Hello all

I have been trying to find a viable means of increasing the diff ratio in my x100 from 3,06:1 (today) to somewhere around 3,50:1. The goal is to increase acceleration of course - I do not spend so much time in top gear. Talking of which, I use a manual T56 instead of the auto box which should eliminate some of the reprogramming issues that other people have discussed in various threads. The current diff also has a Quaife LSD which I would like to reuse! Whilst not a must, it would simplify things if the ´new´ diff fits without needing to modify the car (at least not that much anyway!).

Anyone got any suggestions?

 

I have a 2002 XKR (with a Mercedes transmission) and have made this exact same modification (except I went to a 3.27 vs. a 3.50 rear axel ratio). Based on considerable research on this forum, it is my understanding that if you have the Mercedes transmission (in the US MY 2000 - 2002, in the EU MY 1999 - 2001), then this mod is possible. However, you need to get a new/reprogrammed Transmission Control Module (TCM). I purchased my TCM from another forum member who had it reprogrammed for a 3.27 rear end. It is my understanding that if you have the ZF transmission, then it is not possible to change the rear axel ratio; I can't recall the reason.


I am very happy with my 3.27 and don't think I'd want to go any higher. I like the acceleration with the 3.27 but I took a pretty good hit on gas mileage, loosing 5 or 6 mpgs.


There are forum members, such as avos and XKRacer who are very knowledgeable on these matters so hopefully that will add their 2 cents as well.


Mark

 

A diff from a Mexico XK8 or a 3.2 XJ8 Euro car will be 3.27:1

The Sedan diff will need the Jurid flange removed, the pinion shaft round Jurid extension cut off and the U-Joint flange installed to be compatible with the propshaft.

I had to perform this modification to get the 3.2 Sedan diff to fit the XKR and swap over the QUAIF 'internals' to fit Marks car.

bob gauff

 

The 3.27 diff from a Mexico is a straight bolt in. There is one on Ebay
98 99 00 01 02 Jaguar XK8 Carrier Assembly EXC Mexico MKT 520151 | eBay

This is the same one I sold to a fellow forum member. If you find a sedan diff it will need the flange swapped over. You can also go with a 6 cyl XJR in either 3.58 or 3.77 & they are already pozi. Again you will need to swap over the flange as well as get the front mounting plate for the diff. This will give you the acceleration you are looking for. The problem may be that Norman's black box may not be compatible with a different ratio. You should check with him before swapping out the diff.

 

A Jag xk8 is for "Stylin". Buy a Mustang or Camaro for accelleration. I have 3:73's in the Cobra and first gear is worthless. 3:23's are a good compromise that you can be happy with. The posi makes a huge diference in accelleration due to doubling traction. My old Mustang went from .4 G's to .8 G's of accelleration just by going to a Traction Lok dif. That said, I am swapping out the rear axle to 3:55's this winter.

Just remember, more G's means more wear and tear until something breaks, like an axle, aluminum driveshaft, or a u-joint in your Jaguar.

 

Thanks to all of you for the feedback! The idea that comes closest to my wishes is a 3.58 diff. What do you mean by "already pozi"? It is a bit hard getting that sort of thing done over here in Sweden so I will check with Tom L or RGC if I can get any help from them. Good point regarding Norman´s black box . initially I expected that this would not be an issue but thinking again, I realise that it may be. I will have to check that. By the way, I thought that things like the diff and u-joints never (or at least very seldom) brake - is that so or is there a clear risk when increasing the torque by around 15-20%?

 

The 1988-1997 XJ sedans came with or without limited slip diffs.
The V12, XJR and some VDP diffs were power-loc.
The 3.58 limited slip diffs will have a red and yellow striped decal with a black X or line across it.

The red/yellow decal is the 3.58 ratio and the black line denotes limited slip.

The offset in the older diff may or may not fit into the XK propshaft tunnel without side interference. The sedan tunnel is wider.

The XK was designed for the in-line diff unit.

bob gauff

 

What transmissions did the Mexico market xk8's and Xkr's use? And where would the TCM live in them?

 

Hi Bob

Do you have a source for Mexican diff's for the XK8 ? I have an LS conversion and a 460l transmission I believe the diff is the same and still at 3.06. I would be interested in finding out more about this mod if possible.

yours

Garry (San Antonio TX) gvs1047@gmail.com

 

the XJ8 3.2 also had a 3.27:1 diff. you can find them in europe but you need to change the flange over

if the diff from the XJ40/300 fits it’s a no brainer, much better setup and you can put in any 19 spline dana44 carrier you want