Diff Help!

Fri May 29, 2015 11:25 pm

Hello, after many many years I'm back in an E30! Have a little issue with a diff though if someone could offer their knowledge.

I've bought a 318i with an M50 conversion. I'm told it has a 328 gearbox and the diff is an E30 325 3.61 ratio? As fair as I'm aware BMW didn't do a 3.61 ratio diff in the E30's or E36, so I'm guessing it's a typo and is a 3.91? The revs are stupidly high at cruising speed, I don't even get 60mph to 3000rpm!

Could someone suggest a better ratio diff to give me a better cruising rpm and any help with what diff I actually have?

Cheers

Sat May 30, 2015 12:07 am

Hello Dom_Evola,

You need to find out what your gearbox is, final drive could be 1 to 1 or about .85 to 1, below is a diff calculator, feed in your info and it will give you road speed to RPM in all gears, I'm part way through an m50 2.8 conversion, box 5th gear is 1 to 1, diff is 2.93 and rear tyre 245/30/17, below calc tells me 58mph @ 2500rpm, hope this info can help you, cheers.

http://diffsonline.com/bmw-differential ... ation.html

Sat May 30, 2015 12:11 am

Sorry,

if you check on your diff rear housing, one of the bolts holds a tag with the ratio stamped on it, that's if it's still there of course.

Sat May 30, 2015 8:13 am

Your diff could be either 3:64 or 3:91,niether is suitable for use with the ZF 'box....as you have discovered.
At a guess,first and second gears are redundant,but it gets to 60mph very quickly!

You need a ratio around 3:1,these were standard on E30 M3(difficult to find and expensive)an alternate source is smaller engined E28,the 525e in particular had usable diff ratio's,but now a bit thin on the ground.
Another possible is Z3,they used a similar engine/'box combination with a lightly modified E30 type rear suspension arrangement,but you will need to research to check up on mounting arrangement,I have a feeling that the rear cover may need swapping,but I'm not sure.

Sat May 30, 2015 9:11 am

z3 6-cyl diffs should be 2.93 or 3.15 - perfect ratio for the 328i ZF 'box. Rear cover is fine, it's the output flanges that will need swapping, but this is an easy enough job - just use the output flanges from your existing diff.

Benefit is you should end up with a Torsen (2.8/3.0 Z3) or BMW M (Z3M) diff. Both much better than an open diff for 'spirited' driving

Sat May 30, 2015 11:13 am

Thanks everyone for the very informative replies!

Am I correct to assume the z3 2.8/3.0 came with a Torsen diff as standard then?

The current diff I have is not only ridiculously low ratio for the box, but it's also welded! Prefect for when you want to give it some but utterly useless for anything else!

Edit: yes 1st and second gear are just like a switch on a smoke machine! Gets to 60 very quickly but has a 120mph ish top speed.