re: E30 diff questions

["John C. Hanes" <jchanes@xxxxxxxxxxxxxxxx>]

"Scarface,"

>hello fellow bmw fanatics....i have several questions regarding the
>differential swap on my e-30 325es...

okay..

>i have a 2.79 lsd stock...its slow and i need more pep!  <1> Do u folks
>know the ratio of the differential of a E-30 318 (84-87 model)  and would
>it fit on my car?  

Before 11/83, it was 3.64:1, from 11/83 to 9/84 it was 3.91:1, and
automatics had a 4.10:1 ratio.
NONE of these will fit your car. Four cylinder diffs (with the exception
of the E30 M3) are not compatible with six cylinder diffs. Even if it did,
the ratio is so high the car would be undriveable on the street (or at
least not very satisfying to drive).

><2> Does the diff swap a direct bolt on or do i need to change the
>halfshafts?  what about the 318 diff do i need the 318 halfshafts?

This is a non-issue. The housings and half-shafts are totally different
between 4- and 6-cylinder E30's, so it's not doable. HOWEVER, you CAN use
the diff from an E28 5-series (1982-1988), which is a direct bolt-on once
the flanges and back cover are replaced with the one from your current
2.79 diff. 

><3> Would it improve significant change on my acceleration? 

A 3.25:1 diff from an E28 533i/535i/s is what you want. This significantly
improves acceleration without impairing driveability too much. A much
higher gear ratio will necessitate too much shifting, and too high an
engine speed at highway speeds. 3.25:1 is higher than what you have to
wake up your eta just the right amount to make it a lot more fun.

><4> I found a 3.73 diff from a 325i but expensive and not LSD.....do i
>really need LSD to put in that car?  

Only you can really answer this one. You don't NEED it, but it's nice to
have. You have it now. But this diff is geared too high anyway for your
car anyway, especially if you're not sharked yet. Like you'd get to about
10mph before you had to shift into 2nd at redline, etc....

><5> What's the advantage of the LSD and non LSD?  

The jury's still out on this one. I've asked this before and gotten lots
of real technical answers. Anybody else want to tackle this one? I'd
replace my 2.93LS with a 3.25LS simply to stick with the performance I had
before but at a higher gear ratio. In fact, I have a 3.25LS from a 535i in
my trunk ready to bolt on right now. <sigh> Someday when I have time.

- -John

John C. Hanes
1986 325es 161k 
JC sharked/K&N/Bilstein/PBR/Ansa Sport
Borbet "H"/225 RE71's/custom "BL/SS"
Cook Campus, Rutgers University
New Brunswick, New Jersey
NJ BMWCCA #149905

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