posi rear - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 12-14-2015, 02:41 PM Thread Starter
 
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posi rear

Hi

Yesterday I made some burnouts and I can't explain why sometimes only one wheel is turning and when I try it again, both wheels turn.
It's a 66 GTO with a 3.55 posi rear, 4 speed manual and a line lock to engage only the front brakes.

Is my differential worn out or can you explain that?
I wonder why a worn clutch could work sometimes, and sometimes it won't?! I have to add.. the road wasn't perfeclty even.. maybe weight distribution has something to do with that effect? I'm not sure but maybe it matters how "fast" I start.. if I give it a lot of rpm from the start it seems to work better (turn both wheels instead of only one).



this is my rear end.. with a 3.55 posi.. is there a "test" other than the burnout to test if it works like supposed?


Chris
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post #2 of 9 (permalink) Old 12-16-2015, 07:56 PM
 
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Your posi question has questions

Is this the Pontiac Safe-T-Track? Do you know whats in it. Is it an Eaton or Auburn setup. They use different set-ups to accomplish the same effect. Could be the clutches, but also could be the springs. I have no problem with GM units, but for ease of maintenance, I went with a Moser 9 inch Ford. I would imagine if you open it up and inspect it, if it is the clutch ass., you'll be able to smell the (for a lack of better words) burning of the clutch pack. You can look up an article on the web. What's the Diff?/Hemmings Motor News, Dec., 2008. It has a lot of infomation that might help you.
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post #3 of 9 (permalink) Old 12-25-2015, 09:15 PM
 
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I'll be watching this thread, but for now we're stuck with the ole "peg leg" burnouts...

https://youtu.be/VrJ6LEM9xOo
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post #4 of 9 (permalink) Old 12-26-2015, 05:49 PM Thread Starter
 
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I'll have to open it and check for wear.. not sure what is behind the cover, probably a safe-t-track cone clutch posi. I read some articles about cone repairs and maybe I'll find someone who can do it if necessary I already got some 80w90 and GM posi fluid..

This video was shot on the same day, when occasionally only one wheel was turning:
https://www.youtube.com/watch?v=QNbPznxKoQQ

I'll have to add, the road wasn't even.. maybe it locked "better" when I found a more even spot.
The last few days while I was driving it produced 2 black stripes on the road... but I didn't do "standing" burnouts like in the video.
It seems to lock much better if the car is in motion.. but still I guess the posi might need some work or at least new oil and the posi additive.
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post #5 of 9 (permalink) Old 12-29-2015, 09:41 PM
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Real men don't use the brake to do burn outs. j/k
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post #6 of 9 (permalink) Old 12-30-2015, 08:53 AM Thread Starter
 
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How should I do a burnout without moving the car if I don't use the front brakes?
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post #7 of 9 (permalink) Old 12-30-2015, 09:13 AM
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Check out my burn out videos in my sig.
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post #8 of 9 (permalink) Old 12-31-2015, 03:32 PM Thread Starter
 
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like that? https://www.youtube.com/watch?v=r-CW5e_TmbQ
it's fun as well.. or to make that while driving in 2nd gear.. funny thing is that it seems always to lock the rear if I do that.. don't know why

on the other side it's sometimes more fun if you cover your girlfriend in white smoke while she is shooting the video
(and I can't do that if I don't lock the front brakes)
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post #9 of 9 (permalink) Old 01-03-2016, 09:59 AM
 
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It's not a spool or Detroit Locker, it's a limited slip differential. We'll assume that it's full of engine torque and has enough power to make a habit of spending tire money for little return, but the bottom line is that you're making enough multiplied TQ to put the differential to work. If you have a positraction (limited slip) with 1 tire on snow or ice, apply light throttle pressure, you'll simply drive away. Stab it to the floor and you'll burn right through the differential action with no effort, spinning the tire on the ice as if it were and open diff in a regular car. Let's add another "twist" as well, that is the twisting motion of the axle when power is applied. Looking at the car from the rear, the axle has a natural reaction when TQ is applied heavily to rotate counter-clockwise. In simple terms, it wants to lift the right rear tire. So many go through their car life thinking the left rear tire is the "traction side", when in an open rear diff it's whatever side gets enough weight to do the work. You can make an open diff spin both wheels in some conditions by how you place the car. Hang the rear to the left a bit as you line up, not much, just a bit. When you stab the gas the car shifts extra weight to the right rear, this in turn counters the TQ reaction of raising the rt rear tire.

Short version? Your "posi" is most likely working just fine in normal operating conditions. G.M. didn't design those to work on drag strips with excess traction conditions, nor to be able to paint black lines in urban pavement. They're a traction aid giving an advantage to both rear tires in unequal conditions.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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