How to Identify Differential

[oclee118]

Hello all,

I have a 1991 318is for sometime. Somehow I suspected the ex-owner have replaced the differential. I know there are quite a few of differential for the E30. Is there a way I can found out which differential is on my car.

Thanks in advance,
O.C.

[roguetoaster]

There should be a metal tag on the upper right rear cover of the diff. If the tag is gone you will need to rotate either the input or output and see how many times the other rotates. The ratio should be in the 1:~2.5 to 1:4.4 range and you can find a bunch of charts of all of the available ratios to match to your result.

[turner.e30]

The 318is came with a small case diff as standard, though a medium case will also fit. Probably the easiest way to tell them apart is to look at the drain and fill plugs. On the small case they are off set, and on the medium case they are in line.

[Vincenze]

You can write down RPMs at different speeds and use a gearing calculator to compare.
Moreover, it will give you an idea about the ratio you want.

https://e30goodies.com/gearing-calculator/