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Old 08-12-2018, 12:38 PM   #1
smncutler
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Default Which Amazon diff do I want?

Hey there. My 122S has the M40 and 4.10 gears. I should go crawl under the car to see if it's a Dana diff or what but before I do that I wanna know my options. The engine is all midrange torque and gets ~13 MPG, and it wants to cruise at around 50. What's a slightly taller gear I can swap in in lieu of an overdrive?
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Old 08-12-2018, 01:07 PM   #2
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Wat

13? Something is wrong
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Old 08-12-2018, 01:20 PM   #3
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Yeah it's tuned a little rich. I also have the big Isky cam, velocity stacks and I've been reefin' on it as hard as I can. All in the name of cleaning out my gas tank.
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Old 08-12-2018, 02:44 PM   #4
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Randys should have Dana 30 gear sets for cheap.

{Real world cheap not TB ¢h3@p™}
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how psi stock cna support?

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Old 08-12-2018, 03:37 PM   #5
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Excellent. Thanks!
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Old 08-12-2018, 04:16 PM   #6
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Measure the diameter of the axle tube to determine which rear end you have. 2¼" = Dana 27, 2½" = Dana 30. There are other types of diffs used but those are the most common. Knowing the year of your car might help narrow it down but diffs get swapped over the years so it's best to measure.

There's also a 27 or 30 cast into the housing but it's probably covered in a ½" thick layer of crud.

Agreed that 13 mpg is awful. What cam and carbs are you running?
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Old 08-12-2018, 06:58 PM   #7
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I have the iPd Isky cam and rebuilt/rebushed HS6's with velocity stacks. AC fuel pump.

I have 2.5 inch axle tubes. Excellent
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Old 08-13-2018, 12:29 AM   #8
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Does your rear end have a rear facing inspection cover?
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Old 08-13-2018, 12:36 AM   #9
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With a 4.10 and M40 the Amazon is pretty well balanced. If you go to a taller gear you will curse the (lack of) power off the line.
An M41 or a swap to a 5 speed is really the answer.
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Old 08-13-2018, 03:37 AM   #10
smncutler
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Yeah, it's a dana 30, same cover as my 240. I'm making a long drive soon and I'd like to avoid turning 3000 rpm at 65 for 300 miles if I can avoid it. Bad idea to find a 3.90 gearset? I know the overdrive would be better.
I love being able to wind out 2nd thru 4th in my 240. My 122 makes me row thru the gears pretty rapidly.
My 240 has a 3.31 diff and a 4.03 first gear.
my 122 has a 4.10 diff and a 3.13 first gear.
The remaining gear ratios are fairly similar.
If you multiply the above figures (because the diff underdrives the wheels relative to driveshaft) you get final-drive-to-engine-speed first gear ratios of 13.34 for the 240 and 12.83 for the 122. At 3k RPM they each go about 16.5 MPH in first.
The 14" and 17" wheels with tires are within 3% of the same height so I'll ignore that. I used some online calculators.

240 in 4th at 3000 is 65.2 MPH, bitchin' sweet spot right where the cam wakes up, but still good on fuel.
122S in 4th at 3000 is 52.6 MPH. 3750 RPM would maintain 65.8 MPH (in a scientific vacuum). Call it 3800.

With 3.90 diff gears I would have a first gear 3k RPM speed of 17.7 MPH and a 4th gear 3k RPM speed of 55.38. Not a huge difference but it would help, and 1st is presently a little short anyway.

Next option down would be 3.73.
1st gear 3k RPM 18.5 MPH
4th gear 3k RPM 58 MPH
Would that low of a 1st suck? Probably yeah I'd have to get up to 15 MPH using the little torque between 1000-2800 RPM.

Last edited by smncutler; 08-13-2018 at 05:01 AM..
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Old 08-13-2018, 03:51 AM   #11
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Quote:
Originally Posted by smncutler View Post
Yeah, it's a dana 30, same cover as my 240. I'm making a long drive soon and I'd like to avoid turning 3000 rpm at 65 for 300 miles if I can avoid it. Bad idea
Ehhh doesn’t bother me
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