB V8

From Wikipedia, the ranges for Dana 44 carrier sizes are:

  • Carrier sizes: 2.72:1 – 3.73:1 and 3.92 – 5.89:1

Mike from CWI had some really good information and parts.  I'm not sure what he based the info that Sedat Yalcin posted on.

Also from Wikipedia regarding the differences between Salisbury and Dana 44:
The ring gear in the Salisbury version uses slightly smaller mounting bolts and the pinion shaft is a different diameter than the common version. Naturally the cast housing is also unique to the IRS model. Standard 8.5" Dana 44 ring and pinion gear can be used in the IRS model through the use of a special installation kit which includes special shouldered bolts to mount the standard ring gear to the IRS carrier and a special pinion bearing set to fit the standard pinion shaft to the IRS housing. Gear distributors with knowledge of the Jag IRS should be able to get the installation kit. US Dana 44 gears a slightly stronger than Jaguar gears, and Jaguar gears higher priced, but once the installation kit price is added to the lower US gear prices cost is about the same. If Jaguar makes the needed gear ratio there isn't much difference in strength. The consensus in the hot rodding community is that the Jaguar IRS is good for up to 500 hp from the factory. Most V-12 Jaguar and E-type six-cylinder models used limited slip versions, other models (mainly the XJS) used a standard differential stock with limited slip as an option.


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