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

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

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

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.

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

But the 3.45:1 diff is hard to come by

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.

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?

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.

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

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

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

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 ?

Yeah please can somebody help me out cheers

Jeroen wrote: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!