Coupe ratios ?

J

John Buchtenkirch

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I’m just wondering :confused: what are the rear end ratios are in our coupes with standard shift and also automatic ? On an American car you can spin 1 axle, if the other spins in the same direction it’s a posi, if it spins in the opposite direction it’s a non-posi rear. I would assume this works the same on our coupes ? Thank you :). ~ John Buchtenkirch
 
Auto Diff.

John,

3.45 was the common for CS. 3.25 for CSI. No distiction for autos. The 2.5 CS had 3.63 IIRC.

The lock up is the same principal. Although,there are variations on the percent of lockup, with the most common 45%.
 
limited slips...

Curious if anyone knows for sure but it would seem that all CSA cars have limited slips?
 
I think all N.A. CS automatics got the 3.64 rear end, maybe 4 speeds too? Rest of the world got 3.25 or 3.45 as stated. Don't have any data with me to check though.
 
As I understand it, most coupes used a 3.45, with the same ratio also being found in the LSD units. The later side-loader cars (E12, early E24) used a wide variety of ratios, down to a 3.07 on the close-ratio E24s. The E9 can use any of these diffs, but the problems arise due to speedo correction problems.
 
Gearbox stamped with 10:41 ---> ratio 4,1 = coefficient W=0.854 on speedo

Gearbox stamped with 11:43 ---> ratio 3,9 = coefficient W=0.8125 on speedo

Gearbox stamped with 11:40 ---> ratio 3,64 = coefficient W=0.758 on speedo

Gearbox stamped with 11:38 ---> ratio 3,45 = coefficient W=0.718 on speedo

Gearbox stamped with 12:39 ---> ratio 3,25 = coefficient W=0.677 on speedo

Gearbox stamped with 13:40 ---> ratio 3,07 = coefficient W=0.638 on speedo
 
according to the specs in the blue books - all usa coupes 2800 / 3.0 - manual or automatic trannies came with a 3.45:1

euro cars also came w/ a 3.45:1 ... except for CSi - which was 3.25:1 ... 2.5CS was a 3.64:1
 
LSD vs standard

Sorry for the hijack, but how do you tell an LSD from a non-LSD differential for the sideloaders?

Drew
 
rsporsche said:
according to the specs in the blue books - all usa coupes 2800 / 3.0 - manual or automatic trannies came with a 3.45:1

euro cars also came w/ a 3.45:1 ... except for CSi - which was 3.25:1 ... 2.5CS was a 3.64:1
Click to expand...
Blue books don't always have correct specs for US cars. In fact:
All US 2800CS/A had 3.45
All US 3.0CS/A had 3.64
I have about 7 sets of Blue Books for and over the years have found a number of omissions for US models. Kind of disappointing as the specifications are to me the most valuable part of the manuals.
 
good to know ... that's what makes these forums so fantastic. the wealth of knowledge in enthusiasts heads ... gets written down for others to learn from.

cheers
scott
 
Thanks Bill, was beginning to think I was crazy or something. I thought the US coupes were all 3.64, same ratio in the 2002. Mine now has a 3.45.

Chris
 
3.64 US diff

I'm with Bill.......Maxmillian/Mobile indicates they all had 3.45 through 10/1973, then 3.64 through 12/1974. But I believe all USA coupes and sedans went to 3.64 when the 3.0 landed here - late '71? They still sell the complete 3.45 w/o limited slip for $1200.
 
E3 and E9 automatics: Final drive stamped with 11:40 ---> ratio 3.64:1, W=0.758 on speedo

E3 and E9 4-speeds: stamped with 11:38 ---> ratio 3.45:1, W=0.718 on speedo

CSL: stamped with 12:39 ---> ratio 3.25:1, coefficient W=0.677 on speedo

Of course a car can be found with something different, especially today after ~40 years, but every stock as-delivered E3/E9 big 6 I've owned or worked on since "way back when" came with one of those three ratios, as listed above.

As already noted, the books are wrong.

Now then, anyone have a W=0.638 speedo for me? (took a 3.07 out of one of my euro 635s and want to get the speedo accurate in 2270011)
 
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US 2800 CS had 3.45 diffs and US 3.0CS had 3.64 diffs, regardless of transmission.

BMW had to certify emissions for engine/transmission combinations and the testing was expensive so they minimized the US choices.

The change was an attempt to maintain performance levels for later models when they went to lower compression ratios to enable running on regular gas which was part of the emission requirements for 1972.
 
Just to add to all the variations which make our beloved coupes so distinctive, the diff on my '75 CSi barnfind is 41/10 (!!). Why in the world would a Euro coupe have such a high ratio?
 
Why in the world would a Euro coupe have such a high ratio?

You mean low ratio (but I admit it´s confusing). The Euro CSi always had 3.25. With 200HP and high speed cruising on the European motorways in the early 70s, speed was more important than acceleration.

Why your car has a 4.10 I don´t know. Perhaps prepared for Alpine rallying at some stage?
 
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