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Diff Ratios

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  • Diff Ratios

    I have just fitted a 5 speed sequential dog box where 5th is now 1.08 instead of 1.0 and I find that I am running out of gears.

    My diff ratio is 5.28:1 and I need to reduce this. I cannot remember where I got this crown-wheel and pinion but now I need something lower. The diff housing and LSD are from a 325i.

    Is there anyone with a list fo diff ratios that I can look at to see what I need.

    I have calculated that I need a 4.8:1 or something close as with the extra speed from the sequential box ( nicer ratios ) I am on the rev limiter 3/4 way down the straight.

    ( Rallycross in Europe with top speed of 100mph in case you think I have all my ratios wrong !!!)

    1st 2.33, 2nd 1.78, 3rd 1.51, 4th 1.28, 5th 1.08
    16" wheels with 23.66" dia tyre

  • #2
    I'm not sure whether we're using the same numbers for the diff ratios, but as far as i know there was never a 325i lsd diff ratio of 5.28:1. The 325iSport (UK) had a large case lsd with a final drive ratio of 3.9:1, regular 325i's ran a 3.64 or 3:73:1 ratio but the lsd was optional when new (think the ratios were different depending on manual or auto).

    Then you have the small casing diffs used on the 320i, 318i/iS & 316i. These diffs had ratios of either 4:10, 4.27 or 4.43, none of which were lsd as standard altho it was always an option. The 318iS is most likely to have an lsd.

    Not so sure on the M3 diffs but i think they're 3.15 and 3.25ish depending on model and the 320iS had a 3.46. The US M3 had a 4.10 but with the overdrive gearbox not a dogleg. (If any of that is wrong someone please correct me).

    Hope that helps.