is it true?

[Earendil]

ok, i am fixing a 91 318i 4 door manual tranny. I am trying to pick the best diff for the car, not for highway but for quick sprints. I have the stock diff thats on the 91 318i, i have a diff from an 84 318i manual tranny and another but this one has me the most confused, a diff from my parts car, a 1984 318i auto, i looked on real oem and it says its a 3.91 25% limited slip? any idea if its true and if so is it open or closed? Which diff would be the most likely to help me get from light to light the fastest? the 84 was manufactured in nov. 1983. i am not sure about the ratios on the other two, maybe you can help me out. thanks guys

The best ratio will depend on what speed you want to hit before braking.
25mph? 35? 45? You'll probably want whichever diff tops your 2nd gear out at that speed. No use going super high diff if it means that you (or the car) have to shift 3 times before reaching your final speed.