RE: E36 - Swapping differentials

["Berney, Butch" <butch.berney@xxxxxxxxx>]

John,

First off, not much about your letter was "respectful" ("buzz bomb" "over
the top" "deaf, as well as dumb"). If you had read *my* reply to Chicken, I
said that I had little experience with this, but pointed him to some
*information* on the subject from which he could decide for himself. But no,
you felt the need to belittle people, and equipment suppliers with your
*superior* intellect and *knowledge* in the field of exhaust and
differentials. :(

But so I don't completely stoop to your level, let's do a little *advanced*
math.

RPM = Diff Ratio * Transmission ratio * 336 * MPH / Tire Diameter in inches

Since we are only changing the differential ratio, we can throw most of this
out and reduce our equation to:

New RPM @ 70 MPH = 3600 * New Ratio / 3.46

Let's just say that your number is correct (I think it's probably a little
high) 3,600 @ 70 with a 3.46. A standard 328 manual has a 2.93. That would
put the RPM at 3,048 @70. Not a large difference, but you would notice the
increase in engine speed, and the associated noise. Some won't mind, and
some people will. Let's take it one step further to your current setup, a
3.23...hmm 3600 * 3.23/3.46=3,360. If you ask me (you didn't, but I will
spout anyway) I would hardly notice 240 RPM difference at 70 MPH.

Here is a link to some more good info on the subject of differentials.

http://www.gis.nsw.gov.au/staff/rnott/bmw/BMWText/technical/ChoosingADiffere
ntial.html

You will find some people that love their 3.46, and you will find at least
one person that didn't like his 3.23 and was thinking of switching back to
2.93. Mostly you will find a lot of *good* information.

There is nothing wrong with letting us know that you prefer a 3.23 ratio, in
fact I appreciate data points like that. There is something wrong with
putting it in such a snooty way. I guess there is truth to the BMW
stereotype (that we are all nasty drivers). Fortunately, it is not pervasive
in this list.

Butch (who normally doesn't get bent out of shape, but is cranky today)
94 325is with M suspension (mostly)...*and* a 3.15 final drive

- -----Original Message-----
From: JJINHART@domain.elided [mailto:JJINHART@domain.elided]
Sent: Tuesday, May 22, 2001 1:40 PM
To: butch.berney@domain.elided
Subject: RE: E36 - Swapping differentials


Regarding Korman
    let me respectfully disagree with Korman's recommendations.  If you
want your car to become a buzz bomb, go along with their numbers for
diffs...e.g a 3.46 in a 2.8 will have you doing 3,600rpms at 70 mph,
which is like having a VW GTI out of your 30 grand vehicle.  IF you're
into that sort of thing, go on then, but BE WARNED, their
recommendations are way over the top in terms of what refinement and
quiet you might expect from a BMW--that is, you won't have much left,
especially on the highway.  IF you're after a track car, do it up, oh
and get a UUC exhaust, so you'll be deaf, as well as dumb.
John-96 328is c, 3.23 diff-this is the ticket for a daily driver..sorry,
IMHO!

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