Best diff ratio for e21 323i m20 4 speed manual

[323i-killer]

Hi there which is the best performance diff for my e21 323i m20 4 speed manual? Is it the 3.45:1 ratio diff?

[niske]

3.64 or even shorter ratio... And 5 speed gearbox too

[323i-killer]

Nah I prefer 4 speed unless it's a 5 speed dogleg thanks anyway

[Jeroen]

Well the first 4 gears of the eco 5-speed are pretty much the same as the ones in the 4 speed, you just add an overdrive. The dogleg is a close ratio with a 1:1 5th. Depends on what you're planning to use the car for. On the road a 3.64 can be quite good, also on faster tracks. On more technical tracks -for example- I'd consider a 3.91. Makes a stock box feel like a CR already and you loose quite a bit of top end. Shorter would be more like rally material imo.

[323i-killer]

Yh I want the car for fast road use and track use aswell which is why I'm thinking of using the 3.45:1

[323i-killer]

But the 3.45:1 diff is hard to come by

[Jeroen]

With a 4 speed that may be ok to keep the revs down a bit on the motorway. 5 speed and 3.64 would be an even better combo as niske indicated.

[323i-killer]

Yh my 4 speeder will not be going on the motorway that much just fast road and track, so is the 3.45:1 ok for it jeroen?

[Jeroen]

Depends on the track, on fast tracks with long straights you may need it to get to a decent top speed, on shorter tracks where you'll never get anywhere near top speed a shorter diff may be a better solution. Do you have a diff atm? If so, what ratio? Perhaps try that first? A diff swap isn't too much work. I'll be doing a nother diff swap this weekend and plan to swap in under 90 mins... perhaps aim for 60 mins even haha.

[323i-killer]

I dint know tbh I'll have to look , I hear that a BMW e36 diff internals will fit into the e21 diff casing?

[Jeroen]

Yeah I heard that before as well but no experience there myself

[323i-killer]

I might give that a try as the 3.45:1 for an e21 is hard to come by

[Curly]

Sorry haven't got back to you Killer still have that e21 diff from 323i just havent had time to go and see if it's 3.45 . Can anyone on here tell us how we can tell ?

[323i-killer]

Yeah please can somebody help me out cheers

[petroscf]

Yeah I heard that before as well but no experience there myself

I have a diff bought from Nas, he had said to me that it is like that (small case E36 internals into E21 case)
I saw somewhere that it's doable but the E21 case needs a little gringing in order to fit the E36 internals.

I haven't yet fitted the diff, as my plans wer to store it for a future conversion which has not been done yet
Just for history, Nas was encouraging me to fit it (the car is 316 1.6), but I don't dare yet!