M3 diff ratio question

[Lucius]

What's the best ratio for s50b32 ? Small bit of road usage and plenty (fingers crossed) track time.
Is anything above 4.10 will do or better to keep lower ratio 3.45-3.91?

[kev.skyline]

think its hard enough/expensive to get short ratios for the large case diffs. think a 3.62 would be the handiest out of e46 m3, or you could always try and get a 4.10 bmw motorsport cwp for a few grand.
edit
turner motorsport do the 4.10s for a little over $1000 delivered.

[Lucius]

Sorry, I forgot to mention that engine is mated with 5 speed and is going into compact shell. So probably at the beginning I'll leave 3.45 to see how long it will last. Or it's better to change rear axel at the beginning? I hope small casing will last few weeks?

[kev.skyline]

ah so its e30 diffs youll be using, loads of choice then. if its a welded small case it might last for a bit, cant see a small case lsd holding out for any length of time. if its a small case lsd id sell it and get a medium case. im assuming the 5speed is a zf m3/328 box? if it is i think it worked out with a 4.1 itll top out at 100ish mph, not ideal for a road car.

[Lucius]

I'll be using m50b25 gearbox from e34 525i as I can't get any ZF's around and I've only two getrags from 323 and 523.
I hope it'll will work fine with s50b30 clutch kit? I've used this clutch kit in my e46 328/330 before, and it was fine.

[DaveM-sport]

S50b30 clutch will work fine with the E34 box.

For an Evo road/track car, I'd be using a 3.91 diff (build a 40-75% Evo LSD and use E34 M5 cwp) and a six speed box. That way I'd have good gearing down low but sixth would still be tolerable at 6-80mph

3.45/3.46 is about as short as you'll be able to go on it with the 5 speed without making it a pig to drive everyday if you do any bit of motorway speed driving. Depends what wheels and tyres your using too of course.


Still, with a 3.45... It'll go like scalded cat in a Compact shell

As for the rear axle, what would obviously be ideal is a Z3m rear end matched to Evo front hubs and brakes but at least fit E30 driveshafts as the Compact ones are pretty weak.

Small case diff if welded well will last reasonably well but you should be planning on fitting a medium case sooner than later.


For a medium case 3.45 diff, you'll need an E30 casing and something like an E34 520 cwp. 3.64-4.1 are all easily availible straight out of E30s but ratio might be too short for the road really.

[Lucius]

Thank's lads.