I've searched through the forums and found no references, so straight to the point.
Has anyone out there heard of/done a rear axle/diff ring &pinion swap on an X150 XKR?
Since I have nowhere to safely get up to 225 MPH, it seems like a shame to waste all that power with really tall gears.
With an active diff, I don't even know if it's feasible. It does seem that there has to be SOME sort of way to change the R&P, but heck if I can find any info on the entire interwebz. Salvage parts from some other model??? I have no idea.
With lower gearing, the 0-60 times would be DRAMATICALLY reduced, and since I don't really care much about the fuel consumption, the 10% decrease wouldn't mean much. Sure, the speedometer would need a recalibration, but think of the FUN it would be! Lower gears are ALWAYS a huge kick in the butt.

 

The funny thing is most people here seem to think that the car is geared too short and that first gear is useless. I wouldn't mind shorter gears but I do sort of like hearing the throbbing of the exhaust at low to mid range rpm and moderate to heavy throttle...shorter gearing would make that experience a little... shorter!

Anyhow, good luck!

 

The X150's already have 3.31:1 diffs in them, how short do you want it to be?

1st gear in the ZF's is also a very low 4.17:1 so I can't imagine you'd want to go much shorter than the existing ratio.

Many guys here in Australia have run 4.11's in the Ford Falcon with the same ZF transmission and most have said if they could do it again they would go for a taller ratio more like 3.45 to 3.73 because 1st is just too short otherwise (before you get excited no we can't use the Ford software to change the ratio).

I have been on a quest to find a way of doing a ratio swap in the X350, have a look here https://www.jaguarforums.com/forum/g...ids-sdd-77299/

In the meantime we have a forum member who's an ex-JLR engineer, and he might have cracked it for the X350. https://www.jaguarforums.com/forum/x...ng-sdd-130103/ but it's yet to be tested with a different ratio. So far only looking at the X350 but the X150 might be on the cards, you just have to ask...

I think the bigger problem will be finding a replacement set of gears to suit the E-Diff, i'm not aware of anyone who's opened them up yet. IF (by a long shot) they continued with the Visteon / Ford 8" internals then you can get up to 3.58:1 off the shelf (nothing shorter).

But without seeing an E-Diff in bits, who knows what's inside...

 

The XKR has enough trouble putting down the torque it has with the standard diff; I can't imagine how twitchy it would be with lower gearing. OK, if you are peddling mine quickly, it doesn't change into 6th until you hit 177 mph (GPS measured), but I don't think that shows it's excessively long-geared - it's just a bloody fast car!

I think you're looking at a lot of work, if it's even possible, and you may find the car is less drivable when you're finished. I'm not sure how you'd fool the TCM and Active Dynamics into believing that the standard diff is still there - if you don't, they're liable to keep telling the ECM to back off the throttle. But good luck if you want to have a go at it.

 

With today's "low" fuel prices many prospective owners of high performance cars would tend to care less about fuel economy. Yet, manufacturers are mandated to reach fuel economy marks, thus the overly tall gears in today's cars. Also, you can bet that fuel prices are not going to remain this low for very long.

I think the 8-speed transmission in the newer Jags are a terrific compromise. It has "close/performance" ratios between 1st and 5th and MPG ratios from 6th - 8th. I much rather have that than changing to rear diff. ratio and possibly ending up with a droning issue as the cruising RPMs could fall into the drone-range of the typical V8 engine.

 

Cambo, I think maybe those 3.58s would be about perfect. Not a lot of change, but still noticeable. I could also rig up something electro/mechanical to change the speedometer. A recal on the ECU would be preferable though. Since first gear only lasts about one second at WOT, maybe reducing that to .8 seconds may not be the GREATEST idea ever.
What I may do is a bit of math and borrow some shorter TIRES (and wheels) for a couple days, that would accomplish the same thing as long as the back end doesn't scrape the pavement. Do XK8 wheels fit X150? I suppose they do. Ah crap, I forgot about the brakes. Maybe nothing smaller will work.

 

First thing you have to do is find a donor diff and open it up. They pop up on ebay regularly but aren't cheap. There are some on ebay.uk right now for $750 US plus shipping, it'd want to be cheaper than that just for an investigation...

 

I thought I saw that 8th in the F-type and 6th in the XK were about the same ratio. Both final drive and transmission ratio. The advantage in the 8 speed is the shorter gearing in each gear and not the final drive. I may be wrong here so please chime in. I didn't understand why you needed an 8 speed for normal everyday driving. As it is I rarely get out of 5th while driving around town.

 

6HP

4.17
2.34
1.52
1.14
0.87
0.69

8HP

4.714
3.143
2.106
1.667
1.285
1.000
0.839
0.667

So yeah 1st in the 8 speed is even shorter.

 

You do not "need" the 8-speed for everyday driving. But, since these cars are considered "performance" cars, the 8-speed gives them a set of "close ratio" gears that are much better suited for performance driving. They keep the engine from falling out of the peak power ranges between shifts, therefore, a definite advantage in acceleration. In my XKR I feel too much drop off in power between up-shifts. With the 8sp, this would improve a lot. You have to look at a dyno curve to see the actual power advantage for your particular engine application.

In around town driving you are not likely to use the very top ratios. They are for highway cruising. I was surprised to see in Cambo's ratio post that the 8th and the 6th in the two different transmissions were that close. In my actual driving experiences with both the XJL and the XF the differences in 6sp vs. 8sp top gear RPMs at 70 MPH were 2000 for the 6sp vs. 1500 for the 8sp. The observed actual MPG (highway only) numbers showed more than 10 percent improvement for the 8sp. Makes sense, since it is turning 500 RPM less at the same speed.

The 8sp would have been a terrific addition to the XKR, too bad Jaguar decided that it was not worth the investment for the last couple of years of production.

 

Thanks ax , I got it and yes I guess for a lot of us the difference is insignificant except if you want increased gas mileage. I also see the F-Type SC V8 has a 3:15 differential where the XKR is 3:31. I guess this would account for the 500 RPM difference. If I look at the F-type V6 I see they use the 3:31 so cruising in 8th would be the same as our XKR. I assume that they did that because of the lower HP. I did read that the advantage of the ZF HP8 is it is more efficient and it has a fast response for the stop/start feature that some cars are using. Happy New Year

 

Quite the interesting topic. I would be curious to see how much 0-60 would improve if swapped to a 3.73 or 3.90.
I need to get a hold or a used diff to measure.
But as others have mentioned about, the TCM is going to be the headache. I'm not sure if one of the tuning companies has access to change the rear diff table in the programming.
With FORD setups, you can adjust for tire size and rear diff gear in the ECM program to have accurate speedo etc.
I would think that Jag has a similar setup, it's just figuring a way to access it all. I've been cracking out my electrical engineering and instrumentation books lately to look into the tuning myself.

 

I wouldn't worry yet about the speedo / tyre size as on various models you can set the size but in any case VSS is sent around on the CAN. The TCM I can well believe is going to be a problem but at least some of the ZF boxes can be tweaked. You can use IDS/SDD to reflash a TCM so if you can tweak the files with the data then that's one way to do it. The files are easily found on IDS/SDD but their internal format may need some work.

There are already some firms who can do some of this for some of the jags, such as TL Jaguar who have fitted an LSD to more than one car and reflash the TCM I understand. Or they know someone who can do that. It's worth money to them so they're unlikely to just give this info away.

Anyone who already knows some of this for car X (*) tends not to want to reveal it so unless they move on to jags it all needs figuring out from scratch once again.

An alternative may be to insert a custom-programmed device between the ABS sensors and their module, and have it tweak the pulses to correspond to a different diff. Would that work?

(*) such as Fords with 6HP26

 

I have not yet found a company that can modify the software or maps for any XK module other than the ECM - and even that presents problems, with maps reverting to factory defaults in some cases. I'd love to get my e-diff and ABS modules loaded with the R-S settings, because I know the team ran out of time when creating the 75, but they carried on optimising for the R-S (which has identical hardware).

 

Getting the data in should be easy: rename files on IDS and reflash.

 

I believe there might be issues with some of the files requiring the car's VIN to be embedded in them as an authentication measure. But that may not be correct; it's just something I picked up in passing.

Right, just so I'm clear: it should be possible to take (for example) the software & data files for the XKR-S e-diff, change the filenames so that they appear to be the files for an XKR 75, and flash those files into the 75's e-diff? I guess my issues would be:
1) Finding someone with a proper IDS who's prepared to help out (my indie shop might be a possibility)
2) Getting the XKR-S files, unless they are pre-loaded into the IDS
3) Identifying which file I need
4) Figuring out the correct file name to change it to

 

Yes but it starts in second unless you manually select 1st. I am also interested in a lower (higher numerically) gear set for my F-type v8s with the electronic differential. I can't even use 7th and 8th

 

Interesting discussion here. Somehow I still think if you change the rear R/P the problem would be how to prevent shredding the rear tires.

 

Are you sure? From experience with the XK, people get confused because in manual mode the car will automatically drop to 2nd when you come to a halt, rather than 1st if you don't touch the paddles. It will then pull away in 2nd so they assume it always does.

In auto mode though the XK actually always pulls away in 1st so I wouldn't be surprised if the F-Type does as well. I recorded a video of my 07 XKR to demonstrate it and have also plugged in my SDD and watched the gear changes so I know for certain mine does.

The documentation I can find for the F-Type implies that it only pulls away in 2nd in snow mode.

The documentation for the Dynamic Launch mode on the F-Type says to put it in D or S as part of the process and explains that in Dynamic Launch the engine’s torque is not lowered during the gear change compared to sports mode.

"This delivers maximum through-gear vehicle acceleration, through 1st and 2nd and then through 3rd and 4th gear changes. This allows the car to accelerate more quickly than when in ‘conventional’ Sport Mode."

0.3 seconds quicker apparently but that shows that in D or S it will use 1st gear. Under light acceleration it will change from 1st to 2nd very quickly so may feel like it pulls away in 2nd.

 

In D, the XK/R starts in first (unless you're in Winter Mode). In Manual, when automatically downshifting to avoid stalling, it stops at 2nd. Basically, if you're in Manual, you should be downshifting manually - the car only does it to protect itself if you forget, and it's not optimising for performance.