I am going to swap out the 3.91 Limited Slip Differential to take my 840 CiSport back to stock 2.64 before I put it up for sale. I am now retired and hardly use my car, just a couple of hundred miles in the last two years and can no longer afford to keep it.
I bought the diff from DickB (member of this site) of Holland and have had great fun with it in. It was originally from an E34 M5 3.6. Part No: 33102226601
My car was transformed with it fitted, really hot off the line for acceleration and with the LSD, no tyre slip. It corners as if it is on "rails". Mid-range acceleration is instantaneous, without need for kickdown.
However, I know that the best price I will achieve for my car is in standard specification so hence the swap over.
If anyone is interested, please let me know - I am looking for £900 for this rare differential.
There are a few threads on here about the 3.91 diff. You will need to swap over the rear cover, as I will put it on my 2.64 diff. The bolt holes are identical so this is no issue.