^^^daimlerman wrote:E30 was fitted with two sizes of diff case,we refer to them as 'small' and 'medium',because the 'large' was fitted to E28 and the equivalent '7' series cars.
E30 diffs come in a range of ratios from 3.15 to 4.45.Most people can find something suitable here.
The problems arise when you require to use a ZF gearbox from an E34 or E36,because these use a direct 5th gear,the Getrang 'boxes that E30 comes with use a direct 4th gear,with 5th as an overdrive.
I mentioned that E30 uses two diffs,yes,they are interchangeable!How do you tell them apart?
'Small' has 6 bolts holding the rear cover on,'medium' has 8 bolts holding the rear cover on.
Ratio of the diff will be stamped on a tag attached to the upper r/h bolt.
Now,certain E28 models used 'medium' diffs,ratios such as 3.09 and I think 2.89 can be found.These will fit E30,but the diff will need to be 'dressed' with at least the comparable output flanges and possibly the back plate as well.I have never needed to use an ex E28 diff myself.
Crownwheel/pinion sets from one size of diff will not swap into a different size casing.
Informative post, thanks
Now, if my car has, say, a small casing and I want to use a medium cased diff, do I also have to change my prop/driveshafts?
