lower numerical differentials???

Gary Knox

Gary Knox

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I would like to have a lower differential for the '74 3.0 CS I bought that has the 265/6 transmission and a 3.64 or 3.45 numerical differential. With the high output 3.8+ liter FI engine built in '93 by Metric Mechanics (they called it 4 liter), I have LOTS of torque, and would like lower rpm cruising speeds.

Are there differentials from later cars (e24, etc.) that would be plug and play, or are there lower numerical ring gear and pinions that could fairly easily replace the ones I have? I would like something around 3.0 to 1 or possibly even lower.

THANKS in advance for any information/guidance.
 
I installed a LSD 3.07 from a e23/24 which is a side loader.
Install was straight forward.

With the 5sp upgrade it significantly reduces rpm at highway cruise.
Will cause a 10 mph difference increase in speed to speedometer reading.

Also increases mpg if anyone cares.

I have a stock 3.0 motor and no issues with torque.
 
I would recommend comparing your current diff ratio and RPMs in overdrive with what you expect.

I have a 3.45 limited slip and 3000 RPM is about 78 MPH, that's a pretty relaxed cruising speed. (The original 3.64 is 5% shorter and RPM would probably be 3150 for that same speed)

If I were going from a 3.45 to a 3.07, the difference is 9% so I would expect RPMs to drop to around 2700. Hope this helps.
 
Steve and Chris,

THANKS for your input. I was aware of the speedo change, and actually mine is about 12% fast now!! The mpg, of course is nice, but insignificant vs the cost of the diff change!! The primary reason is engine wear and just the relaxed atmosphere of a lower engine speed/sound. Drag strip performance is not an issue. I have a car with 575 ft lbs of torque for that!

So - a ring gear and pinion for an e-28 (ooops my error, should have been e24) would install OK, or a complete diff from an e-28 (same - e24) would be plug and play - do I have it correct?

Also, is there a marking on the diff that indicates ratios? IF not, just lock one rear wheel, then rotate the drive shaft one (or 10) full turn(s) and determine the revolutions of the other wheel - is THAT correct? Conversely, rotate the one wheel one full turn and check the drive shaft rotation? (OBVIOUSLY, I'm not a very qualified mechanic!!!!).
 
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Ratio is marked on passenger side of diff, 38/11 is a 3.45, an S indicates limited slip. Speedo and odometer can be off different amounts, mine is. Also, I thought it had to be an early e23/e24 diff, not e28.
 
Diffs for coupes don't come from e28 cars.
Ring gears come in different sizes and have either 8 or 10 bolts not interchangeable
Be very careful before spending your money.
 

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HB Chris said:
... Speedo and odometer can be off different amounts, mine is..
Click to expand...

Interesting, I thought they have to be off by the same ratio for example percentage wise at least for the trip odometer.
The main odometer has memory so the error depends on much much you have driven since changing the ratio. The speedo has no memory so it will be off by the same ratio on every measurement.

I did check the ratio before the 3.64 was put in by manually rotating the driveshaft side until a mark I made on one of the axle sides made a full turn. This was on the E24 and because the speedo pickup is at the differential the speedo always works fine.
 
My speedo is 10% slow and odometer is 5% low. Since I have a 3.45 and speedo is for a 3.64, 5% on each would result in the speedo only being off 5% and odometer being accurate, that may be the error factor as delivered perhaps?
 
Gary the e28 is NOT what you want. You need a "side loader" differential for your e9.
I purchased mine off of ebay for $150 5 years ago. No leaks.

If my old memory serves me it was a e23/euro.

Installed it with new diff mnt. An easy install.

The stamped numbers on the diff flange gives you the ratios(see sfdon's post)
 
You need the 40:13 diff for a 3.07. Needs to be a sideloader from an E24 or E12 M535i. Interestingly, I've got two E24 635s in my backyard with these diffs, but unfortunately, they're open, not limited slip.
 
Only turbo m102 automatics and 265/5 cr tranny's would have used 3.07
The math on a 3.64 to 3.07 is about 20%- that's a huge move.
Why not try the csl diff? 3.25
 
3.07 was also the stock ratio for the 10:1 M30B34s in e24s and E28s regardless of overdrive or close ratio transmission. I had an E28 M535i with that combination and really with a motor like Gary is describing, I don't think he'd have an issue with it. Also, I regularly returned over 25 mpg on the highway in that car which I absolutely did not even touch in my M535 with the close ratio. I do agree that it is a big jump, though.
 
That is good info- I killed an '83 635 euro this year and was surprised to see a 3.07 in it.
 
Much like how they went from a 3.73 to a 3.91 from M635/Euro M5 to the US, they did the 3.07 to 3.25 because our cars were down on power compared to the Euros and we'd likely never notice the cars running out of top end with our 55 mph speed limit. :lol:

Of course, none of that is really applicable to coupes unless someone's putting an E28 rear subframe in and whatnot.
 
The one that was wacky to me was the e28 535is.......
5 speed overdrive, 180 hp and a 3.25 diff.
Have to downshift on the freeway to get going.
 
Gary Knox said:
Also, is there a marking on the diff that indicates ratios? IF not, just lock one rear wheel, then rotate the drive shaft one (or 10) full turn(s) and determine the revolutions of the other wheel - is THAT correct? Conversely, rotate the one wheel one full turn and check the drive shaft rotation? (OBVIOUSLY, I'm not a very qualified mechanic!!!!).
Click to expand...

The differential will rotate the axles 720° total for every turn times the ratio. If you have a LSD where both axles rotate equally, for every 3.xx turns of the yoke each axle will rotate 360°, or 720° total. If you have an open differential and lock one axle, for every 3.xx turns of the yoke the free axle will turn 720°, or two full turns.
 
Thank you all for your comments and advice.

Christopher seems to have what I'm looking for, and I'd far prefer to deal with another e9 fan than a wrecking yard or eBay. So, I'll wait until Christopher has less 'heat' in the Valley of the Sun and can determine the condition of his differentials.

Thanks again for all your great input.
 
I just swapped a Getrag 265/5 into my 3.0 CS. At a minimum, I will want to swap in an lsd. What gearing should I be looking for?
 
The close ratio (265/5) came with a 3.07. 5th gear is just like the 4 speed at 1:1 but with a 3.45 or something, the first two years are useless, so not much better than a 4-speed. I do know someone that ran a 3.46 with that transmission in an E28 but it wasn't for long and he was coming from a 3.73 with a 260/6 5-speed (similar ratios to the 265/6).
 
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