what diff should i use?

[new2bmw]

I have an 84 318i auto. I am not sure what diff it came with , i think maybe 3.84? I also have a 325i manual parts car, no idea what it would have in it. Which diff will give me the best results, the one that came with the 318i or the one from the 325i.
thanks

[skatingzooyork]

Results for what? acceleration, top end, mpg?

[new2bmw]

results

i am looking for acceleration, god, anything to help lol

[SoopaGhetto]

im sure your car came with a 3.64 as i had a 3spd auto 318i with one. I swapped in a 4.10 and made a helluva difference in acceleration (5spd swap as well), but if you cruise 70mph+ your rpms would be really high especially since its auto with 3spd (think 3rd gearing is 1.00:1). Thinkin about using one on the eta as well

[new2bmw]

so which one?

so keep the stock diff? or go with the diff from the 89 325i?

[skatingzooyork]

You need to confirm what is in the 325... Any diff that is numerically higher will give you a little better accel.

[MR 325]

The 325i will have either a 3.73 or 4.10.

It was my understanding the M10 318i had a 3.91 diff, correct me if I'm wrong guys...

[SoopaGhetto]

The 325i will have either a 3.73 or 4.10.

It was my understanding the M10 318i had a 3.91 diff, correct me if I'm wrong guys...

i believe the 5spd m10 318i had the 3.91 diff. the autos have 3.64 diff. i searched a lot when doing my 5spd swap a while back.

to OP: it really depends on what you are looking to get out of it. Since you have an auto 3spd, a numerically higher (shorter) diff ie 4.10 would put you at a very high cruising rpm but should give you want you want as far as acceleration goes. most say 3.73 is a good compromise, but if you have a 3.64, .09 of a turn wouldnt yield you that much...

Using a numerically lower (taller) diff would make your car accelerate slower but should yield better cruising rpm...which is opposite of what you want.

if you want to go all out, get a 4.27 from a convertible 318is
(this is off topic but a 5spd swap would definitely help)

[Jean]

My 5spd m10 car came with open 3.64 fwiw. I wouldn't bother changing the diff, and get rid of the auto imho.

[SoopaGhetto]

My 5spd m10 car came with open 3.64 fwiw. I wouldn't bother changing the diff, and get rid of the auto imho.

well damn, guess its kinda like the 325i auto with either 4.10 or 3.73