Diff ratio

MartinClan · #1 10th January 2011, 12:45

Anyone have any idea about what the diff ratio from an 87 320 is likely to be? There is a guy near to me with a low mileage one for sale but he doesn't know the ratio...

Cheers

Robin

cabrioman · #2 11th January 2011, 07:36

from my E30 Bentley Manual:-

"Final drives are identified by a code that gives drive ratio and
manufacturing number. The code is stamped on a metal tag
attached to the final drive. See Fig. 1 -2. The code on the tag
depends on whether the final drive is the original one installed
in the car or whether it is a replacement final drive.
For original final drives, the code gives the drive ratio first
and then the manufacturing number. So code 3.45 4002 identifies
an original equipment final drive with a ratio of 3.45: 1 .
Limited-slip new final drives are identified with the prefix S (S
3.45 4002) .

For replacement final drives, the code also has the drive
ratio and manufacturing number. However, the ratio is given
indirectly, by the number of teeth on the ring and pinion. For
example, 1 1 38 A 405 is an exchange final drive with a ratio of
3.45 : 1 (38 ring teeth divided by 1 1 pinion teeth) . The A is an
additional indicator that it is a replacement final d rive. Limitedslip
replacement final drives are identified by the suffix S ( 1 1 38
A 405 S)"

Don't know if that helps

John

MartinClan · #3 11th January 2011, 11:55

Thanks John. I have the Bently manual but I didn't know that info was in there..

The numbers on the diff, according to the bloke selling it, are 445 8462 which I think makes it a 4.45.

My slightly noisy diff is 3.79 and with the large wheels and tyres I have makes the car rather long legged. So the 4.45 is going to be idea.

Robin

peterux · #4 11th January 2011, 19:36

Hi Robin,
I just dug out may old Haynes manual (the 0815 one for pre- '97 3 series.)
It says 325i cars were fitted with 3.64:1 diff's and most 320i's had 3.45:1, with the 3.64 as an option.

I suspect mine is a 3.45:1 ratio being a 320i donor but I never could read the tag due to rust.

Interestingly, I have the opposite problem to you with my 15" wheels and the lowish profile 50 tyres that came with the wheels. I find 1st gear too high and a feely of the need to change up a gear again once I am cruising in 5th.
I've been thinking of getting higher profile tyres or 16" wheels, but can never make up my mind which ones to get.

Not sure if that helps?

...peter

MartinClan · #5 12th January 2011, 07:35

Peter

I think the Haynes manual is probably wrong. My 325 donor had a 3.79 diff. I find this a bit long legged. The diff I am looking at is a 4.45 - although now I suspect this might be the other way having done the calculation.

With the tyre size I have I reccon that 4.1 is probably the best ratio.

Robin

PS - can you remember what is the standard tyre size for an E30? Was it 195/60/14? It would be interesting to do a comparison.

Sorton · #6 12th January 2011, 13:26

Robin

The standard LS diff fitted to my 325 Sport is a 3.91 with the S prefix.

The standard tyres on 15" cross spokes are 205/55/15 and I believe these were optional on the more basic 325's which were fitted with 195/65/14. These both have an almost identical rolling radius.

Sorton

peterux · #7 12th January 2011, 17:27

I've got the BMW owners handbook that came with my donor and I think that lists the tyre options.

I''ll dig it out and see what it says...

So now we have 3.45, 3.64, 3.79. 3.91 and 4.45 as options, I have no idea what I have in my car

And for once realoem.com doesn't seem to help?

...peter

GreatOldOne · #8 12th January 2011, 21:32

Yep my donor had 3.64 LSD, and it had the 15" BBS wheels (205/55/15)

MartinClan · #9 13th January 2011, 07:30

Quote:

Originally Posted by peterux

So now we have 3.45, 3.64, 3.79. 3.91 and 4.45 as options, I have no idea what I have in my car

...peter

And 4.1 for the 320 manual (according to Unix Nerd's web site)

Robin

MartinClan · #10 13th January 2011, 07:47

For my car with a 3.79 diff.....

Original wheels
===========

195/65/14 = 23.98 dia or 75.345 circumference = 840 turns per mile.
so at 60 mph in top gear (0.8) that gives 2546 rpm

Fitted wheels
==========
205/55/16 = 24.88 dia or 78.173 circumference = 810 turns per mile.
so at 60 mph in top gear (0.8) that gives 2455 rpm

Actually not a lot of difference but considering the use that the car gets (more A and B roads than motorway cruising) perhaps a bit low

with 4.1 diff
60 mph in top gear (0.8) that gives 2656 rpm

with 4.45 diff
60 mph in top gear (0.8) that gives 2884 rpm

So I reccon 4.1 is probably about perfect.

Robin

cabrioman · #11 13th January 2011, 08:50

There is an excellent excel utility for calculating this stuff available for download at http://www.gearboxman.co.uk/inforatios.html.

It allows you to tweak the various parameters and see the results accross all gears etc..

Very useful if you havn't already got it.

John

peterux · #12 15th January 2011, 08:42

It seems like almost any ratio was fitted?

If you try google e30 diff ratios you'll get any answer you want

Wikipedia has this table...(not that I'm saying that anything on Wiki is fact!!)

Differential Gear Ratios and Types:
model(s) ratio(s) case size type(s)
316 before 9/84 3.64 small open
316 after 9/84 3.91 small open
316i M10 3.91, 4.10 small open
316i M40 4.27, 4.45 small open
318i M10 3.64, 3.91, 4.10 small open
318i M40 2- and 4-doors 4.10, 4.45 small open
318i M40 convertible and touring 4.27, 4.45 small open
318is 4.10 small open (USA Spec had an optional 4.10 small case LSD)
320i before 9/85 3.46 small open
320i 9/85 to 9/87 3.64, 3.91 small open
320i after 9/87 4.10 small open
320i convertible and touring 4.27, 4.45 small open
323i before 9/84 3.23 medium open
323i after 9/84 3.46 medium open
324d 3.45 small open
324td 3.25 medium open
325i before 9/86 3.46, 3.73, 3.91, 4.10 medium standard - open
sports - mechanical limited-slip
325i after 9/86 3.64, 3.73, 3.91, 4.10 medium standard - open
sports - mechanical limited-slip
325i convertible before 9/86 3.46, 3.91 medium standard - open
sports - mechanical limited-slip
325i convertible after 9/86 3.64, 3.91, 4.10 (USA SPEC) medium standard - open
sports - mechanical limited-slip
325i touring 3.91 medium open
325ix 2- and 4-doors 3.64, 3.73, 3.91, 4.10(US SPEC AUTO) medium open, all viscous, 10-100% limited-slip
325ix Touring 3.91, 4.10 medium open, all viscous, 10-100% limited-slip
325, 325e US-models 2.93, 3.23 medium standard - open
sports - mechanical limited-slip
325e before 9/85 2.79 medium standard - open
sports - mechanical limited-slip
325e 9/85 to 12/86 2.93 medium standard - open
sports - mechanical limited-slip|-
325e after 12/86 3.25, 3.46 medium standard - open
sports - mechanical limited-slip
M3 (North America) 4.10 medium mechanical limited-slip
M3 (Europe), Evolution , Evolution II 3.25 medium mechanical limited-slip
M3 Convertible 3.25 medium mechanical limited-slip
M3 Sport Evolution 3.15 medium mechanical limited-slip

It's easier to read on the website...
http://en.wikipedia.org/wiki/BMW_3_S...30)#Drivetrain

Now I really do have no idea what is fitted in mine

And now I think about it, I don't know what gearbox I have either.

I also found this article...which lists even more options.

http://www.e30eta.com/ecdiff.htm

good luck Robin....

MartinClan · #13 17th January 2011, 10:26

While we are on the subject I always wondered what was the actual difference between the "small" and "medium" cases that is often mentioned? I guess that one is bigger than the other (lol) but are there any other differences? For example are they 100% interchangeable or are the drive flanges/mounts any different?

Cheers

Robin