M50 Gearbox/Diff ratios

Post Sun Jan 25, 2009 7:11 pm

I'm currently doing an M50 conversion into my E21, and at the moment I'm using an M20 flywheel and M20 Dogleg close ratio gearbox. I'm planning to use a 3.45 323i diff. I also have an M42 gearbox sitting around and since we all know M20 boxs sit at an angle and therefore the gear linkage and mounts will need to be modified, I thinking this may make things easier. I don't pretend to know anything about ratios so will a 3.45 diff with an M42 be compatible?

Thanks

Post Sun Jan 25, 2009 7:12 pm

Not really, you will need a 3.64 or 3.91..

Is that 3.45 a small case diff? If so, don't expect to last long if driven properly..

Post Sun Jan 25, 2009 9:32 pm

I would say that a 3.64 is the ideal box to use with a M50 and a direct 4rth gear box.
A 3.45 is a little low, but not excessively so. It'll certainly help with top gear cruising, though.
If it is a small case diff, it won't take kindly to too many hard starts!

Post Mon Jan 26, 2009 2:04 pm

E21s only came with one size diff and one type of mounting, so yes it's a small case. Nothings straight forward when it comes to swapping diffs, it will have to be lots of custom fabrication and jigging to cut and shut a diff centre section off another car. Fritz bitz do an E12 LSD subframe conversion, but I expect the diff ratio will be even lower than 3.45 and it's over £600. The car itself is a 316 came with an open 3.91, so I can use that diff. The diffs will have a hard life and will be welded since I can't justify ruining an E21 LSD that I have, so I expect I'll breaking a diffs before I can afford the time/money for a custom diff swap. I have a few spare 3.45s, and if I swapped an E12 diff in the dogleg would be the better suited box it seems.

Thanks for the advice.

Post Mon Jan 26, 2009 2:12 pm

Are E21 diffs identical to E30? E30's use two case sizes that are interchangable... though I am starting off my M50 with a small case 3.45,I will instruct the wife NOT to drop the clutch at high RPM

Post Mon Jan 26, 2009 7:38 pm

As far as I know, they are not. People cut and shut E30 centre sections into E21 rear subframes. Although the innards definitely swap from E30 small case to E21 diffs, but this makes no difference to the strength of the diff. I could narrow an E28/E12 rear axle and this has the benefit of making it 5 stud at the same time, or cutting and shutting the E30 centre section.

Looks like I'll be testing the durability of E21 diffs soon enough...