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What differential is this?

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All right, stamp says 11 and 40 on top, and below this is a 1 over 7. So if I'm interpreting the coding correctly this means I have a 3.64 ratio built in January '77. Right?

 

I'm neither excited nor displeased to know this isn't a 3.91. I've got that in my '76 with a 5 speed and it's great, but this car has a 4 speed which I don't have any immediate plans to replace so the 3.64 will be nicer on the highway.

 

 

IMG_3472.jpg

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(edited)
49 minutes ago, otto said:

All right, stamp says 11 and 40 on top, and below this is a 1 over 7. So if I'm interpreting the coding correctly this means I have a 3.64 ratio built in January '77. Right?

 

I'm neither excited nor displeased to know this isn't a 3.91. I've got that in my '76 with a 5 speed and it's great, but this car has a 4 speed which I don't have any immediate plans to replace so the 3.64 will be nicer on the highway.

 

 

IMG_3472.jpg

But what this does mean, is you have a set of half shafts and inboard stub flanges that will work if you swap to an LSD without needing redrilling for larger diameter bolts

Edited by Guest_anonymous
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(edited)
17 hours ago, otto said:

All right, stamp says 11 and 40 on top, and below this is a 1 over 7. So if I'm interpreting the coding correctly this means I have a 3.64 ratio built in January '77. Right?

 

I'm neither excited nor displeased to know this isn't a 3.91. I've got that in my '76 with a 5 speed and it's great, but this car has a 4 speed which I don't have any immediate plans to replace so the 3.64 will be nicer on the highway.

 

 

IMG_3472.jpg

 

That’s right: it’s a 3.64 manufactured in January 1977!

 

And, yes, I agree: with a four-speed, the 3.64 is certainly a better highway ratio. The original 3.90 from my ‘76, for instance, is currently resting up while a 3.64 LSD serves a tour of duty in the (four-speed) car. 😉

 

By the by, the 3.90 and 3.91 diffs represent different gear sets. The 3.90 is largely an oddity of the U.S. 1976 2002 market, intended to recover some acceleration lost over the years to emissions controls, and has a 39/10 ring/pinion combination. I’ve not yet seen a factory LSD example of this ratio. The 3.91, on the other hand, is an e21 differential, a common mate to an overdrive five-speed, and has a 43/11 ring/pinion combination. LSD versions of the 3.91 are relatively common.

 

Regards,

 

Steve

 

Edited by Conserv

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8 hours ago, Conserv said:

The 3.91, on the other hand, is an e21 differential, a common mate to an overdrive five-speed, and has a 43/11 ring/pinion combination.

 

My thought is the 11 tooth pinion offers more tooth contact with the ring gear than the 10 tooth pinion in the 3:90 diff and maybe a stronger set up as far a tooth loading is concerned.

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2 hours ago, jimk said:

 

My thought is the 11 tooth pinion offers more tooth contact with the ring gear than the 10 tooth pinion in the 3:90 diff and maybe a stronger set up as far a tooth loading is concerned.

 

Thank you, Jim. I never got past “the two gear sets are different”! 😉

 

Best regards,

 

Steve

 

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The tooth contact patch is going to be roughly the same regardless of the number of teeth. Remember that the ring and pinion are numbered and matched to each other for a smooth mesh. Usually the last 4 digits of the ring gear # is scribed on the pinion, either on the gear end or the bearing mount area. 

 

A pinion gear with fewer teeth will by definition be stronger because each tooth is larger. Hence, 39/10 would be stronger than 43/11.  Check out the Metric mechanic torque ratings on diffs and see how they decline as the ratio rises. Hence a 4.27 ratio in any diff has less torque capacity than a lower gear such as a 2.93. 

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20 hours ago, RussTii said:

Check out the Metric mechanic torque ratings on diffs and see how they decline as the ratio rises. Hence a 4.27 ratio in any diff has less torque capacity than a lower gear such as a 2.93.

Not convinced that it's the pinion/ring gear that determines those torque limits and that one can't surmise from those tables that the pinions are or are not stronger for either end of the table ratios.

As I read the tables, it's the input torque limit and the input torque is multiplied to the output side by the speed reduction ratio.  There may be other elements of the diff that set the limits, like the spider gears that seem to break.

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