Bad posi? What do you think.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 246
Likes: 12
From: middletown, de
Bad posi? What do you think.
I've done quite of bit of work to may 70 and its where i feel comfortable driving it fast now. I checked the tires before i did any highway trips and saw that the tires are about 15 years old so i want to replace them before i hit any type of high speed or long distance driving. I think we all know the best way to get rid of very old tires 
Here's the question. After an extensive burn out i went to admire my work and it was not what i was expecting. The passenger side tire made a nice long 50ft tire mark, as expected. The driver side had about a 6 foot tire mark but then just stopped. So the driver side tire spun for 6ish feet and the pass tire spun for 50 ft. Does this mean the posi is bad/going bad? If its bad, i'd expect just 1 tire spinning. Also the driver rear also has a weird squeak when i hit the brakes now despite me not using the brake during the burn out. I just stopped then matted the gas. I'll check the squeak next week but pulling the rear cover is more involved, as we know. I did just replace the rear end fluid and diff lube about 100 miles ago so that should be all good. Thoughts?
Here's the question. After an extensive burn out i went to admire my work and it was not what i was expecting. The passenger side tire made a nice long 50ft tire mark, as expected. The driver side had about a 6 foot tire mark but then just stopped. So the driver side tire spun for 6ish feet and the pass tire spun for 50 ft. Does this mean the posi is bad/going bad? If its bad, i'd expect just 1 tire spinning. Also the driver rear also has a weird squeak when i hit the brakes now despite me not using the brake during the burn out. I just stopped then matted the gas. I'll check the squeak next week but pulling the rear cover is more involved, as we know. I did just replace the rear end fluid and diff lube about 100 miles ago so that should be all good. Thoughts?
Drifting
Joined: Oct 2020
Posts: 1,567
Likes: 576
From: Wisconsin
I’m not sure I’m the posi question, but I can say for sure that you are playing with fire doing burnouts on 15 year old tires.
Ever see what a blowout can do to a fiberglass car body like a Vette? It ain’t pretty.
Ever see what a blowout can do to a fiberglass car body like a Vette? It ain’t pretty.
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
Was the diff ever worked on? If so what was done to it, what type of clutches used and was the case in good shape?
If it hasn't been touched your clutches are worn and slipping, possibly the posi case is cracked as cross shaft play will click before it cracks implodes. Changing the oil was just good practice as it won't fix this. If you are going to push the car hard you need to build everything South of the Flywheel to application or you will continue to find the weak links. Original 70 posi cases are not that good, have weaker spiders, single OD case, but have(had) good clutches. You can always do a break away test on the posi if you like but like the oil change it won't make much difference.
As mentioned, 15-year-old tires are a time bomb just driving the car.
If it hasn't been touched your clutches are worn and slipping, possibly the posi case is cracked as cross shaft play will click before it cracks implodes. Changing the oil was just good practice as it won't fix this. If you are going to push the car hard you need to build everything South of the Flywheel to application or you will continue to find the weak links. Original 70 posi cases are not that good, have weaker spiders, single OD case, but have(had) good clutches. You can always do a break away test on the posi if you like but like the oil change it won't make much difference.
As mentioned, 15-year-old tires are a time bomb just driving the car.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 246
Likes: 12
From: middletown, de
Was the diff ever worked on? If so what was done to it, what type of clutches used and was the case in good shape?
If it hasn't been touched your clutches are worn and slipping, possibly the posi case is cracked as cross shaft play will click before it cracks implodes. Changing the oil was just good practice as it won't fix this. If you are going to push the car hard you need to build everything South of the Flywheel to application or you will continue to find the weak links. Original 70 posi cases are not that good, have weaker spiders, single OD case, but have(had) good clutches. You can always do a break away test on the posi if you like but like the oil change it won't make much difference.
As mentioned, 15-year-old tires are a time bomb just driving the car.
If it hasn't been touched your clutches are worn and slipping, possibly the posi case is cracked as cross shaft play will click before it cracks implodes. Changing the oil was just good practice as it won't fix this. If you are going to push the car hard you need to build everything South of the Flywheel to application or you will continue to find the weak links. Original 70 posi cases are not that good, have weaker spiders, single OD case, but have(had) good clutches. You can always do a break away test on the posi if you like but like the oil change it won't make much difference.
As mentioned, 15-year-old tires are a time bomb just driving the car.
I know the tires are trash and other than the traction test i haven't driven it any farther than a few miles at a time. I'll be replacing them soon.
Burning Brakes
Joined: Oct 2002
Posts: 1,040
Likes: 109
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
Every vette differential from 1963-1982 had one or more issues. Revisions were constant. Posi cases were revised 3 times during the run. Some held up great, many did not. Power, traction, trans, an use will determine how long they last. The 72-79 and current Chinese Eaton cases are the best. The Chinese cases are machined better than original but the USA made casting from 72-79 and service case sold until discontinued were better. Clutches had 4 revisions, the current fiber coated ones are junk and the old snowflakes that broke with abuse are still sold and used by some rebuilders today.
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
It is shown the GM manual, basically you hold one wheel and using a fixture on the other and a TW see how much force is needed to slip the wheel. This only applies to spring loaded posi's not tuned ones.
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