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Subject: Determining rear end ratio and diff types. Author: kff (moderator) : member since February, 2004 : 3593 posts Posted on: 2005-10-09 14:02:28

BMW made a number of differential ratios for their cars. This is most of them:

2.93, 3.07, 3.25, 3.46, 3.64, 3.73, 3.91, 4.11 Many are abailable in Limited Slip (LSD). Some of them are not available in the correct case for our use. The older diffs are considered side loaders as the gears go in the side. Our diffs are loaded from the rear and the rear cover comes off for this.

BMW installed an aluminum tag on the rear cover attached to one of the bolts that has the ratio on it. Something like S3.25 xxxx, where the S means limited slip and x is some number. This ratio is 3.25:1. A replacement diff will have a number like 14 41 A xxx where the ratio is the second number divided by the first, in this case 2.93. If limited slip the number will have a suffix S.

To tell if you have an LSD without looking at the diff and cleaning the tag which may not be there, with both rear wheels off the ground, turning one will cause the other wheel to turn the same way if LSD and the opposite way if non-LSD. Or with one wheel on the grass and one on the pavement hit the throttle and see if one wheel spins. Remember these LSDs are only 25% meaning they will only transfer until the torque difference exceeds 25%. If you need more info on that, the internet will explain it.

There are two diffs commonly used in the E23, the Big diff and the small (regular really) diff. The big diff has the drain on the side and the regular diff has the drain below the fill plug on the bad cover.

Common E23 ratios are 3.25 and 3.25. Well there are others, but these are less common. The 728 had a 3.64, the 732 had the 3.46 the 745 has either the 3.07 or the 2.93 (long legs in that autobahn cruiser). The US cars were usually 3.25 until late in the production when the automatics would have the 3.46.

Now if no tag exists, rotate the wheel one complete revolution and observe the rotation of the driveshaft and determine how far it went. It should match one of the numbers in the first paragraph.

• Determining rear end ratio and diff types. - kff 2005-10-09 14:02:28 (294 views)
  • Determining rear end without tags - doug 2005-10-17 09:14:44 (121 views)
  • Re: Determining rear end ratio and diff types. - kjkumar 2005-10-09 15:12:31 (140 views)
    • Thanks Keith, I think this clears things up a bit (nt) - chad20 2005-10-10 15:04:02 (92 views)