I was driving spiritedly through some switchbacks today, and got the rear end to step out on a few occasions, in the dry. While I could control it, the most aggressive of those incidents caused the car to flop from left to right as one tire gained grip while the other lost grip. I know a limited-slip differential would have made all the difference in the world. Is our only option the Quaife unit?

 

No quaife is not the only option.
The the 15HU offset pinion diffs which predate the centreline pinion 14HU in our cars can be swapped over with a little work.
Quaife would probably be the easier option but then you are still left with the 14HU which is a somewhat weaker diff.

 

So, the 15HU diff would be a stronger option, but harder to install? Will the diff being offset cause reliability problems?

 

No, the offset is only 1", but it does require the 2 piece XKR prop shaft if the conversion is on an XK8.

 

But it's an XKR, so I already have the propshaft? What else would I need? Reflashing the TCM? The 15HU has a different ratio to the 14HU?

 

Ratio change is in the works currently. The Mercedes TCU is problematic in that reprogramming is no longer an option. A work around is available, (search this forum for "chirpy" post recently)l will know for sure soon enough. You can stick with same ratio but would need to fit appropriate ring gear and pinion to the 15HU.

 

My car has the Zf 6hp26, is a ratio conversion possible for my car? Is there any 15HU with the same ratio as the 14HU? I wouldn't mind some more oomph tho.

 

I don't think you will find a 15HU easily with a 3.06 ratio. Getting a 3.58 ratio lsd 15HU from a '95-'97 XJR X300 seems to be the easiest.
With an HU15 you effectively have access to the Dana44 world of re-build/upgrade options, which are extensive.

If you change ratio, then it's possible the ABS spoofing work-around could work, but no-one has done it as far as I know.
The TCM does not know how fast the wheels are going around, it only knows the speeds of some of the input, internal shafts, and output shaft, depending on model.
So it seems to me that adjusting the ABS values fed to the TCM should work. An open question is if the ZF TCM does other checks (e.g. ABS reported speed, engine rpm correlation, or something else, in which case more signals might have to be manipulated - which could be done in the same way). So my guess is it will <probably> work, but I might be missing something.

 

The mechanical aspect of this swap I think I could manage, but spoofing the abs is out of my skill set and being the first one to try it is not very appealing to me when the result could be a bricked car and a loss of money I don't currently have. Is there a way of determining if this could work before going out to hunt for a suitable 15HU diff?

 

l will be in a position to answer that soon, (in large part due to Chirpy's help) but only as it relates to the earlier MB transmission cars.

 

I have started in the same place - try to prove out the electronics before tackling the mechanicals.
The way I have been doing it is to use a TCM that is programmed with the wrong ratio, and then adapt to it with the ABS signal modification.
But I was a lucky that I had a non-standard diff ratio and modified TCM, so I could buy a breaker yard stock TCM and work with that, without messing with the mechanicals.
So if you could find a different ratio TCM and install that, it would provide a way to test it out, if something suitable exists?
Not sure since I don't have a 6HP26, but I think the ZF TCM might be located inside the transmission, which would be inconvenient.
I think this transmission was used in BMWs, and maybe OEM'd by Ford? So perhaps there is someone out there that can reprogram the TCM to a different ratio, as an alternative?

 

xjr6 is 3.27


you can gut the 15hu housing and put a 19 spline auburn dana44 carrier with a set of gears that match your current ratio. 1310 yoke on the diff and convert your current driveshaft

no programming required

 

That is correct, the TCM for the 6hp26 is inside the transmission. The Ford 6r80 is essentially a licensed copy of the 6hp, so there are likely companies out there that are capable of reprogramming it, but I doubt any of them know how to make it speak with a British accent.

The spoofing approach should theoretically work, all of the communication is CAN based.

 

Interesting information from all of you. I will have to see if converting the way xalty mentioned using the Dana 44 gears is less expensive than the Quaife ATB diff. On the other hand, if the spoofing works on the MB trans cars, it may provide some hope for the ZF cars as well. I would be nice to have a better ratio for getting the car moving... Also, I read somewhere that not all the 15HU diffs have Posi-trac. Is there any way of knowing if the 15HU in question is a Posi-trac?

 

They are marked on side of rear cover with stickers of various colours signifying ratio, if this sticker also has a black cross it means LSD. A metal tag across two of the diff's rear cover bolts also has that Info. Easiest way is still to jack both rear wheels of the ground and turn one wheel, LSD if both raised wheels rotate same direction, open diff if they rotate opposite each other.
Definitely generalising here but start your search looking at XJR's and XJ12's, the others were mostly open diffs.

 

A search among local junkyards reveals those are few and far between here.

 

xj40 3.58 powerlok

xjr6 3.27

these are the only ones you will find

 

I had one of the most highly modified XKR'S at one time. My car had the first of Avos's twin screw kits in the US. You can change the diff ratios from 3.27, 3.58 & 3.77. I was had all 3 fitted to my car over a period of time. The 3.27 was simply a Mexican spec XK8 unit & a straight bolt in. The 3.58 & 3.77 require the front diff mounting plate from a X300 as the diff is offset as well as a non jurid coupling. The 3.77 diff along with the blue top solenoids fitted to the transmission made the car a monster. The issue with changing the ratios is you need to reprogram the TCM. The only person that was able to do this was Phil @ Eurotek Designs. I would send him the TCM to someone in Germany that reprogrammed the unit. If I remember correctly it was about $2000. I always had a spare to send of so I was able to drive the car until all parts were assembled. The source for the 3.58 & 3.77 was Coventry West. I will say the LSD & gear ratio changes the dynamics of the car.

 

The reprogramming options seem to have dried up some time ago, however Chirpy's CAN workaround could be done for $50 to $100 as DIY and works perfectly with the 3.27 15HU i have now fitted. Other ratio algorithms are available but I have found 3.27 to be noticeably quicker pick up while only upping the cruising revs by around 150 RPM.
Totally agree re the dynamics. In addition to acceleration improvement mentioned, my car is an R features vehicle and came with more direct steering rack so always had good feel but after fitting the LSD along with new clutch packs the steering feel improved even further. Highly recommend this conversion.

 

If thought the 3.27 made a difference you would be blown away with the 3.77 & blue top solenoids.