Anyone know about Diffs/LSD/Final ratios?

[Gibbo]

Hi there

Being in a couple of M3's now running 4.10 and 3.91 final ratios and the extra get up and go is noticable. Owners who have done it say it improves acceleration more so than a remap which is impressive as remap transformed my car so if I can achieve similar again then great.

With gearing over 200mph and 8300rpm redline loose some top speed is no concern to me though I do feel 3.91 gear will be better suited for a track car than 4.10 otherwise on some tracks I'll need 5th and potentially 6th.

A video that's being floating around the web showing three cars:

Why the thread?

Well I am clueless on Diffs and in case I diss like it then it's hard to reverse once done.

So I've being offered a complete E46 M3 differential supposedly in working order for £150, so I could buy this and then I assume I need:-
BMW Motorsport 3.91 or 4.10 crown and pinion kit
Refurb/rebuild kit new bearing or whatever, the LSD itself?
Professional fitter to do it all.

So is that all I need if this £150 is a duffer will the above make it like new or are more items required?

Sorry if dumb questions but differentials I am not so sure about, some M3 guys even change the LSD type too for more traction out of low speed corners if that makes any sense?


With the car nearly done, I am thinking that with it around 1350kg (with cage), 3.91 or 4.10 gears and 380BHP I'll have one extremely quick car, weight should be bang on 50/50 too.

 

[Gibbo]

4.1 FD best for track imo, if your gonna do it go all the way. Pig on the road though so take that into account. I seem to remember the gear kits costing a fair bit.

Bear in mind SMG though. I remember when folk started fitting these there was some issue with changing gears at full throttle. That was some time ago now though so there's probably a solution out there.

Will be an issue with SMG if in auto mode, as the RPM it thinks to change out would be different with a different ratio fitted, so simply do not drive it in auto flat out, which I never do anyway.

In manual an SMG works just like a manual.

 

[Gibbo]

Yeah there's no way i'd do it in a car which would ever see normal road driving.

Its quite fine in the M3 to be honest, due to how high its rev limit is and very tall stock gearing. A lot of guys use 4.10 even in road cars with no issue, but I am steering towards the 3.91

 

[Gibbo]

I would just go down the 4.1 route. Even then it's only 3200rpm at 70 so completely fine on the road still.
While you have the diff apart, ditch the visco lok centre and put the M3 evo salisbury type one in, it will give the diff a more consistent and predictable locking.

Cheers m8.

I believe it is something along the lines of 80mph is 3000rpm stock on E46 M3 and 4.10 takes it to 3500-3600rpm at 80mph in top. 3.91 a little low.

Top speed in 3rd is like 100mph stock and with a 4.10 around 92mph I think.

So yeah its not a drastic change but the acceleration difference is quite noticeable.