So here is basic information about the diffs:
318Ti's use a small case diff
4cyl E30's use a small case diff but you wont find a highway friendly ratio there
6cyl e30's use a medium case diff which is a direct bolt on. You can find a 2.93 on ETA's
Most e28's use a medium case diff which is not a direct bolt on. You would need to take the cover and flanges off an e30 diff for it to work. You can find 3.25's on 528's
z3 4cyl use a small case, direct bolt on, not sure on ratios but i believe there isnt a highway friendly option there.
z3 6cyl are a medium case, direct bolt on. You can get a 3.15 ratio on any of the manuals
Here is where this fitment issues come into play. Because you are putting a medium case diff in, the width is slightly larger than the small case diff. Some people seem to have no issues using the Ti half shafts, while others either cant get them to fit or they explode rather quickly. If you want to be safe pick up a pair of e30 half shafts with the diff and you will be set. __________________ 
-Alex |