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From digest.v6.n878 Mon Jul 14 11:08:05 1997
From: Kevin McClure <>
Date: Mon, 14 Jul 1997 02:31:55 -0500
Subject: differential upgrade (long)

A little over a month ago, I asked this group for their opinions on changing the differential on my 1984 633 from 3.25 to a higher numerical ratio, for better acceleration. There were a number of very helpful responses, and the upgrade is complete. Here's the story, in case others may be considering this.

New and remanufactured diffs were well over $1,000. However, since they usually are bulletproof parts, several sources on this list and elsewhere reccomended going with a used diff. Mine came from Active Foreign Auto Parts (in the Roundel) and cost $300.

The most important question was how far to go. The Korman web page suggests a 7% change, or roughly from my old 3.25 to a 3.45. Some of you suggested this as well. However, this is for a 633, a car that could use a little more oomph, IMHO, especially for the large prepondrence of around town and short highway driving I do. A couple of you had made the upgrade to a 3.73, and liked the results. I decided on a 3.73, just short of 15% more than stock.

On the down side, highway RPM's are up. Using finely calibrated instruments (eyeballing the gauges), for the 3.25 at 70 and 80 MPH fifth gear RPM's were about 2450 and 2800, respectively. For the 3.73 at 70 and 80 MPH this increased fifth gear RPM's to about 2750 and 3150, respectively. If this were Germany with the Autobahn, this might be something of a problem, but sustained cruising at 80 or 85 is about the most I'm likely to do.

On the up side, the car now feels much more responsive, definitely quicker. Above all, there is the curious feeling that the car is now significantly lighter. I am tickled to death with it.

For any stoplight racers (few and far between in this company), 0-60 now requires a shift to third, as 2nd hits the rev limiter at 58 MPH rather than 65. However, I need to look into this, as the limiter comes on at an indicated 6000 RPM, while the tach shows a narrow red band at 6250 and a broad one at 6500. Can anyone shed any light on this?

Caveat: Part of my decision to go with a fairly radical change was the discovery that Dinan chips don't go in my Motronic unit, and the King of Chips Conforti said he might get to it sometime, but "don't hold your breath." Weeping and gnashing of molars and fillings ensued! You folks with more powerful engines contemplating a differential upgrade might not need to trade up so high. This was my one chance at a significant perfromance boost of the relatively inexpensive variety.

Thanks to all the excellent responses from the BMW Digest and the Big Coupe Groupe, including Samir Shah, Norm Grills, Terry Lee, John Csuri, Mark Child, Andre Grandi, Gerald Brown and Scott Buchanan. What a great resource these groups are!

YMMV, IMHO, not legal in France, etc.

Hope this is of interest to some folks.

Kevin McClure
(Reborn Bimmer champing at the bit, looking for a chance to carve up some Texas Hill Country roads).

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