M73 + 3.45 lsd = holy smokes!

[dragon850]

Built a custom 3.45LSD for my buddy's 1996 850ci (black one). Installed it today, turned off ASC and went for a drive, all I can say is HOLY Smokes! Had a grin ear to ear the whole time driving it, the car is completely transformed!
3.15 lsd is nice but not even remotely close!


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[ColtonBH]

What are highway cruising revs? XD

I'm looking forward to the slight boost once I get the 2.93 in my 750il

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[Fatandre]

What would be ideal for a csi?

[dragon850]

What would be ideal for a csi?

3.45LSD is also amazing for the 6 speeds.

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[72tpik5]

My 97 M73/5 speed auto came to me with a 3.15 LSD. I swapped in a 3.64 LSD and the difference it made really is amazing. No regrets here.

[m6bigdog]

What are highway cruising revs? XD

I'm looking forward to the slight boost once I get the 2.93 in my 750il

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Easy fix.. The E38 gets a 3:38 diff in the Euro 750iLS. It is an open diff but the E38 traction control was designed to limit tire spin with the AST.
I installed that diff in my 740I and the boost in acceleration is wonderful.
To import you will need to purchase it from outside of the US as the US dealer will just tell you the part is NLA.
Schmiedmann is a parts supplier that can get it to you.
https://www.schmiedmann.com/

[sofiabghome]

Hi Dragon850,

If you are Wow on 3.45 gear..

You should see when I did upgrade the
differential to 3.91 (E34M5) to a customer
with 1997' 850Ci 5spd late version tranny.

The Car is used only on weekend in LA fun toy
not Long trips LA to SF or so.

Car had 295 200 T-ware tires in the
back and off the line when you punch the gas
all the way down, you really feel this
car like I removed 1000pounds.

The power deliver to the rear wheels felt
like not real for E31 at mere 4200pounds
The car felt more like 3200pounds car, the gear
ratios are very close to each other and it really
had very little rpm drop when the engine was
on the red line and then upshift.

Yet, around town on a mild throttle you can
still be cruising in 5th gear at 45mph and
slowly accelerate on the 5th gear whiteout
downshift.

On the other 93' 850 4 speed tranny I upgraded
3.91 (E34M5) and never felt this nice way because
the gears are waay too tall.

3.45 is good upgrade on late version M73 5 spd
but 3.91 is personally I think the way to go.

On my E31 840 97' I had 3.15 but due to Daily
Driving condition and trips to Central Coast area
I build for myself a 2.65 gear 210mm LSD.

Whit the 5 spd late version it does not feel bad
at all as I really thought... I am close
to start a process of swiping S62 in it and the G420 has
much nicer and even closer ration to each other
than the 5spd auto and then will be really win win.
400hp quad vanos system and nice absolute flat
torque curve compare to M60/M62 non vanos engine.

Knowing most E31 owners thinking 3.91 gear is
perhaps absurd.

If I were you I would swap the 3.62 vario LSD
from E46M3, I have done it already to a customer's
E34M5 3.8 6 speed and also to 3) E31 840Ci's great
choice of quick E31 and the same time cruising option.

Regards
Anri

[ChrisFuture8]

"Knowing most E31 owners thinking 3.91 gear is
perhaps absurd."

It's quite the contrary. Folks here with the 850 model loves to get their hands on an E34M5 3.91 Large Case LSD. The problem is the price and rarity. Dragon850 is all about build/rebuild differentials...it's his specialty. Sounds like you have plenty of experiences with swapping differentials, Anri, so maybe you and Dragon850 should hook up and exchange/share knowledge and expertise in that area. It would be a win-win for all. @:-)

Cheers.

[dragon850]

Hi Dragon850,

If you are Wow on 3.45 gear..

You should see when I did upgrade the
differential to 3.91 (E34M5) to a customer
with 1997' 850Ci 5spd late version tranny.

The Car is used only on weekend in LA fun toy
not Long trips LA to SF or so.

Car had 295 200 T-ware tires in the
back and off the line when you punch the gas
all the way down, you really feel this
car like I removed 1000pounds.

The power deliver to the rear wheels felt
like not real for E31 at mere 4200pounds
The car felt more like 3200pounds car, the gear
ratios are very close to each other and it really
had very little rpm drop when the engine was
on the red line and then upshift.

Yet, around town on a mild throttle you can
still be cruising in 5th gear at 45mph and
slowly accelerate on the 5th gear whiteout
downshift.

On the other 93' 850 4 speed tranny I upgraded
3.91 (E34M5) and never felt this nice way because
the gears are waay too tall.

3.45 is good upgrade on late version M73 5 spd
but 3.91 is personally I think the way to go.

On my E31 840 97' I had 3.15 but due to Daily
Driving condition and trips to Central Coast area
I build for myself a 2.65 gear 210mm LSD.

Whit the 5 spd late version it does not feel bad
at all as I really thought... I am close
to start a process of swiping S62 in it and the G420 has
much nicer and even closer ration to each other
than the 5spd auto and then will be really win win.
400hp quad vanos system and nice absolute flat
torque curve compare to M60/M62 non vanos engine.

Knowing most E31 owners thinking 3.91 gear is
perhaps absurd.

If I were you I would swap the 3.62 vario LSD
from E46M3, I have done it already to a customer's
E34M5 3.8 6 speed and also to 3) E31 840Ci's great
choice of quick E31 and the same time cruising option.

Regards
Anri

I agree that 3.91 would be a game changing upgrade to 5 speed M73 but it is an overkill, on the freeway car would be cruising in close to 4k RPM. Even 3.64 is too much IMO for the M73, the 3.45 IMO is best all around gearing. I do believe 3.46 is great for M62. The 3.91 lsd is a great option for a 4 speed M70.
Yes I have the custom built E46/E31 hybrid LSD with 3.45 gearing in my SC 850 had to step up the gearing due to power.
Please keep us posted on you engine swap! Pics would be great!

[sofiabghome]

Hi ChrisFuture8,

Hear is were you are confusing.
Std. 4spd 850i is 3.62 gear ratio
then 3.91 is the most desired option.

But, but, but, most E31 840/850 5spd
owners I have ever speak to most
don't like to use 3.91 gear because
of the nicer 5spd closer ratio compare
to 4spd, the most of 5spd owner they
go is 3.64 gear and they are all happy
and they say anything shorter is going
to be way too short is what you hear.

I know well Dragon850 works on this
cars from a decades ago.

Hear is the proof to you. The owner
of this rare M73 5 spd decided perhaps
to use on his own 3.45 gear ?

If I were to do this on a late version
M73 V12 5spd and if say 3.91 1) is hard to
find or 2) more money then the Secondary
and only choice of mine will be from E46M3
compete unit 3.62, then you have the gear
ratio of a std 4spd earlier version M70 at
cruising speed. On 4spd version tranny
4th is overdrive at mere 0.73 ratio
on the 5spd the 5th is 0.79 ratio(a hair of
a difference) but then the 3.64 is from E32
so it makes the very little shorter by .2
and the minimal hair difference, offsets.

So by swapping 3.62 ratio is basically
at the same gear ratio on cruising speed
as a std M70 4 spd. But then you step on
the gas WOT and there are 2 different cars
in how one will accelerate compare to the
other.

On earlier 4spd M70 the only one option is
3.91 (M5)
Should I even mention that I can do 4.10
Motorsport gear 210mm into E46M3 casing
and then swap the entire unit to the E31
casing and then better than 3.91.
But But But I have never seen
yet an owner of E31 4spd whom will spent
around ~$2000 on 4.10 differential and
then freeway cursing speed will really suck !


And one reason I like 5 spd cars over at 4spd
even when people say oh 4.0 does not have
torque but its 5spd, I get it its a little down
on torque to M70 and then M62 but then swap
3.62 and then then things are changing..

I always wanted to try to do a pulls between
840ci m60 4.0 with swapped 3.62 compare to
E31 std 4spd 850Ci std 3.62.

I did pulls between my 97' 840Ci when I had
the 3.15 vs std 850 4spd and I was pulling very
steady and progressive.

I can imagine if I were to swap the the same
gear ratio at say 3.64 then the difference will
drastic in car lengths.......

Regards
Anri

[sofiabghome]

5 speed M73 but it is an overkill, on the freeway car would be cruising in close to 4k RPM. Even 3.64 is too much IMO for the M73

Hey Dragon,

Let's do the math together, because
I am not feeling the love on this miss-concept.

So, from the factory std 4spd E31 comes at 3.62
gear ratio. Then every 4spd owner is wishing on
or upgrading the gear ratio to 3.91 M5


So say let's build 2 Model cars, both will be put on M-parras
at 285/35/18 and say both Models will be put on
the same tire Brand to get the same tire diameter.

So say will pick up Continental at 25.7 Inches diameter.

E31 M70 4spd 4th gear is 0.73ratio on the Freeway will
at say cruise speed of 93mph I mean this cruising
speed is Absurd on public roads...but let's hypothetically say.

M70 4 spd gearing+rpm

At 25.7 Inches diameter,
4th Gear ratio is 0.73 Ratio
and at speed of 93mph the
engine RPM will be at
Mere 3200rpm so not even near 4000rpm as you say.

25.7+3.62+0.73+(say 3200rpm) =93mph cruise speed.

For M73 5spd is much the same due to the same over drive
at 0.79 ratio

25.7+0.79+(say 3200rpm) =86mph
(basically we are splitting hairs hear)

So as you say we raise the engine rpm to 4000rpm then both
set up will be at 107mph of were the only place you can cruise
this speed is back home on the Autobahn.

Hear in US at 107mph cruise speed one will meet the Judge
inside the Court, in very short...

Again, at the Shop when people ask me which one do I buy
V8 or V12 the Endless debate.....between us hehehehe

I always answer back buy 5spd car I don't care V8 or V12 they
are all confused...and say what do you mean....

Regards
Anri

[dragon850]

Hey Dragon,

Let's do the math together, because
I am not feeling the love on this miss-concept.

So, from the factory std 4spd E31 comes at 3.62
gear ratio. Then every 4spd owner is wishing on
or upgrading the gear ratio to 3.91 M5


So say let's build 2 Model cars, both will be put on M-parras
at 285/35/18 and say both Models will be put on
the same tire Brand to get the same tire diameter.

So say will pick up Continental at 25.7 Inches diameter.

E31 M70 4spd 4th gear is 0.73ratio on the Freeway will
at say cruise speed of 93mph I mean this cruising
speed is Absurd on public roads...but let's hypothetically say.

M70 4 spd gearing+rpm

At 25.7 Inches diameter,
4th Gear ratio is 0.73 Ratio
and at speed of 93mph the
engine RPM will be at
Mere 3200rpm so not even near 4000rpm as you say.

25.7+3.62+0.73+(say 3200rpm) =93mph cruise speed.

For M73 5spd is much the same due to the same over drive
at 0.79 ratio

25.7+0.79+(say 3200rpm) =86mph
(basically we are splitting hairs hear)

So as you say we raise the engine rpm to 4000rpm then both
set up will be at 107mph of were the only place you can cruise
this speed is back home on the Autobahn.

Hear in US at 107mph cruise speed one will meet the Judge
inside the Court, in very short...

Again, at the Shop when people ask me which one do I buy
V8 or V12 the Endless debate.....between us hehehehe

I always answer back buy 5spd car I don't care V8 or V12 they
are all confused...and say what do you mean....

Regards
Anri

Anri, so today for the sake of argument I got actual RPM from the M73 850ci that had the 3.45LSD swap, results:

80MPH= 3000RPM
100MPH = 3800RPM
120MPH= 4500RPM

This is my personal opinion but cruising at anything over 3k RPM at 80MPH is going to the other end of the extreme and is not optimal. That is why I recommend 3.45LSD as a golden middle.
I absolutely agree with you that 3.91LSD is a perfect upgrade for 4 speed automatics.
As far as what car to buy, it's a matter of preference, me personally , I like the the OBD1 850s the most, for a few boosted reasons

[sofiabghome]

Anri, so today for the sake of argument

Hey Dragon850

Please do me a favor you should
understand I am not hear to argue
nor I want to. We are gown people
whom share E31 and car passion.

We are just discussing what few
parts put together will do as Final
result.

I do understand your concept of what
you are saying in regards of 3.45
does get both worlds cruising and
around town comfort.

But, I have tested Customers whom
were so paranoid that the rpm will be
screaming in the red line when they drive.
I always say if you don't like it I will
replace it for free of charge no problem
so drive it for a month and if you don't
like it I will change it.

They never want to change..

Every single E31 840Ci 5spd owner hear ever
ever replied in any regards of the differential
upgrade subject there is not even one person to
say absolute anything agains 3.64 E32 swap
all love it and none say it's too short.

Dragon, much the same for V12 same 5spd
tranny ratios, just attached to different engine.

Again, all downs to absolute personal taste
if you think at 86mph 3200rpm is way too
much then another E31 owner may think
it is totally fine.

Will give you again my 840Ci as an example
I have swapped
2.65LSD+25.6+0.79+3000rpm==109mph.
at 2200rpm=80mph.

See you have different concept on your OBD-1
pick to chose..boost

Remember as they say "Different Horses for different courses"


Regards
Anri

[dragon850]

It's just a saying, not trying to argue....
It's a matter of personal taste.