I've decided to swap to steeper gears and 3.58 seems to be the simplest swap. From what I've read and been told, an X300 diff is the easiest solution as it has the taller 3.58 gear ratio and LSD, which is another reason for moving forward with this project.

Has anyone actually completed this swap in an 03-06 XKR (perhaps with a write-up of the procedure)? I understand that I need to use the front plate of the sub-frame as the nose of the diff is 1" off center. What other parts are necessary to complete the swap?

And what else would one suggest servicing while the rear end is disassembled?

 

Not all, in fact only a few of those differentials have LSD. Usually the V12 and the XJR.

The XJR unit has 3.27 gears.

 

True. I should've specified that I already located a low mileage, used X300 diff with LSD and 3.58 gears. I haven't pulled the trigger yet as I was hoping to receive some feedback from the gentlemen here...in case I'm missing a crucial piece of the puzzle.

 

Can anyone tell me if the X300 diff is the same as the X305 diff?

I think I've found all of the information that I need to perform the swap, but haven't found a concrete answer on the X300 vs X305 diff. If I understand correctly, most of the XJ12 (X305) have 3.58 gears and powerlock, which is what I want, but everything I've read indicates that I need an X300 diff. Are they the same?

EDIT: I received an email from BritishParts.com, which states, "All the VDP models had the power lock and our Jaguar catalog says they are 3.58...the Base models came with non-power lock and different ratio"

What are VDP models?

 

I would assume that would be the "Van de Plas" (Spelling?)
I also assume you would also have to have a "serious talk " with your computers, due to the change in gear ratios.

 

You will need to swap the pinion 'companion flange' as the Sedans (X300/308) use the pinion shaft for the JURID coupler.

You will also need to CUT the pinion shaft JURID coupler 'extension' that goes into the rubber coupler to match the X100 pinion shaft.

I have had to cut a sedan pinion shaft (X308) to accept the X100 'companion flange' so the 'U' joint/propshaft would bolt up.

I don't know if the pinion shaft splines are the same (X308/100 vs X300/305) so this whole idea might not work.

It WOULD work if you were doing this on X308 because it uses the JURID coupler like the X300/305. You would NOT need to swap the yokes for the JURID coupler, they both use the rubber coupler.

This is JUST the physical aspect, there might be ratio programming/configuration issues as others have mentioned.

good luck.

bob gauff

 

This was my concern when first considering a gear swap. Contrary to what I’ve read in other posts, gear ratios are not programmed in the TCM. I’ve since spoken to another forum member whom says that he can do the reprogramming remotely, stating the following:

“Yep, it is possible…I can program any gear ratio to your car. After you have a new rear diff we can program it to your car. Gear ratios are not programmed in TCM, they are programmed in a VID block. I can modify it. No TCM flashing is needed. To modify it I need a laptop, with IDS or SDD installed, a cable (Mongoose or VCM) connected to laptop & vehicle.”

If anyone disagrees, please speak up.

 

XKRacer & Cambo seem confident that the X300 works. Aside from the front plate of the subframe, I was told that it "bolts right in" but I will "have to change the coupling for the prop as the X300 is a UJ set up and your XK will have a rubber mount."

I haven't read anything about spline compatibility. This brings me back to my original question:

Has anyone successfully installed an X300 diff in an 03-06 4.2 X100? Or is all of the information that I've read just speculation/theory?

.

 

Bump. I hate to feel like a nuisance but does anyone have any valuable feedback before I drop the loot on a differential that may not work?

Has anyone successfully installed an X300 diff in an 03-06 4.2 X100? Are the pinion shaft splines the same on the X100 and X300 diffs?

 

x100 vs x103 doesn't matter, the rear is the same whether the car is a 4.0 or 4.2.

There are several members who have done it to their 4.0 cars, JgaXkr is one that I know of.

 

Thanks, Charlie!

Regarding programming, it would be a big bummer if I installed the new diff and the vehicle was inoperable. Does anyone disagree with the following?

Contrary to what I’ve read in other posts, gear ratios are not programmed in the TCM. I’ve spoken to another forum member whom says that he can do the reprogramming remotely, stating the following:

“Yep, it is possible…I can program any gear ratio to your car. After you have a new rear diff we can program it to your car. Gear ratios are not programmed in TCM, they are programmed in a VID block. I can modify it. No TCM flashing is needed. To modify it I need a laptop, with IDS or SDD installed, a cable (Mongoose or VCM) connected to laptop & vehicle.”

 

You might have seen, i was trying to find a way to reprogram the diff ratio in the 4.2L cars for almost 3 years.

It's a very new thing to be reprogramming the VID block for the diff ratio swap. The preliminary testing on X350's and X308's is promising. Basically the ratio was reprogrammed leaving the original ratio diff in the car, and it gave the "right faults" so yes, should work...but not 100% proven in practise yet. Someone has to go first

 

Thanks Cambo! My fingers are crossed!

Now I'm driving myself crazy...I ran across a post from XKRacer on another forum where he states,

"for an XK you have to change the diff and if you do it to anything over 350bhp (XKR) forget using an XJ LSD they wont take it...Have fitted XJ diffs already and they simply are not man enough for the job."

Is the X300 diff too weak for a modded XKR? I drive semi-aggressively; occasional WOT blast-offs for the hell of it and a once a year visit to the drag strip. I'm not building an all out race car, just a red light warrior which is a daily driver in spring/summer. Bettering my 0-60 is the most important to me, but my goal is mid-12s 1/4 with the typical bolt-ons...+3psi upper AND Bigger Lower pulleys, 2.5" cat-back, re-worked 3.5" CAI, Maxbored TB, 92mm MAF...and X300 3.58 LSD (I hope).

.

 

The general consensus is that the 15HU (x300) is stronger than the 14HU (XK8/R).

I have heard that the usual failure mode in the 14HU on an XKR is to break teeth on the pinion gear. I have also heard that the XKR has this happen more frequently than the other models that used the same diff (x308).

My experience is that if you just bury the throttle from a standstill on the open diff you get a 1 tire fire and the car starts to do something the feels like axle tramp or wheel hop.

I can imagine if you just plowed through all that hammering away at the driveline, then you might break the pinion gear, and it probably wouldn't matter whether it was a 14 or 15HU.

As a case in point, JgaXkr has one of these with a 3.77 gear and a twin screw, so upwards of 550hp. Hasn't mentioned breaking it.

 

Yes, the acid test is to have it fault free with a different ratio.
and not the other way around.

 

Pretty sure the designations are reversed. 14HU preceded the 15HU.

In any case the 14HU is pretty much a Dana 44, and popular in the
Cobra kit car world. There are cars running 600+ BHP in 1/4 mile
competion without problems in the differential. The problem for them
is the halfshafts. The one that comes to mind is a stroked Ford 427
side oiler. Lots of torque and the driver sidesteps the clutch at high
RPM against massive drag radials. His differential is fine, but he has
twisted or snapped several halfshafts including custom made.

In the case of the 15HU, some credence can be given to a loose setup
as being the base cause of shedding teeth. A loose setup is a quiet setup.
That is what Jaguar might prefer in production.

 

Thanks for the correction.

 

Is the Trans in an 03 xk8 different then a xkr? I've been told you can't do a gear swap with the six speed.

 

No, when the trans changed to a 6spd the XK8 and XKR used the same 6hp26.

 

Appologies for resurfacing this old thread but I've been getting a few IMs about fitting 15Hus onto X100s and X308s recently. I want to make sure the correct information is out there. The above post is WRONG. Plums you are wrong, if you don't know- then please don't put down 'pretty sure'. 14Hu with 8 inch crown wheels was in the later V8 cars, the 15 HU was in the XJ40s and X300 series (including X305s and X306s).
The 15HU can be adapted to use Dana 44 internals (as I have done on my X308) . As well as the diff above-
- you need to get the triangular brace plate that the 'hogs head nose' sits in, the earlier 15HU is offset to the vehicles centerline while the later one sits on the centerline. The the other aspects to be aware of are
-That the exhaust hangers are in a different location on the earlier triangular hogs head brace plate and this needs to be moved/looked into and finally
- the X100 is a shorter car- so the rear part of the prop shaft will now be at an angle. This is not a problem on the X308, I don't know if there are clearance issues on the X100. Can anyone who's done this answer that?