Different diffs?

[glenbat]

My 325i cab started life as a 318 (i think) and had the 2.5 lump put in last year before i bought it, a friend tells me they should of changed the diff also otherwise the gears will be too short when pulling away?
Is this right and also is there a way of telling easily?
Cheers
glen

[320Touring]

if it was originally a 318i it'd probably have had a 4.10 or 4.27 diff fitted

the 325i should have had a 3.64 or 3.71(3?) or 3.91 diff fitted as standard.

Basically the lower the number, the higher the gearing of the diff (ie a 3.64 is "taller" than 4.10)

there are a few ways to tell what diff you have

1-see what rpm your car is pulling at an indicated 80 mph, the shorter the diff, the higher the revs (my 4.10 pulls 4krpm at 80mph, a 325i with 3.64 should be pulling just over 3k rpm)

2-jack the car up and read the tag attached to the diff casing-the last 3 numbers indicate the ratio e.g 4253614373= a 3.73 diff (look out for an S on the casing (means LSD )

3-jack up the car and see if you have a 'small case' diff (ie one that is fairly compact-these tend to have less bolts on the casing than large ones) if so it'll be a short diff-4.10/4.27

hope that helps!

[dannosail]

Slightly off track but what's the little sensor plug that goes into the back of the diff for?

[M3Compact]

Slightly off track but what's the little sensor plug that goes into the back of the diff for?

Speedo.

[glenbat]

cheers guys, couldnt locate the tag but found 66640 on the casing?
Does this help?

[stuartgallafant]

i have a 318iS and i wanna LSD for it

whats the best ratio to look out for:
a) acceleration?
b) top-end?

[stulane]

http://www.jaredsquires.com/ndp/content/view/17/31/1/2/

^ Good guide to diffs at the bottom of this page ^

[Simon13]

i have a 318iS and i wanna LSD for it

whats the best ratio to look out for:
a) acceleration?
b) top-end?

these 2 don't go i'm afraid for if u want long legs and top speed u will lose acceleration. The easiest diff for u to find is a large case sport 3.91 mate

or if u are lucky a 4.10 LSD but good luck finding one!

[tomtomiS]

i had th emisfortune of my diff going in december last year and the only one i could get at the time was a 3.64. my car hits 100 mph in 3rd gear and motorway driving isnt quite so drony plus i get roughly an extra 50 miles out of a tank. BUT its not as eager through the gears and thats what iS's are all about IMO so i might be having the old diff rebuilt when some more money falls from the tree outside my house

[stuartgallafant]

cheerz guys. i'll be looking out for a 3.91 or a 4.10 and i'll let you know how i get on.

where'd you get the seeds for that tree?

[tomtomiS]

i know of somebody selling a 3.9 lsd for Ԛ£210