3.0CS DIFFS & RATIOS


E28 Diff Swap Options for 535i, 533i, 528i.

There are two reasons most people change their differential ratio. Either the original gearing
is too tall, or they are sports oriented drivers wanting a lot more power in 3rd and 4th gear.
The people who benefit most from a ratio change are those whose car is too heavy for the
engine, such as the E36 320, 325 or the E28 528.


Your Gearings Too Tall:  Originally the 528i, 535i & 735i were sold with a 3.25:1 diff ratio.

Whenever you approached a small incline with a headwind the cars were too heavy to maintain the speed and it was
necessary to change down a gear. In some markets, BMW later changed the ratio to 3.46:1 to overcome the
problem. They didn't go to the 3.64 because this would have hurt fuel economy and made the car a lot noisier. The
later E34 535 was heavier than the older E28, so BMW was forced to select the 3.64 ratio for the manual and 3.91
for the automatic, while using the same ratios in the gearbox as the older E28.

Some E28 528i owners changed the diff to a 3.46 and removed the air pulse tubes from the exhaust manifold. This
transformed the car. It no longer ran out of puff in 5th and became a strong motor with plenty of power in all gears.
On the US M5 the factory selected a 3.91 to achieve similar performance to the higher power and lighter European
M5 with 3.73 diff ratio.
The M5 used lower gearing because they were made for Sports Oriented Drivers.


Sports Oriented Driver:   People who want more power for sports driving and like to listen to the engine don't mind
the extra noise. By going from 3.45 to a (lower) 3.64:1 ratio you benefit most in 3rd, 4th and 5th Gears by the engine
accelerating more strongly. The disadvantage is that 1st and 2nd Gears are much lower and are no longer matched
to the engines power. For example: you'll feel that if 2nd was a fraction lower, you wouldn't need 1st at all, meaning
that on an everyday basis you have to change from 1st to 2nd before attaining any real speed... and wasting time.
This is the compromise you make when

Choosing a Performance Differential / Date: Sun, 24 Jan 1999 (Update 11 Jan 2001) / From: Richard Nott
<Richard.Nott@dpws.nsw.gov.au>



What Works Best In Your E28

General consensus suggests the best final drive ratio for a manual E28 535i/528i is 3.64.

Many argue that the 3.73 will provide the best 'Bang for the Buck' because of the slightly higher acceleration... "if
your going to generate extra cabin noise, you may as well select the ratio that gives the best acceleration". However,
going higher to a 3.73 means 1st and 2nd become too low, and this can actually harm your acceleration from the
lights and make driving around town a real pain... The benefit though is on the track, where the car will have
noticably better pulling power through 3rd and 4th gears. Using a higher ratio means your engine is working harder
all the time and generating a lot more cabin noise. If you want your car to be more suited to highways than the track,
the 3.46 is the best choice because it will minimise noise and you really don't gain that much pulling power going to
the 3.64.

Of note, BMP recommends the 3.64 as the best performance ratio for this model. If you own an automatic 535i, 528i
or 735i however, the best performance ratio is the 3.73. This is because the automatic gearbox is taller geared and
therefore acceleration through 1st and 2nd is still good. The penalty here is that revs will increase by 500rpm and
your car will always be noisier inside the cabin. Personally, I chose the 3.64 for my 535i automatic because I wanted
to keep the noise down... (Revs increased by only 300rpm) If I wanted maximum performance I would have chosen
the 3.73 (but I have a later 155kW engine with Schrick cam), so I thought I'd have all the performance I'd need
without it.

Apart from performance, the other considerations when choosing the best differential ratio are engine noise,
additional frequency of gearchanges (esp. in lower gears), and your cruising speed. Do you want your car to be
quiet, do you want more power in 3rd, 4th and 5th, do you want 1st and 2nd gears to be too low... The additional
performance isn't free. It comes at a cost. Consider the power of your engine (higher power means you don't need as
big a change. What will your RPM's be with this new ratio?. What ratio will you be most happy with over the long
term... This is the hard part.

It costs quite a bit to change your diff ratio, so be warned, if you select a ratio that's either too tall (low number eg
3.23) or too short (high number eg 3.91) you'll end up with a costly mistake that's actually worse than your existing
setup. Begin by deciding whether you want to build the car for driving around town or long highway trips. My final tip
is too look at what BMW chose for your model. They spent a lot of time and money deciding which ratio would be the
best 'compromise' for both city AND country driving. Deviate too far from this and you will impair one or the other.
Sometimes even BMW gets it wrong and will alter the ratio at a later time, but this will usually be no more than one
step in either direction. Because this is so important, it is my final tip for you: When you change ratios, only go up by
one or two steps. Be very careful when you go further than this. I can almost guarantee you, any more than two steps
and you won't be happy with the result.

Real World Examples: (E28, E23)  Choosing a Performance Differential - E28 9/10/08

Leaded 1986 E23 735i Automatic with 10:1 compression, ported head, free flow exhaust and the 3.73 final drive,
delivering 150kW (201HP) to the rear wheels. This is an impressive car with the heavy 7 easily able to spin the
wheels when pushed. Surprisingly the 3.73 does not feel too low geared in the Seven with the sports auto. I was
surprised, as the 3.91 in a manual 5 Series with 5 speed overdrive was too low and unpleasant to drive in lower
gears... This emphasises just how different the gearing is for Automatics compared to Manuals and shows that what
works for one, will not for the other. The Dyno Shop that tested the car said they'd never had a six cylinder put that
much power to the rear wheels... impressive.

Of interest to Automatic drivers will be BMW's decision to use the same gearbox ratios for I, II, III, & IV in all their
Automatics (E28-535i, E23-735i, E34-535i).

Surprisingly, Porsche also uses the same gear ratios in their Tiptronic S - 911 with 3.64 Final Drive Ratio...

the E34 Auto uses a 3.91. The " Tiptronic S " accelerates 0-100km/h in 6.4 seconds with 210kW@6100rpm.
(3.6 liters, 340Nm@5250rpm, 11.3:1 Compression. 1425kg.)  The BMW E34 535 accelerates 0-100km/h in 9.5
seconds with 155kW@5700rpm.(3.4 liters, 305Nm@4000rpm, 9.0:1 Compression. 1540kg. )

Earlier I wrote about my 1983 E28 528i 5 speed with 3.25 diff ratio. This gearing was too tall and the engine could not
maintain speed up slight inclines at 100 km/h, (60mph) unless the pollution equipment was removed!. Later, BMW
changed to a 3.46 ratio after realising 3.25 was too tall for Australian Conditions: (110 Km/h max).  3.46 seemed an
ideal compromise for low noise, good fuel economy, and good power for general highway driving. I wanted the car to
accelerate up hills in 5th gear, so I decided to change the crown wheel and pinion (diff ratio) to 3.91.  Unfortunately
this was not a good choice and actually slowed the car down because reaching 80 km/h required 3 gearchanges
instead of 2... Not good for city driving. On the highway it was also bad, because of the extra noise generated by
the engine revving hard. It was simply too low geared. If I did this exercise again, I would choose the 3.64 as the best
compromise... good power up hills in 5th and not too low in 1st and 2nd driving around town.

For my larger 535i auto, I was hoping to keep the 3.46 final drive ratio. But the 3.5L (E32-155kW) engine, even with
a free flow exhaust and cam, was unable to accelerate up hills in top gear... Therefore, I chose the 3.64 knowing
there is a noise penalty...

(I'd rather have the engine quiet to enjoy a good sound system). This resulted in a very slight improvement up hills,
but no difference in 0-100km/h times. At this time, I still haven't had a custom chip made for the engine conversion,
and the standard Motronic program is not able to benefit from the higher compression engine. So time will tell how
big a difference this makes and whether I can go back to the 3.46. Companies such as Alpina or Hartge in Germany
fit even taller diff ratios than the 3.25 because their engines are more powerful and they are doing higher speeds, so
can sit in the peak torque band at around 3500 RPM's doing 160-200 km/h!...



Final Drive Ratios
BMW occasionally change these to suit the individual market. There are lots of other ratios fitted. In E23, E24, E28,
E30, E36 the Crown Wheel and Pinion that determine the Final Drive Ratio are interchangeable, even though the
housings are different and not interchangeable. eg E28, E30, E36.

Hints, Tips & Tricks:
1. The ratio is always stamped on a metal tag, and can be found on the back plate of the diff joined by a bolt to the
housing. Don't trust this though, it could be swapped.

2. An S stamp at the beginning of the number indicates that the Diff is limited slip, eg: "S 4.10" indicated Limited Slip
with 4.10:1 Final Drive Ratio. These are also marked with a large white S spray painted onto the diff housing.

3. The E28 520i diffs have different half-shaft output flanges compared to the E28 528i, but these can be simply
prised out from the side and swapped for the original.

4. Someone suggested that E30 differentials were interchangeable with E28's by swapping the rear plate with the
original. I can't verify this. It may be possible, as long as you know the trick of prising out the wheel flanges to the half
shafts,to swap with your originals.


E36 Differentials -
Model (E36)                                 Stock Gear Ratio         Recommended Ratio

318iS, Ti Manual Transmission                 3.45                         3.90
318iS, Ti, Automatic Trans                       4.44                         4.75
325 Manual Transmission                         3.15                         3.46
328 Manual Transmission                         2.93                         3.46
325/328 Automatic Transmission                 3.91                         4.27
M3 3.0                                                         3.15                         3.46
M3 3.2 , Auto and Lightweight                 3.23                            3.46

The above information comes from Kormans' website catalogue. I am not associated with Kormans in anyway.


M6 - Low Ratio
I'll assume your wanting this for the M6? In that case, you only have one option. There are only three cars in the US
that come with shorter gears than 4.10 and none of those diffs will actually bolt into your M6. The E30 318ic cars
came with 4.27 gears and the 318ti and 318i AT cars (E36) both came with 4.45 gear ratios. However, these are
"small case" differentials and they will not bolt into your car. The ring and pinion set is also smaller and will not
transfer to your diff case. That leaves the late model E34 535i AT cars. The later years (like 91 or 92 and later) came
with a 4.27 in a larger case diff that would fit into yours. You'd have to pick one of these up used and swap the gears
into your case (or another that fits into your M6) because the E34 differential case will not fit the E24. BMW no longer
sells ring and pinion sets so your only option is used. I've heard you can buy some 4.45 sets directly from the
Motorsport division, but I'm sure they ain't cheap! Good luck.


What Does a Limited Slip Differential Do?
Date: Thu, 18 May 2000   From: Joe Tan <jtan71@yahoo.com>  Subject: limited slip differentials

A limited slip differential (LSD) will not add any power to your car. It will however make better use of the power you
already have. When cornering a car with an open differencial (non-LSD) the inside wheel will get too much power
and it will smoke your tire thus wasting your power. With a LSD, the power will be transfered to the outside wheel
where it can make better use of it and help you power you out off the corner faster. The only way a LSD will add
more "power" to you car is if the differential is numerical higher where you would get better off line accleration in
exchange for a nosier ride at highway speed. If you are looking to add some real HP you will need to do some engine
work or forced induction.

Joe T.
(