HI-
I am going to look at a rear end out of an 1985 Camaro Z-28 HO that has supposedly had 3.73's and posi installed in it not too long ago. How can I tell this unit has posi? It is still in the car but it is not running.. I was told if I jacked the car up and put it in neutral and rotated the tires both should move the same way. Is this true? Or if i took off the axle cover should I be able to see anything that would give it away being a POSI and having 3.73's??


Thanks in advance for any help and information,

Dan

 

If you jack up the car, you can rotate the driveshaft by hand, both tires will spin in the same direction. If you just grab a tire, the other side should spin in the OPPOSITE direction.

 

If they move the same direction- posi. If they move in opposite directions- not a posi.

 

If it has posi, when you rotate one tire the other will turn the same direction, if it doesnt it will turn the other. If they turn the same direction you can figure approximate gear ratio by putting a mark on the drive shaft and having somone turn the wheels one turn and counting how many times the driveshaft turns. If it turns about 3.73 times it is .... 3.73. If it is not posi, you will have to have one person at each wheel turn them the same direction one full turn and count the turns on the driveshaft the same as before.


Ben

 

that is correct, in neutral they will turn the same direction if it is a limited slip. the best way to check is leave it in park and try to turn a wheel. the wheel should have alot of resistance, the more resistant the wheel is to turn, the stronger the clutches (or cones) are.

 

im confussed.... some people are saying that when you spin them and they move in the same direction, that it is posi....and some are saying if they go in the OPPOSITE direction,, then its posi....which one is right????.....cant you just smoke them and see if they both light up?? well that is if the car is running

 

If they move the same direction it is posi. The one guy that said otherwise probably just accidentally said the wrong thing.

Ben

 

just do a brake stand for a block or two. if you have 2 black marks, its a posi. if only 1 mark its not :-(

 

You guys are correct. I mis-quoted the posi, I was thinking limited-slip. Sorry for the confusion.

 

Limited slip is posi. Open differentials (one leggers) are the ones that spin the oposite direction.

Ben

 

WHAT?????...now im lost again.. please explain

 

Yup, that's quickest way I know. Of course, if you just leave 1 mark you may have a blown posi/limited slip.

And yes, limited slip IS Positraction. Positraction is GM's trade name for limited slip. Chrysler called theirs "SureGrip" and Ford has their own name (which I can't remember). But they are all "Limited Slip"....which means they only allow the other wheel to slip a limited amount (clever how that works out).

A "one wheel peg leg" is an "open" or "unlimited slip" because one of the wheels can have "unlimited" slip.

 

Posi and limited slip are two very different things, ive had both, the limited slip only turns one wheel as your driving down the road, but if the one wheel starts slipping then the power is partially transfered to the other wheel thus giving more traction.

Where as with a POSI unit, both wheels turn all the time, resulting in greater traction, but lower gas mileage because your turning 2 tires not 1

 

once again another damn amature typing in an answer when he obviously has no clue what the hell he's talking about. posi (shortened from the term positraction) is gm's name for it's limited slip differential. it originally used the name positraction in chevrolet's, but the name became popular enough that it allowed it in others. olds, buick, cadilac, etc. all had different names for their version of limited slip differential. since the name became so popular, pretty much anything spinning two tires became a "posi". the name is and always will be gm's with only one company besides them allowed to use it and that being eaton. eaton is the company that made the carrier's back when it was given that name. the newer camaro's as an example use the torsen differential which is why the positraction name isn't use for it's advertising anymore. even in the 80's when the auburn and borg waner were used, it was advertised as a limited slip. without getting into this way passed what some may understand, it's simple. "posi" is the name chevrolet used for it's limited slip.

 

WOW I've never seen so many people have different variations of how POSI works. Please someone just describe it correctly! But I'm afraid that won't happen.

 

If its posi, the car will have RPO G80.

 

i dont appreciate being called a "damn amerature" i was just trying to help unlike the rest of you, nobody seems to know whats right so they just go around calling everybody else names, and saying the same thing that was said before,

 

Whatever the case may be.... Posi = Limited Slip = Both Wheels will spin (assuming clutches aren't toast).. end of story.. end of thread.

 

OK, here's the deal. Zippy is right, he may have been rude, but right.

Ben

 

nice reply caleb, but if your going to try to help as your saying you were, then just speak on the stuff your sure of. calling you an amature isn't exactly calling you a name, obviously you are and you shouldn't take offense to that. you also stated no one seems to know what's right, well we already stated what was right and you tried to screw that up. i'm not being an *** for spite, but more for the point. a few weeks ago we discussed people learning from this site and i feel this is a small thing that can be learned, but when someone who hasn't learned yet puts out and answer that is incorrect it confuses the right answer until it has been proven again. it's like giving the wrong answer in school after someone has already given the right answer, the one's who know are going to correct you.

 

Females have "POSI's" and males don't.

 

i wouldnt think of anything else, i mean what do i gain be lying to you? and further more I apologize for confuseing you

Caleb

 

More food for thought,

The "spinning the tire" rule doesn't work for Zexel Torsen posi units. If I spin one tire, the other stays put.

 

Is there not some numbers stamped on the passenger side of the axle that tells your all about the axle? But if some one has gone into the diff. and changed things then I guess this would not work. The numbers are stamped on the axle housing that faces the front of the car.

 

Yes, but zippy and I are trying to tell people the truth. Positraction IS limited slip. Your information is dead wrong and we are trying to ensure people don't give out incorrect information.

In the GM shop manual, the description for RPO G80 (Positraction) is "Axle, Rear Locking Type (Limited Slip)". I suggest you do some research on the subject, but Positraction is JUST GM's Trade-Name for "Limited Slip".

Think about the term "Limited Slip". What does it mean? The answer is "it limits the amount of slip on the other wheel", which is what Positraction is. You MUST have SOME slip for turning corners, but the rear axle will only allow it to slip so much and that is why BOTH wheels will leave tire marks on the road when you boot it (provided you have enough power).

An open rear end (non-limited slip) will let the one spin while the other wheel does nothing. Hence, when you boot it, you leave only 1 tire mark on the road.

Now, what hasn't been discussed is GM has had many different types of Limited Slip/Positraction rear ends over the years. Some better than others. But regardless of the type of Limited Slip, GM has always called them Positraction (except as noted by zippy).

PS: I've been driving and mucking with cars for over 30 years (i'm going to be 47 yo shortly) and zippy is a qualified GM mechanic. I would drag Bernard (GMTech) into this to bring another qualified ASE GM Master Mechanic to confirm what we are saying. But unfortunately, its posts like this where people ARGUE over their "mis-information" that has caused Bernard to boycott www.thirdgen.org.

But the simple fact is, Positraction is just GM's tradename for Limited Slip, nothing more. The particular TYPE of Positraction used on 3rd Gen F-bodies is the Auburn cone type unit - which isn't a particularly good one IMO.

And GM's name for an "open rear-end" is "Standard".

 

Basically, I think everyone knew who was right as soon as caleb said you get "lower gas mileage because your turning 2 tires not 1". Obviously this makes no sense, or all wheel drive would cost quite a pretty penny at the pump...

zippy, although completely correct, probably could have used a little more tact and patience with the mistake and a little less abusiveness; but hey, we're only human.

And not to mention the fact but this post probably belongs in the Transmission section of the tech board.

 

If Zippy was a moderator, I would say that he should "bite his tongue a little bit", but he's not and free to say as he wishes IMO.

I can also sympathize with Zippy (and many other of the knowledgeable posters) who try to "set the record straight" and then have people refuse to believe what he says and argue with him.

There have been some very knowledgeable people that USE to post at www.thirdgen.org but found the same problem that Zippy is finding - people that really don't know the facts and yet, making statements that are factually incorrect. Zippy could have easily said "Look, I am a qualified ASE GM mechanic and I have worked on these things for years", but he did not.

Zippy and I have had our disagreements on certain aspects. But because it was an "opinion" on what is a better mod (i.e. not a black and white fact), our discussion stayed at the "debate level". A great example was almost two years ago when Zippy and I debated t-stats. At that time, I was "pro" 160* T-stat and Zippy was "pro" 195* T-stat.

Later, when I got deep into eprom burning, I saw some of the "problems" Zippy mentioned within the code of the eprom and I found that IF you run a non-stock 195* T-stat that it in deed does cause problems for the ECM.

I then did further tests and found that it really didn't matter WHICH t-stat you use (160, 180 or 195) - if you tune your eprom properly and change all the little "gotchas" inside the eprom. Based on my tests, I found from a "consistent performance" perspective that a 180* actually gives much more consistent performance AND I don't have to freeze my butt during winter.

Conversely, Zippy has done some testing of his own and now leans toward a 160* T-stat. So Zippy and I STILL disagree on which T-stat to use, except we now debate the opposite point of view. But this is a "subjective opinion" based on our own independent testing WITH appropriate changes to the eprom.

But on this "black and white" fact (Posi=Limited Slip), I'm with Zippy.

 

well import guy going to chip in also


I'm going to have to agree with zippy on this
LSD =posi
just thinking ab out the name on LSD makes me think that it is going ot limit the amount of slip that the rear is going to allow

open diffs give more power to the wheel with the least amount of traction right, never fully giving you the best grip? while a LSD gives it to the wheel with more traction and I think depending on the type of LSD (clutch, viscious, torsen) they can even fully lock up the wheels where some still bias it more twords one side or the other

wouldn't that be more of a locker unit? not something I understand you want to drive on the street

but the rest of the guys I think are right as far as telling if you have LSD or not by spinning one tire does the other one move in same direction if so you have LSD

now guys if I am wrong please let me know. these are only things stated as I undertand them

 

Not to mention that GM uses a small metal tab to identify posi on the differential cover. At least on all the cars I've owned they did.

 

Far be it for me to contradict anyone but I recall from my old MoToRs manuals, that the term "positive traction" was associated with Jeeps first, and was actually a locked rear where both axles drove at the same time, all the time.

Anyway, to answer one question above of "how does it work", in regards to LSD's, is in essence, both axles are coupled via metal plates (called clutches) or the "cone" (as in our cars) where pressure on the plates and cone assembley locks and forces both axles to drive at the same time, until there is sufficient torque to overcome the force of the cone/clutches contacting each other (like going around a corner).

This is easily understood if you can either rip a rear apart, or look at an exploded diagram...but what's really interesting in rears (except for the BW in our cars) is how the spider gears interact and apply force to one side or the other..and how they can have the rear act like a "posi" when there's equal resistance to both tire contact at the same time.

This would be a neat explanation for someone to type out (not me) but as you sit there and spin the rear and watch it (internally), you get an idea of how it works.



Funny how these topics that seem sipmle to answer turn out to be the longest threads on the board

 

www.howstuffworks.com

that can kinda give an idea howthey work

but also that is only one type of LSD
there are many different types
some are clutch only, then there are torsen diffs that bias the power through a mesh of gears I think. viscous where the fluid inside the diff causes friction and that helps all the plates spin and I am sure there are many other types out there also
but the clutch type I think is the most common.

 

Pukka is dead on about the torsen unit test. You can't CAN NOT turn one wheel by hand to see if you've got posi or not. You need to turn the driveshaft to be sure. Lots of people are going to the torsen units because for a daily driver they seem to be the most solid piece (don't have clutchs). Not to mention they're cheaper ($100 compared to $300+).
I like the test about the do a burn out, 1 wheel turns, open diff or burnt out posi. 2 wheels turn = more smoke and you know it's posi. About that stupid open diff gets better gas milage because only turning 1 wheel.... you had better jump off a cliff or take back that stupid statement.

 

well, since the torsen keeps becoming a point even though it's not something that came in the car in question i'll comment on it. not only have i worked with it, but i use it in my s10. the way to test the locking capability in the torsen is to have someone hold the wheel on one side while turning the other side with the trans in neutral. it should lock after about a 1/4 turn. the torsen when engaged is locked, although the version the f body is currently using has a low breakaway and will allow just one tire to spin under a slight uneven tire load. one very big difference with the torsen is using synthetic which is why they began using them. synthetic helps them engage easier and quicker when cold. the auburn unit on the other hand is not recomended to use synthetic (this information right from gm and auburn) since the synthetic is slippery enough to cause a glazing of the friction material they use.

 

dont think you have to worry about glazing the clutch on a torsen since from what it looks like they dont have any

and also a torsen doesn't fully lock so not quite as good for a drag race but great for road racing


how exactly does a torsen work though
I understand it is based on gearing but what all moves and how does it function

 

TNN just had a special on all of this today (saturday)...keep a eye out for it, they go throgh all of the differant types and explain it all... you might beable to find it on http://phrtv.com/

 

Ahem. Lockers. Ok, many people, when I tell them I bought a Detroit Locker for my car, say "OHHH, you don't want that. They are verry bad in the rain. NO NO, sell it and buy one of these slippy thinggies..." I call them "Locker haters" (or Ford Haters, one or the other) Anyway, I find that my locker is no problem what so ever on the street to drive. I think many will agree, whether posi or open or anything, if you screw around in the rain, and get punchy with the throttle, you will have trouble. My experience with road racing is similar. I can apply full power in long turns, and the locker happily clicks away while switching between powering both tires, then just the inner one while the outer over rotates, then back to both for a while... I would recommend one any day. Clutches can't be any good

Oh yea, the coolest way to show off, is when everyone is hanging around at the gas station, and you go to make a big u turn to get to the pump at full lock, and your locker is going klunk.klunk.klunk.klunk.... and everyone looks over. If they are stupid, they will say, "what's wrong with your car, sounds broken" if they are smart, they know you have something going on back there...

 

well i think it's time to put this thread to bed. we've all had soem fun here, found out the technical knowlege of some of the members here and found out the lack of technical knowledge here of some of the other members. all of us should try to remember we're guest here and this site does belong to someone else, so play nice. to me there is only one accpetable way to tell if you have posi or not, pull the cover and look.