LSD ratios etc, M635 & standard 635csi - differences etc

[UKDaveJ]

I read in a magazine article that the LSD as fitted to the M635csi is a very special unit.

How much does its' LSD differ to that fitted on late Highlines, or earlier 6'ers for that matter?

Were all LSD's the same final drive ratio, or did they vary over the years, different models etc?

Maintenance tips?
Lubricants?
Do's & don'ts?

Any info much appreciated.

Thanks,

Dave

[KC Ron Carter]

The e24 M series uses a larger differential in size.
The same as the e23.

The normal after 1982, e24 shares the differential case with a e30-6, e28, and z3.

Before 1983 the e12 side loader was in most e24.

Now the ratio:

Most Euro Sharks are equipt with a 3.07 to one ratio. 180 mph maximum.

All e24 M series use a 3.91 to one ratio, 180 mph because of higher RPM.

Most USA models from 1982 to 1989 e28 based e24 got a 3.42 to one.

Taking the non M model and installing an M 3.91 to 1 ratio, from an e30, will make the standard e24 a monster.
You will be limited to 140 mph unless you chip the ECU.

Many in the US install an e30 3.73 to one ratio differential, me to.

Wakes up the around town driving.

[ScottAndrews]

3.42 or so might be nice. My 1980 Euro has the 3,07 LSD and it is pretty damn tall. Not so tall that I can't get a good launch, but jeez, you can't even think about 5th gear under 80-85 mph.

S

[KC Ron Carter]

I have a way to show the lurkers what we are typing about.

Here is a Dyno run in third gear on my 1985 635 csi with a 3.73 to one differential.

It shows 120 mph at 5400 rpm in third gear.



Later,

[jrcalvin]

Maybe have bad information on the e24 ratios? The 3:07 was a standard ratio on regular euro 635 cars. However, the euro M635 had a 3:73 with the 286hp motor. The US M6 had a 3:91 to try to make up the difference of fewer hp at just 256hp.

As a side note, my US 1988 blk/blk M5 has the 3:91 ratio and I always think that 1st gear is too low with the Getrag 280. Although I have seen an indicated 150mph with the M5 (pretty solid at that speed but the windshield wipers were sure rattling around!)

I find my 1978 euro 635 to be just right with the C/R and the 3:07 lsd.

[Mountain Shark]

I posted these numbers on roadfly today (as they relate to the 3.73 Euro vs. the 3.91 US...)

BMW played with the ratios a little so the US M6 could compensate for the 300 pound / 30 pony deficit. Plus you can't drive 150 here so what's the point? It was suggested on Roadfly that the Autocar testers didn't get their foot all the way into the M635 back in 1984. Regardless -- these cars are amazing and I'd be honored drive drive either!

M635 based on Autocar (UK) April 1984
M6 based on Car & Driver (USA) 1987

I'm sorry the numbers all run together. I type them with spaces but they all run together when posted.

1984 1987
M635 M6
mph time time
30 2.4 2.1 the 3.91 US M6 rear end screams off the line!
40 3.7 3.4
50 4.8 4.6
60 6.1 6.1 Dead even!!!!
70 8.6 8.3 don't understand this one. the curve reverses
80 10.5 10.6 M635 starting to pull away with the extra ponies!
90 12.7 13.3
100 15.6 17.1
110 19.1 21.6
120 23.4 28.4 the 3.73 M635 has longer legs and it shows
130 29.7
140 41.8

Standing 1/4 mile
M6 14.7 sec at 94 mph
M6535 14.7 sec at unknown

[ScottAndrews]

Here is an interesting thread from earlier this year. The BCG guys were discussing gear ratios. Frankly tim eto speed is not a very good way to approach this since it introduces a lot of other variables (road incline, wind, the weight of the driver's foot, shifting speed, gas, etc...)

What we did was go runb our cars in each gear at 3K RPM, and note the speed. In addition, Metric Mechanic has a nice handy chart on their web site that shows speeds in gears for variousdiff and trans combinations.

Bottom line it that there is something weird about KC's graph.. 120 mph in 3rd at 5400 RPM implies some pretty high gearing. My Euro has a non-dogleg CR trans and a 3.07 LSD (about as high as they come). It calculates to a theorectical 113 mph at 6500 rpm.

Anyway, here is the thread:

S
Quite different from the a US 5 speed. I expanded your table.

mph at 3000 rpm 6500 rpm Gear ratios (calculated)

1st 18 39 4.33
2nd 40 87 1.95
3rd 52 113 1.50
4th 78 169 1.00
5th 93 202 0.84

If we assume that you have a 3.07 differential, then 4th gear at 3000 rpm would
be 72 mph ( 3000 / 60 / 3.07 x ((225 / 25.4 + 16) x 3.14 / 12 ) x 60 / 88 =
72 ).

Either your speedometer or tach is off.

For comparison:

US 635 Ratio Calculated mph at 6500 with 3.46 rear end

1st 3.83 36
2nd 2.20 63
3rd 1.40 98
4th 1.00 139
5th 0.81 171

Bill

----- Original Message -----
From: "Scott Andrews" <[email protected]>
To: "bcg members" <[email protected]>
Sent: Wednesday, June 16, 2004 4:50 PM
Subject: bcg: Those Lanky Autobahn gears


I have always marveled at the ability of my Euro to go fast..Not real quick
but real fast.. She can just kiss 40 in 1st at redline (6500). So On Nate's
question, and following Reverend Bill's admonition to "take data"..I went
out for a little spin..Here are the approximate speeds at 3K RPM (these are
with 225/50/16 wheels and tires).

1st 18
2nd 40
3rd 52
4th 78
5th 93

So, this car would theoretically redline at 172 mph..if it were not for that
pesky wind resistance...I have never had her over 125 though.

So yes, Nate.. She does seem to have some lanky autobahn back end.
S


On 6/16/04 3:38 PM, "Nathan Beachey (A)" <[email protected]> wrote:

> Um, no, that's 80mph in 5th turning 3K RPM. You sure you aren't talking
> about 80kph or do you have some lanky autobahn diff?
>
>> -----Original Message-----
>> From: Scott Andrews [SMTP:[email protected]]
>> Sent: Wednesday, June 16, 2004 4:55 PM
>> To: Nathan Beachey (A); bcg
>> Subject: Re: bcg: Re: what kind of diff?
>>
>> I assume 80 mph at 3K is in 4th... I can't even get my euro into 5th below
>> 85..85 is about 1700 RPM
>>
>> I suspect mine also has the LS. It does the same thing you mentioned. You
>> shift hard and it squats and sort of kinks to the side, and yes, the tail
>> out is instant in the rain..as in whoo hold on..crank the wheel over hard
>> to
>> get that rear end get back in line...
>>
>> Also the traction is pretty impressive. As I said, I have chirped the rear
>> on shifts, but you really have to work at it, and it is clearly not just a
>> single wheel.
>>
>> S
>>
>> On 6/16/04 2:34 PM, "Nathan Beachey (A)" <[email protected]> wrote:
>>
>>> I've never actually confirmed that I have a Limited Slip in my U.S. '85
>> by
>>> looking at the diff itself, but I'm GUESSING I have one by how it acts
>> in
>>> the rain.
>>>
>>> 1. Turning a corner in the rain with an excessive foot in the throttle,
>>> there is absolutely no delay between tire spin and tail out like I would
>>> expect if just the inside tire lit up and the outside tire slowly lost
>>> traction. This is true of "whoa, I didn't think I'd break a tire loose"
>>> slightly-overpowering throttle applications as well as the "Man, these
>> TRXs
>>> are fun when they're free" burn-'em-off-the-rim full-opposite-lock "yep,
>> I
>>> meant to do that" applications.
>>>
>>> 2. When travelling in a straight line, again on wet pavement, the rear
>> end
>>> steps to the side if I overpower the tires. This is different from the
>> old
>>> Volvo 744 Turbo I drove many years ago with an open diff that would sit
>> and
>>> spin that one rear tire for a day and a half on a wet road while the
>> other
>>> one kept the rear end in check because it still had traction and wasn't
>>> spinning.
>>>
>>> I think I got lucky and whoever the original owner of the car was
>> ordered
>>> the LS rear end option because it's also not a 3.73 or 4.10 or something
>> as
>>> evidenced by my recent gas mileage post and how I can still cruise at
>> 80mph
>>> spinning 3K RPMs.
>>>
>>> Nate.Six
>>>
>>>
>>>> -----Original Message-----
>>>> From: Estrada, Erwin F. [SMTP:[email protected]]
>>>> Sent: Wednesday, June 16, 2004 3:55 PM
>>>> To: Nathan Beachey (A); bcg members
>>>> Subject: RE: Re: what kind of diff?
>>>>
>>>>
>>>> The LSD tag on the back of the diff will have an "S" designation.
>>>> Otherwise, odds are that your diff is a standard "open"-type. And the
>>>> peel-out test shouldn't be as dramatic as that. Pour some water in
>>>> front of the rear wheel and slip the clutch a bit. If you leave two
>>>> stripes, you have an LSD. If you leave one stripe, it's an open
>> diff...
>>>>
>>>> -Erwin
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Erwin F. Estrada
>>>> Operations Administrator
>>>> Sonnenschein Nath & Rosenthal LLP
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: Nathan Beachey (A) [mailto:[email protected]]
>>>>
>>>> Sent: Wednesday, June 16, 2004 1:26 PM
>>>> To: bcg members
>>>> Subject: bcg: Re: what kind of diff?
>>>>
>>>>
>>>> There really is no difference between Posi-trac and Limited Slip...
>>>> Posi-trac was just someone's brand name for a limited slip diffy.
>>>>
>>>>
>>>> If your wandering, that's all I got. Your going to have to wonder
>>>> around
>>>> until someone who knows how to decypher the tag and/or stamp on the
>> diff
>>>> responds as far as ratio and weather or not its a limited slip.
>>>> Whatever it
>>>> is, being a BMW part, it probably is not a complete POS.
>>>>
>>>> That hurt to type. And now I'm sure my spell/grammar check will be
>>>> blowed
>>>> up.
>>>>
>>>> Quick way to check if it's a limited slip... find a parking lot, come
>> to
>>>> a
>>>> complete stop, rev engine to 4500 RPM, sidestep the clutch pedal and
>>>> floor
>>>> the gas pedal at the same time, then go back and count the stripes.
>>>> Hopefully it's stripes you're counting and not parts.
>>>>
>>>> Nate.Six
>>>>
>>>> P.S. I really don't mean to hack on anyone, I just have fun with the
>>>> language sometimes. I think I inherited it from my College English
>>>> Professor grandfather.
>>>>
>>>>> -----Original Message-----
>>>>> From: Eric Pearl [SMTP:[email protected]]
>>>>> Sent: Wednesday, June 16, 2004 2:44 PM
>>>>> To: bcg members
>>>>> Subject: bcg: what kind of diff?
>>>>>
>>>>
>>>>> I got around to going through my car and compiling a
>>>>> list of things that need to be fixed. WOW! Im really
>>>>> in for it...
>>>>>
>>>>
>>>>> Now I'm wandering what kind of diff I have. With the
>>>>> rear end off the ground, when you spin one tire the
>>>>> other one goes the opposite direction, when you spin
>>>>> the drive shaft, both tire rotate the same direction.
>>>>> I'm thinking this is bad because I was hoping it had a
>>>>> POS or LS, and what is the difference between
>>>>> positrack and limited slip any how? Can someone please
>>>>> fill me in on the diff scene when it comes to the 6
>>>>> series? The sticker on the car says 633csi.
>>>>
>>>>>
>>>>
>>>>> This really bothers me becuase if it's not a locking
>>>>> diff it may be financially unfeasable for me to get
>>>>> the car where I want it w/o spending too much money.
>>>>> Why have a lot of HP and only one tire spinning?!?
>>>>
>>>>>
>>>>
>>>>> Thanks
>>>>>
>>>>
>>>>> -ep
>>>>>
>>>>
>>>>> 84 633 csi w 3.5ltr

[KC Ron Carter]

I have seen 165 mph in Oklahoma, Sunday morning and the road was 130 miles of four lane straight as an arrow.

Yep my tow mule was still pulling, not hard, but still increasing when I ran out of nerve.

The telephone poles show up like lines on the pavement at 165 mph.

The pavement marks are a sold line.

With a JC chip and a 3.73 to one I would suggest 180 as possible.

No snow plow, front spoiler just the early Euro fairing, yep it slows them down.

Later,

[Quendil]

thats one thing I need to get a LSD

[jrcalvin]

Open or LSD? Maybe it is easier on the drivetrain to just jack up the rear of the car - both wheels off the ground. Turn the left wheel forward by hand and see what the right wheel does. If it spins opposite to the wheel you are turning by hand it is an open diff. If the opposite wheel turns in the same direction as the one you are turning by hand it is a LSD diff. Easier than crawling under the car trying to find the S (in white paint or imprinted) on the right side of the diff.

john

[ScottAndrews]

I second that. What posesses automotive engineers to use tags to ID a critical feature of a car? On my 65 Mustang the Diff and trans are also supposed to be identifiable by tags.. Long gone of course. You would think they could just stamp it into a clear panel using metal stamps...it would sure help out those of us decades hence.

On this note.. Having restored several old cars, I am now very sensitive to issues that will not age well. On the new BMW 645, for example, the spare tie well is filled with a large styrofoam block that holds, you guessed it, a secondary battery and a fuse/distribution box. Apparently they ran out of room, and so, rather than moerate thier appetite for electronic gizmos that will bedvil owners and service technicians for years, they simply dumped the spare, put on run-flat tires and use the space for this cheesy, wont-last-10-years electrical travesty. I can see us in 30 years trying to restore one of these and having to custom mold a filler for the spare tiure well to hold the aging fuse box..

S

[indychris]

are you driving a 635 or an M? If its a US 635, your math is way off, no way it can hit 180 with a 3.73 rear end. 5th gear is 0.81, with a 3.73 rear end and a 7000 rpm redline you get 172mph on 25" tall tires. Thats assuming you have enough horsepower at 7k rpm to push the car thru all that wind, which you don't no matter what chip you put in a 3.5 liter m30. 6000 rpm (the likely hp peak for the 635) plus the 3.73 and 25" tires equals about 148 mph. . . again, I doubt it since wind and tires would be fighting you.

Fact of the matter is that the BMW racing 635s used anywhere from a 3.07 to a 3.91 rear end, with a close ratio gear box (1.00 5th gear) and the factory said that 155mph was the fastest speed they could push with the 3.07 rear gear (the 3.91 would result in better acceleration but much lower top speed). The reason? Horsepower. . . the racing BMW put out 285 horsepower at 6000rpm. . . single overhead cam M30 just can't breathe deeply enough to crank out the super high rpm horsepower that the 4 valve per cylinder M engines can. But even then, the aerodynamics become a limiting factor in top speed. It is seriously hard to push that car thru that wind.

I'd say a top speed of 140 or so is about as good as it gets. 150-155 may be possible with a tall rear gear and PLENTY of horsepower (285 hp like the race car. . . try driving that on the street).

Chris

[indychris]

For US 6 series (auto transmissions may be different, but not that I know of):

US M6 (1987-1989) 3.91 diff
US 630 (1977) 3.45
US 633 (78-82) 3.45
US 633 (83-84) 3.25
US 635 (85-87) 3.46
US 635 (88-89) 3.64


Euro M635 reportedly had a 3.73 but I can't confirm. Many Euro 635's were geared taller for Autobahn, likely using 3.07 or 3.25, maybe even higher gearing than that, depending on type of transmission. I don't have a good source on Euro cars though, so I can't say for sure.

Chris

[Quendil]

How stupid am I! I checked the brochure I have for my year of 635 to see what the LSD ratio would be if fitted and it states that all manual models have a 25% LSD. So i did the wheel check on mine and both wheel turn in the same direction so I already have a LSD!

[Quendil]

On another note, I was planning on putting a LSD from an E12 on my E9 3.0CSA I'm restoring. I know it will fit ok but I have been told it will make the acceleration very slow. Any comments?

Cheers
Dave

[Brucey]

my early 87 euro owner's manual says;

628 CSi 3.46:1
635 CSi 3.07:1
635 CSi (cat) 3.45:1
M635CSi 3.73:1

My 635 has the 3.07:1. Odometer is geared for 640mm diameter wheels. Speedo is 10% optimistic.

cheers

[cornershop]

Digging up and old thread...

Is this a 3.42 LSD I removed from my 88MY highline? I replaced it with a 3.46 unit, can't really tell much difference in day to day running

[hornhospital]

You mean because it has a 42 cast into the ring? No. IIRC that same number is on most (all?) the differential retainer rings I've seen.

[cornershop]

Thanks Ken, yes because I saw the 3 and 42.

Are there any other markings I should look out for?

The 1207768 doesn't return anything on google

[cecotto479]

There is or should be a tag attached to one of the cover plate bolts.

It should say "S" or "Z" and then numbers that will be the ratio. The "s" or "z" denotes limited slip diff.

I think the open diff ratios for Highline cars are the same, so you'd not expect to feel much difference unless you're really pressing on in corners. You'll have replaced like for like in terms of ratio and therefore final drive. It'll feel,pretty much the same in day to day driving.

To be fair, I may have assumed that it was a 3.46, because it was an LSD and because it came out of a Highline. If the tag isn't there, I'd not have take the cover off to count the teeth, but I'm pretty certain it's a 3.46.

[cornershop]

Thanks Boyd - the above is the one I removed.

I don't see a tag on any bolt but will take another look

[cecotto479]

I edited my post after I saw the one above was the one you removed. As I say, I think you'll have swapped like for like in terms of ratio and will only notice a difference when pressing on in corners or if you partake of the current drifting craze.

[cornershop]

Thanks Boyd

I'm assuming the above is an LSD based on the options list and no records of it being swapped out previously.

Do the markings confirm this?

[ron]

I'm assuming the above is an LSD

To check if it is an LSD rotate one of the output flanges and if the other flange rotates the same way you have an LSD. On an open diff. they rotate in different directions.

[cecotto479]

That's how I check them generally too. Spin one wheel. If they both rotate the same way, it's an LSD if they counter rotate it's open. Sometimes it doesn't work so well lying on the ground. I think the extra weight and resistance of driveshafts and wheels helps. In that case, drain it, whip the back cover off and look out for the clutches.

I wouldn't rely solely on the options list unless the car has a huge uninterrupted service history.