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Re: Swapping out diff in 325es - Summary



Due to the overwhelming interest in this topic, I've summarize my findings
below (so far). First, many thanks to the contributors who have performed
this swap with various ratios (3.25 & 3.73) and sharing their experiences.

Background:
The 325 "eta" (efficient) was imported to US from 1984 thru 1988 with the
high torque low revving (redline @ 4750rpm) 2.7 liter engine. This provided
a balance of performance and economy. Transmissions found on this model was
either a 5-speed manual or 4-speed automatic. The gearing had been
intentionally set for economy especially with the final drive ratio of
2.93:1. This drive ratio was good for touring but not for quick
acceleration off the line.

Potential Modification:
A potential modification would be to swap out the diff with a higher ratio
and leverage the horsepower at the higher rpms but keeping in mind the
relatively low redline @ 4750rpm (or 5250rpm with Dinan chip). There are a
variety of possibilities which could be extracted from the following
year/models (US). Obviously, there are limited-slip and non-limited-slip
versions. The 3.25 and 3.73 can be found quite readily but the 3.46 is rare
to find. Also note that the 3.25 must be altered by swapping the OEM 2.93
back cover and flanges onto the 3.25. The 3.73 is a direct swap, no
alterations.

        3.25    88 E28 528e/manual, 85-88 535i
        3.46    87-88 E28 528e/auto, 87 635/L6,
        3.73    87-91 E30 325i

Net Wisdom:
There are several fellow 325 "eta" owners which have modified their drives
to either the 3.25 or 3.73 ratio. And to paraphase their experiences, it
all depends on what you are wanting to achieve. If for track, the 3.73
nicely shifts into the higher rpms but as a daily driver vehicle, 2800rpm @
70mph is abit high and noisy. For a more conservative alternative, the 3.25
lifts the rpms abit, increases acceleration and compromises with 2500rpm @
70mph. BTW, the 2.93 spins at 2200rpm @ 70mph.

Personal Choice:
At first I had the hunger to aggressively swap in the 3.73 but decide not
to after some thought. My usage would be a daily driver and not on the
track. I also noted that my original diff was NOT a limited-slip (I had
thought the 325es version was equipped standard with LS). The decision
wasn't quick for me, I generated MS Excel calculations of shift points
between the various ratios. Keeping in mind that my 325es is 4-speed
automatic transmission. Its gearing is very different than the 5-speed
manual. Simply stated, the automatic is like the 5-speed manual without 1st
gear, starting in 2nd! Also the automatic overdrive is even lower than the
manual, .73 vs .81. Here's glimpse of all the data I've been reviewing;
four tables, one for each ratio...

ratio vs gear vs rpm = calculated mph


2.93    manual  1600    4750    2340    2.93    auto    1600    4750    2110
- ------------------------------------    ------------------------------------
    1   3.83    10.1    30.1    14.8        1   2.48    15.7    46.5    20.6
    2   2.20    17.6    52.4    25.8        2   1.48    26.2    77.9    34.6
    3   1.40    27.7    82.3    40.6        3   1.00    38.8    115.3   51.2
    4   1.00    38.8    115.3   56.8        4   0.73    53.2    157.9   70.1
    5   0.81    47.9    142.3   70.1        R   2.09

    R   3.46



3.25    auto    1600    4750    2340    3.73    auto    1600    4750    2680
- ------------------------------------    ------------------------------------
    1   2.48    14.1    41.9    20.6        1   2.48    12.3    36.5    20.6
    2   1.48    23.7    70.2    34.6        2   1.48    20.6    61.2    34.5
    3   1.00    35.0    103.9   51.2        3   1.00    30.5    90.5    51.1
    4   0.73    47.9    142.4   70.1        4   0.73    41.8    124.0   70.0
    R   2.09                                R   2.09

So finally just today, I've purchased a 3.25LS version and awaiting for its
arrival next Monday! I keep you posted after install it.

Leon
86 325es; Dinan Chip, RD Springs, Bilsteins, K&N, (soon to be) 3.25