Diff rebuild / ratio swap

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E30 Zone Newbie
E30 Zone Newbie
Posts: 64
Joined: Tue Jan 26, 2010 11:00 pm

Sat May 18, 2019 9:57 am

Hello all

I’m looking for a bit of advice in regards to rebuilding a differential with a different ratio.

I have just finished an M50 swap, I’m currently running a ZF 5 speed gearbox (5th gear is 1:1) and have a 3.45 small case diff from an e36 compact.

The ratio is acceptable I guess bringing 70mph in 5th at about 3.3k, but I feel that I want a longer final drive to make cruising a bit more bearable.

I’m looking at changing to a 3.15 or even a 3.07 but have found these ratios really hard to come by for sensible money.

My question is this:

Can I rebuild an existing diff with a new ratio? Can I mix and match parts, ie a medium case piñion set in to a small case diff? Or even an e36 pinion / gear set in to an e30 casing?

Ideally I’d like to build e36 gearing in to an e30 casing for ease of finding parts etc.

Anyone with experience of this?

You can't polish a turd, but you can paint it matte black

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E30 Zone Newbie
E30 Zone Newbie
Posts: 145
Joined: Wed Apr 10, 2013 11:00 pm

Sat May 18, 2019 11:13 am

Easiest option is find a z3 diff from a bigger engined z3. 188 diff. You can't mix and match medium and small case but can e36 Medium and e30 medium. And same for small if you wanted. Changing final drive ratio isn't really a diy, it requires shims and dial gauges to get get the correct mesh. And the crown and pinion have to match you can't mate a 3.07 pinion with a 3.15 crown wheel.
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