Well I am own another mission for my 02 xk8 and that is to change the single trac to a upgrade posi trac rear. I seen a few folks on here who have said that is a project on there to do list. I just sent out a email to a company called Quaife about there product for the changeover. Any of you fellas who may know a lot more than me on this changeover would be greatly appreciated as usual. From just looking at the rear differential on the car looks easily to get to and out.

 

David,

I've moved this question about your 2002 XK8 from XK/XKR 2006 onwards to XK8/XKR forum. Members here with the same model will be able to help.

Graham

 

Hi mate these are very much like x300 rear ends and the easyest way to do it is to drop the full rear axle an part the diff when the axle is on the floor as these can be a right pain as I found out

 

One of the first mods I did to my 2002 XKR was to add a Quaife ATB (gear type) limited slip differential (LSD) because I planned to upgrade the s/c, which I have now completed, and I wanted the extra power to go into acceleration and not into leaving 20k miles of tire rubber on the highway. I am very happy with this mod.


I bought my LSD from a company called Pro Street (Part Number QDF5W). This link is to the XK8/R compatible LSD. Pro Street had the best price, beating everyone else by $500 or more. I don't know if this is still the best price but it's a good place to start looking.


One thing, you should check with forum member avos, as he is very knowledgeable on this (and many other subjects regarding the XK8/R). He favors the clutch type LSDs because they will lock up the rear end after a limited amount of slip, thereby allowing you to "drift" through corners. I don't drive that aggressively and didn't want to "drift" through the corners so I went with the Quiafe which automatically biases the torque between the two rear wheels and does not lock up.

 

The best way to go is by installing a 3.77 LSD from an XJR6. You can buy the diff thru Coventry West & all you need is the front mounting plate. This will give you LSD with a large increase in acceleration from the higher ratio. The install is straight forward with the only mod being to swap over the exhaust mounts. The TCM does not need to be touched as long as you have a 5 speed trans.

 

Jaxkr, Does this work as well for a 2000 XJR, or is it only for an XK? Then purchase a front mounting plate as well from Coventry? Thank you, Louis

 

The XJR is more expensive as you have to reprogram the TCM. I only know of one company that is able to do it.
Automotive Tuning and Electronic Engineering. ECU programming and tuning

 

When I had the Quiafe LSD installed, I also swapped out the stock 3.06 rear end for a 3.27 but with the Mercedes transmission on the 2002 XKR, this meant a reprogrammed TCM. I bought both the new differential and reprogrammed TCM from another forum member.


I have been very impressed with the increase in acceleration from a standing or rolling start and am not sure I'd want to go to a much higher ratio. Also, just this modest increase in the rear axle ratio took my gas mileage from about 22 mpg to about 19 or 20. I realize that these cars are not economy cars and mpg has never really been a consideration for me. However, based on my experience with the 3.27, going to a 3.77 rear axle ratio I expect would drop your highway mpg down into the 14 to 16 mpg range. Remember, these cars are GTs, not drag racers. I am pleased with the 3.27 but am glad I didn't go any higher.


Mark

 

You are correct about the fuel mileage. The best I see on the highway is about 15 mpg. The saving grace with me is that I drive less than 5000 miles a year. I think on an XK8 which has far less performance upgrades the 3.77 is still the way to go.

 

Did you mean to say 3.27? That's what the XJR6s came with.


Cheers
DD

 

The diff I had built by Coventry West was a LSD from an XJR-6 that we installed a 3.77 ring & pinion. The 3.27 mentioned in an above post is a Mexican market part for an XK8.

 

I'm very interested in this too and am curious how it compares cost wise to the LSD mjlaris had done. I drive aggressively and low speed cornering demonstrates repeatedly a delicate balance of throttle to stay in first gear through turns and not have traction control abruptly dial back power or spin tires with traction off. That said, my Michelin Pilot Sport A/S are holding up nicely after 16 months but I've wondered if the cheapest option is simply to try super sticky tires for summer such as the Michelin Pilot Super Sport. I had both on a Porsche Boxster and at the limit the Super Sports were beautiful.

 

I installed an OS Giken LSD. It's great. The problem is that I may have bought the last available one?? Not enough people willing to pay for quality, I guess.

Good luck.

 

So what are you fellas paying for labor costs to have this alternate differentials and rear gear ratios installed?

$1000.00?

How about differential and related items net numbers?
XJR6 rebuilt differential from Coventry West is $1095, + $150 for a front mounting plate + $125 for shipping.
What have you paid for your Quaife, OS Giken and Aisen parts by comparison?

I am compiling a summary sheet for this whole thing.

 

I paid $1500 for my Quiafe LSD, $2000 for the 3.27 rear axle ratio with differential (bought from another forum member who had upgraded to a higher rear axle ratio and most of this was for the reprogrammed TCM, which is necessary on my 2002 XKR with the merc transmission). The installation was performed by another forum member, motorcarman, who runs an independent Jag shop not far from where I live. I believe the installation cost about $800 but I can't remember for sure. All in all, NOT a cheep mod. but worth it in my mind.


Mark

 

You also need to remember that if you are using a sedan diff with a rubber JURID coupler, then the 4 bolt U-Joint flange will need to be installed and the pinion shaft modified (CUT) so that the propshaft will bolt up.

The XJ40/X300 diffs have the pinion shaft 'off-center' and the XK diffs are 'centerline' as are the X308. That is why the front plate needs to be replaced. The hole is moved slightly to one side.

bob gauff

 

Ah motorcarman, perhaps you can help clarify some things for a summary that I and other non-mechanics can comprehend and then pitch to our local mechanics (I am in Denver CO). Please feel to correct errors.

1. XK8/XKR low speed performance greatest inhibitor is the stock rear ratio. So for those of us looking for stronger low-end pull, especially in 0-50 MPH environments, supercharger upgrades not as effective as rear ratio change. Downside is lower top speed and worse gas mileage on highway (neither of which I get to do much anyway).

2. A 2002 XK8/XKR in the USA or Canada has a rear ratio of 3:06 while Mexico had 3.27 . (Much earlier cars had a variety such as 3:03 or 3:31)

3. Changing the ratio to 3.27 (from Mexican market for XK8, e.g. mjlaris), 3.58, 3.77 (e.g. Jaxkr) or even 4.09 requires an alternative rear differential such as the Jaguar 6 cylinder XJR sedan differential which Coventry has ($1365 parts and shipping) more expensive ones such as Quaife ($1500), OS Giken, Aisen (clutch-type good for drifting according to AVOS)

 

While the 3.27 rear axle ratio does lower the top end, which Jaguar claims to be 175 mph, the ECM actually limits the top speed to 155 mph, which I estimate is less than my top speed with the 3.27 rear end. Therefore, there is no "real" loss in top speed. Also, I don't know about you, but I run out of b*lls far before I run out of hp!


As for the gas mileage, with the 3.06 rear end I got 22 to 24 highway mph (which is most of my driving). With the 3.27 rear end I get 19 or 20 highway mph. Not a huge difference, which I can partially offset if I can get real gas instead of gasohol.


Mark

 

You are correct when I ordered the diff it was with a 4 bolt flange. The other mod to do at the same time is to install blue top solenoids in the transmission. This gives a much more positive shift.

 

Can someone clarify the relationship of changing the rear axle ratio and rear differentials including limited slip differentials (LSD) ?

Rear axle ratio change requires a new differential?

I don't understand parts and labor for this versus supercharger upgrade costs...

Goal is much snappier 0-50 as launch off-the line when I'm first at the light is the big thrill I get with my XKR (besides its beautiful appearance inside + out).