How to ID 8-3/4?

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Captainkirk

Captainkirk

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I've got an 8-3/4 diff in my Duster 340 4-speed, always assumed it's a 3.91 Sure grip, but how can I positively ID it? Thanks in advance!
 
If you are lucky, the original metal tag might still be attached to one of the housing studs. Otherwise, pulling out the pumpkin to look at the numbers on the ring gear.

If you want to ball park it just count the amount of times the driveshaft spins during one revolution of the tire. Or vice- versa. I forget.

To test for sure grip- raise only one rear tire off the ground. With car in neutral the raised tire should not spin or at least be very difficult to turn.
 
Open diff
block on wheel, rotate opposite tire two turns ( :1) and then how many times the drive shaft turned ( x.xx:)

Sure Grip/Spool
both tires off the ground and one revolution of the tire, count driveshaft rotations
 
Rear end is out of the car.
I did the whole rotation thing, but that's sort of a hand-grenade toss....close but no cigar thing. How do you measure ".91" accurately?
 
If it's already out of the car, just pull the center section out of the housing. The ratio will be stamped on the ring gear.
 
OK, determined the pumpkin out of The Duke is a Sure Grip 3.55. Pretty sure the one out of the long-departed Li'l Red Minx is a 3.91....it's very close to 4:1 rotation. So next question: Which would you prefer to run with 340 and A833 4-speed?
 
What is the cam size and tire size? Then I'll feel you my answer.
 
3.91 = in town/track fun, 2800rpm at 55
3.55 = good all around choice for a driver, 2200rpm at 55

Just ball park figures with no overdrive.
 
I have a 340 with similar specs, single plane manifold, pretty big cam, and a 600DP carb. I also have an A833 4-speed and 3.55 gears. The motor will like the 3.91s a lot better (I am in the process of putting in an 8-3/4" so I can change gear sets) but you will like the 3.55's a lot more on the highway.

-Mike
 
Captainkirk said:
OK, determined the pumpkin out of The Duke is a Sure Grip 3.55. Pretty sure the one out of the long-departed Li'l Red Minx is a 3.91....it's very close to 4:1 rotation. So next question: Which would you prefer to run with 340 and A833 4-speed?
Click to expand...

If you rotate the driveshaft close to 4.1 times for 1 tire rotation you have a 4:10 gearset, and even though you cannot exactly measure the driveshaft rotations you can accurately determine what the gearset is without pulling the pumpkin because there are only so many ratios that it will fit in to - 3:55, 3:73, 3:91, 4:10 etc. You can make an educated guess at which one it is.
 
Captainkirk said:
Tires=14
Cam, haven't chosen yet but likely will be close to .475 lift 290 duration
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Is that cam spec duration @ .050 or advertised and by whom is it made by. Because advertised duration specs differ greatly between manufacturers.

Your wheels are 14 inch. Now what about your tires? Wheel size is useless! Tire size is everything else!

73SwingerBuild said:
I think he was looking for outside tire diameter, not what size wheels you have. A 28" tall tire versus a 26" tall tire will make quite a difference.
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BINGO! Tires are measured in two ways. The metric way. Like this; P225/14/70 or the old way of letters. F70.
 
rumblefish360 said:
Is that cam spec duration @ .050 or advertised and by whom is it made by. Because advertised duration specs differ greatly between manufacturers.
Click to expand...

Just a generalization at this point. Have not chosen make or exact grind yet

rumblefish360 said:
Your wheels are 14 inch. Now what about your tires? Wheel size is useless! Tire size is everything else!
Click to expand...

Haven't picked out tires either. Old ones are 30+ years old-not gonna run those
 
If I may hit the chime –in bell…. Keep the type of exhaust system you choose in the equation. I had a 340 Duster (Ruster) with a 3.55, R 50’s & F 60’s. A good gear for the e-way, just don’t pick the wrong exhaust….. The drone will drive you and/or your passengers insane in about an hour.
My 2cts.
P.S. You guys rock!
 
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