| Re: [GMCnet] ~ Final Drive ~ [message #225160] |
Thu, 10 October 2013 22:54  |
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hnielsen2
Messages: 1434
Registered: February 2004
Location: Alpine CA
Karma:
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Senior Member |
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We run 4:70 and never have had a tire spin problem
No posi.
Unless I stand on it.
We returned today from Coos Bay returning the long way back on US 395 eastern Sierra.
Snow north of Bishop on the passes over 6,000.
Never a tire slipping/spin problem.
Howard
Alpine Ca
74 Canyon Lands
455 not quite stock running 3:70 final drive.
All is well with my Lord
> On Oct 10, 2013, at 19:30, Bob Heller <rheller@cfl.rr.com> wrote:
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>
>
> I've heard some people say the 3.21 has 'posi' characteristics but having had one apart to replace bearings and now running it on the GMC I can't see how it's any different from any other open diff. There is nothing in the 3.21 to couple the two outputs in any way. A LSD has something, like clutches, to allow some additional torque to be applied to the 'stuck' wheel instead of letting the spinning wheel determine the max amount of power available. The 3.21 has nothing like that, it's an open diff that just happens to use a planetery gear set instead of the more traditional spider gears like everything else. From what I can see, the 3.21's only 'advantage' is that it's narrower but in our application there is plenty of room so the 'narrowness' doesn't matter.
>
> On my GMC the 3.21 spins the tires easier than the 3.07, probably due to the gear ratio multiplying the torque a bit more. It really sucked when I was at Speed World and, where with the 3.07 I got good consistent launches with minimal wheelspin, now with the 3.21 the tires go up in smoke far too easily. With the 3.21, flooring the pedal with the gear lever in '1' results in a tire spinning, the speedometer at 44mph, and the engine making some noises I don't feel comfortable with (it's the original engine and I don't like to float the valves on it). Even in normal driving when it's wet out the tires (actually one tire) spin(s) with less pedal than before. Since I don't tow and mostly drive in Florida I'm ready to put the 3.07 back in.
>
> SO, tell me again, how is the 3.21 the better diff, other than the ratio?
> --
> Bob Heller
> 1974 X-Canyonlands 26ft
> Winter Springs FL
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