Rear differential change 3.38 to 3.91

If anyone else has done a direct drive conversion and wants more power. One way to do it would be to put a shorter differential in the car. As my diff needs replacement, I’ve organised a new differential to be installed.

The ratio of the current differential is 3:38. The BMW 3 series automatic has a 3:91 ratio differential which should give me about 20% more power at the wheels.

Below is the current diff

2 Replies to “Rear differential change 3.38 to 3.91”

  1. Hi
    I have a E90 320i 150 Hp manual with a diff problem.
    It’s a strait swap with the automatic diff of 3.91, or need modification?
    Thanks.

  2. The diff is the same – 3 large bolts to hold it in.

    However the CV joints on the manual are wider and won’t fit into the socket of the auto diff.

    So you will also need to get the automatic CV joints.

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