Hey Guys... Just as a disclaimer, I do have a website that sells Jaguar parts, and will likely use the info I'm posting as a page on my site at some point.
What I am researching is the differences in the Jaguar IRS as it pertains to the XJ6 (68-87), XJ12 (72-92), and XJS (75-96). The reason for this is because in the US we can simplify the offerings and often think "if it's this year, it should be this gearing", etc.. So what I'm trying to write up is a brief explanation of why different sources cite different info, most likely based on market variations, engine offerings, or transmission offerings.
Most of this is just from notes from Nigel Thorley's books, as well as notes in the parts manuals (that aren't great at noting market differences)...
So I want to offer this info to the forum, and also ask for editing where necessary... In reality it's such a vast discussion there could likely not be a concise listing...

Thanks!
David

Jaguar Independent Rear Suspension was introduced with the Etype in 1961. It was also used on the Mark 10, 420G, Stype and 420. On these cars, a Salisbury axle was used with either a 3:77 for manual 4.2L cars, or 4:27 for the 2.8L manual models. Cars with the automatic transmission had a 3:54 or 3:31 for the 4.2L, or 4:27 for the 2.8L… The manual cars, and 2.8L cars weren’t available in all markets. These were all narrower in width than the suspensions used in the later XJ6, XJ12 and XJS.

The XJ6 was introduced in Oct 1968… These later became known as the Series 1 XJ6, that was produced from 1968-1973. The XJ6 was never given a power lock (posi) option in the US, and was only available in the 4.2L engine coupled with an automatic transmission in the US. These cars were given the 3:54, non-locking differential.
Outside the US, there were manual transmission cars, and 2.8L engines that were coupled with lower gearing. The parts manuals show these to be 4:55, 4:27 and 4:09, both in open and locking differentials. These cars were produced mainly for the European market, and all XJ6s going to Austria and Switzerland had locking differentials.

The Series 1 XJ12 was introduced in July 1972. It was given Salisbury Power-Lok differential, with 3:31 gearing, and only available in with an automatic transmission. Not counting the Daimler Double Six, there were only about 3200 Series 1 XJ12s produced.

The Series 2 XJ began with the 1974 model year, and ran through March 1979. The US only received the 4.2L and 5.3L cars, with auto transmissions. Initially the XJ6 was offered with a 3:31 non-locking differential, while the XJ12 maintained the 3:31 Power-Lok. Somewhere in production, the gearing on both was changed to 3:07. This was likely around the time that the fuel injected cars were introduced.
In May 1975 other markets outside of the US were offered a 3.4L engine. These cars were coupled with a 3:54 geared axle.
The Series 2 parts manual continued to show both open and Power-Lok offerings for the 2.8L (4:09 amd 4:55), 3.4L (3:54), 4.2L (3:07, 3:31, and 3:54) and 5.3L (3:07, 3:31 Power-Lok only)

In March, 1979 the Series 3 XJ was introduced. The US Series 3 ran from then, until May 1987. Most of the US Series 3 cars were XJ6s, and only had Automatic transmissions. A handful (around 12) XJ12s made it to the US in 1979, but changing US regulations stopped the import of the XJ12 to the US. The XJ6 initially continued the 3:07 gearing, but changed to 2:88 around 1980, and was only available with non-locking gearing in the US.
Outside the US, the XJ6 3.4L and XJ12 5.3L cars were still available. The 3.4L cars continued with the 3:54 in both locking and open differentials.. The 5.3L XJ12 was coupled with a 2:88 Power-Lok differential.
The Series 3 XJ6 production ended in May 1987. Outside the US the Series 3 XJ12 was still produced until Nov 1992.

The XJS was introduced in Sept 1975. It was only offered as a 5.3L V12 all the way through the 1993 model, and always had a Power-Lok differential. The majority of XJS were delivered with an Automatic transmission, with the number of manual cars released at 352. The earliest XJS had 3:31 Locking differentials, but this was changed to 3:07 probably around 1977. With the introduction of the HE version of the V12 in July 1981, the gearing was changed to 2:88.
Outside the US, a 3.6L version was introduced in 1983. This was coupled with the same Salisbury Power-Lok as the 5.3L cars. Originally the 3.4L cars had manual transmissions, but automatics were available after 1987. There is some mention of Salisbury vs Dana units being used, and some notations of gearing changes, there are likely due to market variations and gearbox variations. The majority of the US based XJS after 1981 will have 2:88 Power-Lok differentials.

In May 1991 the Facelift XJS was introduced. With that came the 4.0L XJS powered cars, along side the 5.3L V12. The 4.0L was offered in both a manual and auto transmission.
In May 1993 the 6L V12 was announced. With introduction of the 6L came the introduction of the first Jaguar IRS to offer outboard brakes and revised calipers.
XJS production ended in Oct 1996.

 

Timely information. I just pulled my rear cage to replace the calipers and have the diff rebuilt. Was considering a diff change - mine is the 2.88 non powr-lok.

Presumably the 3.54 will result in a screamer at highway speeds with even worse fuel consumption. Any opinions/recommendations?

 

Phil...
Some general thoughts on changing diffs, mostly from chatting with XJ owners and Hotrod builders...
A lot of that comes down to whether you want/need a locking diff..
From a US perspective, a 3:54 locking diff is a tough one to get your hands on, while a 3:31 is easier to find... Unless you change the gearing.. (With changing the gearing, I've often heard that you need a power lock diff housing to change to power lock gearing... but even that has been challenged to me by someone saying there are aftermarket parts that can put a posi gearing into a non posi case... Anybody know for sure???)
If you aren't looking for a posi specifically, the US Series 1 XJ diff has the 3:54 gearing, and can still be found (though it is getting a little togher to get good S1 parts cars for me anyway)... As far as gearing changes I've been told that without some sort of an adaptor, you can't change the 2:88 geared diff to another gear set... Again, that could use some editing because i get conflicting reports from people...
One thing that may help you more than changing diffs is looking at the transmission. I have a buddy on here from GA that kept his stock engine and rear end, but when to a GM transmission (I think it was the T350, as suggested by John's in TX)... His car was quick, and I think it helped on his gas mileage too...
John's Quarterbreed Kits seem to make a huge improvement over the BW66...
http://www.johnscars.com/qb/xj6qb.html
Hope that helps a little...

David

 

As it happens, I'm also in GA and also have the John's Cars kit with a TH350. Per John's recommendation I have a high stall speed converter which does indeed seem to make the car more sprightly (based on my seat of the pants recollections from when the BW66 was in there) but gas mileage is pretty horrendous. I get 12 if I'm lucky (around town).

 

David...

All that that is consistent with my experiences. I would only add that in 1993/1994/1995/1996 when the 3.54 power lock was offered across the XJS range, they were all low bias power lock. This is why the later IRS units are so desirable even though it's a low bias power.

The low bias is somewhat better on a track. It will offer improved traction coming off a corner with out the tenancy to induce a spin when adding the power early or in the wet. Low bias is also more forgiving when an inexperienced driver makes mistake becasue there will be less tenancy for the rear to come around when lifting off the throttle abruptly in a turn as well.

A traditional power-lok with the normal high bias is only necessary for a more skilled driver with over 400 HP and sticker tire. And very necessary for straight line performance.

You would need low bias power-lok with traction control so each rear wheel could be independently controlled with the brake system. Traction control was a feature which never made it to the XJS but was probably considered.

 

The offset on the 2.88 carrier is different from the 3.31 or 3.54. I have no idea about the 3.07 as I've never seen one.

If you want to use Dana or Jag gears, numerically higher than 2.88 you need to correct the offset issue, or how far way the ring is from the pinion. It's often said you need a "thick gear" or a "crown spacer". I was never able to find either. The availability of low cost aftermarket carriers with the correct offset have obviated the need for these trick fixes.

 

Depends how much time you spend on the open road.

In my Series III V12 I went from the 2.88 to a 3.31 and view it as a positive change. Certainly much 'busier' at freeway/highway speeds but I seldom take long road trips these days. And the RPM at 65-75 mph cruising speed is right at that sweet spot where power and response is very satisfying. And, naturally, acceleration, particularly off-the-line acceleration, is markedly improved. For me the change was a net 'plus'.

There was a time when 500-700 mile road trips were common for me. If I ever go back to that I'll probably find the 3.31 gearing unsatisfactory.

Cheers
DD