Jag Diff Suggestions (SuperV8 Content)

Hey all, I don't post much here, but I read quite a bit.

My quandary is thus: I've got a great car (2006 XJ SuperV8) that I absolutely love with the exception of a couple little things. The biggest is that the supercharged V8 (rated 400hp, 415tq or so from the factory) sits in front of an open dif. It's a travesty that Jag didn't put LSDs in these cars, even the flagships.

Searching the Jag forums, I've only found out two things. 1 There is a Quaife unit that supposedly works, however I can't even find it in stock (I believe it's Quaife QDF3W) It's also more than a dollar, so I think it's expensive. I'd have to pay for installation as it's 120 degrees and I've also never installed and shimmed a dif before. 2 Most of the people on the Jag forums either like theirs stock (wine and cheesers in miataturboland), or have way more money to mod with than I do.

Anyone know of any solutions? I believe the stock dif is a lovely 2.87:1. Obviously, if I could get that ratio up a bit it could really wake the car up! I don't care about mileage as I can't keep my foot out of it anyway.

Any known GRM solutions?

I thought the early X350 Rs had limited slip?

I gave a friend that specializes in jags. I should run in to him in the next week or so. I will ask him about this.

What differential do those Jags come with? Ford 8.8"?

Relevant to my interests... Looking forward to hearing more about this luxo-rod. have been watching really nice examples in the low teens and lesser ones for 8-9.

RossD wrote: What differential do those Jags come with? Ford 8.8"?

I'm not sure if the X350 cars use the same diff, but the older Jag IRS setups (both the early inboard brakes version and the later version in the XJ40 / X300 / X308) used a Salisbury diff very similar to a Dana 44 (with some parts interchangeability). IIRC, some years may have had an actual Dana 44 in them.

Thanks Dean, and thanks everyone!

As I continue to research this, I'll post here with findings. I've got a half dozen threads to dig through on the various Jag sites, but work is busy so I can't slack too much today.

As far as the aluminum body XJs go, they are pretty fantastic. It's a huge car with the footprint of a decent sized truck, but it feels smaller the faster you go. It handles well, but it's not exactly auto-cross material. It will happily go 90 on the highway for hours and I feel refreshed at the end of a long journey.

The trouble area is generally the air suspension, but if you want coilovers, there is an aftermarket solution for that. Also, there are remanufactured units that seem to last longer. I've replaced one, it was pretty straightforward. General wisdom is if you replace them when they start to go, you won't burn out the compressor. However, there is a rebuild kit for the air tank compressor so it's no longer a thousand bucks if that goes out. (A side project that I haven't started is to see if I can run a train horn off of the tank) Also, since it's a Lifetime* fluid transmission, they die if not maintained. So, change the fluid and filter and have the computer reset and all is dandy for pretty much ever.

As far as power goes, you can only go so far with the stock supercharger. Pulleys spin it faster, but there is a max efficiency limit. There is an aftermarket upgraded supercharger good for a couple hundred more horses, but it's an expensive endeavor. However, this is GRM and a 4.2 liter low compression overbuilt engine could go a long way with enough creativity (remote turbo set up, perhaps? replacing the supercharger with a turbo would free up the hundred or so hp it takes to spin the dang thing without putting any more strain on the motor)

Whatever, it doesn't really matter since with an open dif it can't put the power down that it has.

I got mine with about 80k, it now has 125 or so on it. Only issues have been a cracked coolant expansion tank, some bushings, and engine mounts (nothing out of the ordinary for 10 years of use). (And brakes, and tires, and windshields, Pro Tip: if you live in AZ pay for the glass coverage!) Also, the stereo stuff is okay stock, though dated, and kind of a pain to upgrade. Doable, just pricey (like most modern luxowhatevers)

*no such thing damnit, give me my dipstick back!

So, based on research so far, including many conversations with a jaguarforums.com member Cambo (who most if not all of this info is derived from), it looks like an LSD doesn't have to be terribly expensive after all. There are a couple of options besides the Quaife. I've only been researching this for the XJ (X350) cars, though S-Type info can be extrapolated.

First, a note! Changing ratios cannot be done to all of these cars, apparently the computers freak out and put the car into limp mode when they see conflicting signals about how fast different things are spinning. However, the 2006 and up has this info stored in a different file that supposedly can be changed using a dealer-quality computer (though dealers won't do it, you'll have to find an indie). I'm still researching these details.

Adding an LSD, but keeping the ratio should be straight forward as far as the car's computer goes since it simply doesn't know anything is different. It seems at this point that the pre-2006 are stuck in this boat.

That brings us back to the Quaife, the only one that is known to fit in the stock pumpkin, or switching the pumpkin and half-shafts to something that fits and has the same ratio or fitting your current stock gears into the new (used) pumpkin.

The first gen S-Type has a rear that can fit a ford 8.8 LSD unit, but not the 8.8 gears. The half-shafts will also need to be sourced. I believe the gears from the later difs will fit just fine. These 4.0 differentials and half-shafts supposedly bolt right up and accept the ring and pinion from the gen 2 allowing you to keep the ratio.

The second gen cars have the same goofy offset, unequal half shaft units as all of the x350s, so that's why 1st gen only.

Another option is the first gen Lincoln LS. It is reportedly the same size as the first gen S-Type's just doesn't have the cooling fins.

So, if you want to keep your ratio, get a quaife. If you don't want to go the quaife route, get a pumpkin and half shafts from a first gen S-Type (or try the Lincoln LS parts), swap in the gears from your car for the right ratio, and source a 8.8 LSD. Depending on how good you are at scrounging, this is significantly cheaper! Quick ebay search, it looks like about $500 for S-Type parts and a 28 spline LSD(not including any rebuild parts though, if you want to do anything 'while you're in there') the Lincoln stuff is cheaper yet.

If you have a 2006 and up, do the same, but keep the 3.31 gears in it, or change to whatever else you find that fits, then have some electrons rearranged to keep the computers happy.

This is all for informational purposes only, by all means go buy parts (and whole cars) and wrench away, just know that you are out on the front lines with me here. Cambo tells me that one of the UK forum members has added the 8.8 LSD on his S-Type, switching out the first gen S-Type's gears for the ones in his 2nd gen, to keep the computers happy.

Hope this helps!