Positraction / differential issue
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
Positraction / differential issue
The last time I was racing my car I noticed, and my friend who was watching confirmed, that during my stationary water burnout both tires spin normally. When I release the line lock and continue spinning the tires to come toward the starting line, sometimes the left tire will stop spinning while the right tire continues to spin till I let off the gas. After that. the car always hooks when I launch, with no traction problems there. So the posi unit is working normally at that point.
This is with drag radials on and 60 ft times in the 1.72 to 1.74 range. On the street, on street tires, the car will always spin both tires continuously - not that I would ever do that. No strange noises or unusual operation.
The rear was rebuilt professionally about 10 years ago and has about 240 runs and 10,000 miles on it. I have changed the oil twice and used GM additive during that time. (Its due for an oil change now.) Any ideas about the one wheel spinning issue? Is the posi unit just worn out?
This is with drag radials on and 60 ft times in the 1.72 to 1.74 range. On the street, on street tires, the car will always spin both tires continuously - not that I would ever do that. No strange noises or unusual operation.
The rear was rebuilt professionally about 10 years ago and has about 240 runs and 10,000 miles on it. I have changed the oil twice and used GM additive during that time. (Its due for an oil change now.) Any ideas about the one wheel spinning issue? Is the posi unit just worn out?
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
That would be my guess as well, something to do with the clutch pack is worn, maybe when it gets hot enough it slips. Im Guessing there is a line lock and it’s not working against the brakes.
could be it’s not so bad that a fluid change will make a difference.
could be it’s not so bad that a fluid change will make a difference.
Last edited by REELAV8R; Oct 27, 2019 at 08:10 PM.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
Thanks guys. I'm thinking of just trying the fluid change. Its been 6 years since I last changed it. The trouble is I won't be able to test it on the track till next season and if I need something more serious I'd rather do that over the winter.
Reelev8r - I am using a line lock for the burnouts.
Reelev8r - I am using a line lock for the burnouts.
Is my vette stock?? HAHA
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
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When you race again mark the tire with chalk at the valve stem to see if it's the tire problem
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
I have the stock C3 slotted wheels so its easy to see if the wheel is turning or not. I know its possible, but probably not the answer here.
Gary - Bairs did the rebuild. Really just a stock rebuild with a change to 3.73 gears. It has held up well.
Gary - Bairs did the rebuild. Really just a stock rebuild with a change to 3.73 gears. It has held up well.
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Tech Contributor
Joined: Aug 1999
Posts: 15,173
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From: Connecticut, USA
WIth the amount of runs you have on it and age I bet you need to go over the posi again. You also need to check it for cracks and make sure solid steels are used- no fiber coated or snowflakes. In your application I would run ARP RG bolts, Steel cap, solid sleeve, and a good polished and tuned posi. That will hold up to some abuse, if you are at any swap meets look for 30 spline spiders and you can move up to Tom's axles if you want. The good spiders are long gone but I bet there are guys out there like me who stocked up on them so who knows maybe you'll find a nice set.
Tech Contributor
Joined: Aug 1999
Posts: 15,173
Likes: 3,982
From: Connecticut, USA
PS - I was thinking of holding a tech seminar late Spring 2020 on polishing and tuning posi's here in CT. If you are interested in coming up it might be just in time for you to build the diff yourself.
Disabled by request 8 JUN 2025
Joined: May 2019
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From: St. Augustine, FL
Setting Up A Posi
When I set up the 12 bolt posi for my 10.8 second SBC Camaro, I shimmed it up so tight that I had to put a slight chamfer on the pinion shaft to get it in. In doing so, it compressed the clutch packs even tighter. It was as close to being a spool as you could get out of a posi.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
Well thanks for all the suggestions, Gary particularly. I'm probably going to hold off till Spring before I pull the diff out. I'll just change the oil over the winter. I might race it once in the Spring, see what happens and attend Gary's tech seminar if I'm lucky. Its fine as is for the limited driving I do with it.
Tech Contributor
Joined: Aug 1999
Posts: 15,173
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From: Connecticut, USA
Ok if you or anyone else wants to see how to really polish and tune a posi, let me know. It is a free deal and you won't get this experience anywhere else. Just email me directly and I will see how the calendar looks around the end of May.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
After mulling this over for a while, now I'm considering buying a new Eaton posi unit. Then I think I will only have to install carrier bearings and bolt the ring gear on and set the backlash - which I have done in the past. Jegs sells them for $594, free shipping. I don't really know if these are at least as strong as the factory posi unit, which I've been using to this point. Any thoughts from you guys?
Tech Contributor
Joined: Aug 1999
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From: Connecticut, USA
They come with huge springs, fiber coated clutches, less per side, powder metal spiders, cast in China, assembled in Mexico. The last one I took apart, new out of the box had excessive endplay in the side gears once the springs were removed.
I recall having a conversation with Tom about them 10+ years ago and he would use them only for the case, he never used those spiders or clutches. Will it work, yes- for your use I don't think it would be my first option. I would rebuild yours over that any day of the week, using the 10-17 spiders and new solid steels, polished and tuned.
The last one I installed per someones request was in a 69 427-390 M20 308 that was street driven less then 500 miles a year.
Remember the old saying, " Fool me once.........
Kind of like all the guys that over paid for what they were told was a real super-10 diff by a known con man but that's a different story being played out in the transaction forum.
I recall having a conversation with Tom about them 10+ years ago and he would use them only for the case, he never used those spiders or clutches. Will it work, yes- for your use I don't think it would be my first option. I would rebuild yours over that any day of the week, using the 10-17 spiders and new solid steels, polished and tuned.
The last one I installed per someones request was in a 69 427-390 M20 308 that was street driven less then 500 miles a year.
Remember the old saying, " Fool me once.........
Kind of like all the guys that over paid for what they were told was a real super-10 diff by a known con man but that's a different story being played out in the transaction forum.
Pro
Joined: Mar 2014
Posts: 529
Likes: 113
From: Texas
There are parts suppliers out there that have 30 spline axles, spiders and steel clutch packs. One of them is Vansteel. When you decide to go through the differential check the case and posi unit for cracks and/or other damage. If everything looks good, add the upgrades and blueprint the assembly. There is also some better bearing caps out there that you may want to invest in just to finish the whole thing off. If you decide to change your gear ratio you should think about having the gears coated. ( Mikronite Technologies, Micro Blue, REM Coatings)
Last edited by Gunfighter13; Oct 31, 2019 at 04:42 PM.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
OK thanks Gary and Gunfighter. I guess I'll go the rebuilding route. The 3.73 ratio I have now is perfect for my setup. I will probably pull the diff out in a couple weeks and come up with a plan.
Last edited by BKbroiler; Oct 31, 2019 at 07:15 PM.
Tech Contributor
Joined: Aug 1999
Posts: 15,173
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From: Connecticut, USA
You can contact me with any questions when you get it out and I will tell you what to look for.
NOTE. 30 spline axles are either Tom's or Drag vettes regardless of where they are purchased. There are some differences to them. The 30 spline spiders required for them, the good ones, are long gone now. Tom had the best 10-17 x 30 HT spiders and US Gear stopped making them about 7-8 years ago. FInding them is hard and expensive, using the new powder metal or new DANA made in India are not the same quality. The posi case will need to be bored to use them as well. Same for a steel cap. They are available from Tom's, Mark Williams, and others and all have to be machine fit. If you have access to a machine shop they can also make a cap but it will cost more in the long run.
NOTE. 30 spline axles are either Tom's or Drag vettes regardless of where they are purchased. There are some differences to them. The 30 spline spiders required for them, the good ones, are long gone now. Tom had the best 10-17 x 30 HT spiders and US Gear stopped making them about 7-8 years ago. FInding them is hard and expensive, using the new powder metal or new DANA made in India are not the same quality. The posi case will need to be bored to use them as well. Same for a steel cap. They are available from Tom's, Mark Williams, and others and all have to be machine fit. If you have access to a machine shop they can also make a cap but it will cost more in the long run.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Having been to Garys shop and talked to Gary himself I would go with his advice. He is spot on C3 diff rebuilds. If you want it to last either have him do it or do it yourself with his advice. He is not that far away from you and a visit to him is well worth the drive.
Thread Starter
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
Thanks for your input. I do intend to have Gary involved in this process. I'm moving kind of slow on it now but I will be contacting him to help make some decisions.