Ok how do you do it if you cannt find the axle code or no RPO code. Like if I take the wheels off and turn one side the other moves the oppisite way it is standar or if it moves the same way is it posi, am I think right here?

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87 Z28 305 TPI, Auto, 2.77, Gutted air boxes, K & N fillters, Dyno Max 3" cat back, Accel 8.8 mm spirals, Accel cap and rotor, Accel coil, rapid fires, and 160 Stat.
Best 15.414 @ 87.47, 3.23 and SLP airfoil on the way!

 

well i 've read all kinds of things about turning the wheel and watching the other. i know for a fact that doesn't always work. best 100% sure method is to pull the cover and look.

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ICON Motorsports
1st & 3rd

 

if it's still in the car do a burn out and see, but that only works if the posi is working.

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ICON Motorsports
1st & 3rd

 

You should have a small tag hanging from one of the diff cover bolts. The tag signifies a limited slip unit. Doing a burnout will work too unless it's very high mileage and/or was abused which could have worn out the clutches in the diff.

 

Before I installed the powertrax, I could do a 2 wheel burnout about 1/3 of the time. Pull the cover to be sure. Next best method is put in park and lift up the back end. An open rear end will allow one of the back tires to spin easy while the other spins opposite. I would pull the cover, chances are it's past time to change the fluid anyway.


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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

82camaro i've heard that method over the years and have saw several posi rear NOT do it like you said that worked just fine. not sure why it happens but it does.

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ICON Motorsports
1st & 3rd

 

ede, if these posi units you've seen do not function like 82camaro mentioned, then they were either shot or not at all posi. You see, because the function of the posi is to limit rpms to (tranfer torque from) the tire with less traction by using torque distribution (I'm not going to go into specifically how a posi works because there are different methods, ie: clutch, gear, etc ...). So if the posi were to allow one tire to spin in the opposite direction, it (theoritically and mechanically) would not be able to limit rpms to (tranfer torque from) the tire with less traction. The reason why an open allows such behavior is because the spider gears have no "pre-load" set on them to "lock" them together. Using this example should explain why a shot posi will act like an open. Mind you, on a level road, a shot posi will still spin both tires easily (provided you have enough TQ to spin the tires in the first place). But I have spun both my tires with my open rear also.

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George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB
Chevy Info --> MadMike's

 

george, yeah they were posi, i held the carries in my hands. they left 2 long black streaks every time you did a burn out. they had clutches just like every other posi unit. no idea why they did that but they did. i'd put very little faith in watching which way the tires spin while turning them by hand. maybe i had the only 2 to ever do it but i know what i saw and i know what they were.

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ICON Motorsports
1st & 3rd

 

I seen that before ede. If I remember correctly, in that type of posi the clutches tighten as power is applied from the driveshaft. At rest (without power from the shaft) the clutches release and the wheels can spin free. I think some shade tree mech. told me this, so I can't say it's accurate. Like is said above, pull the cover to be sure. My open 3.08 open used to burn both tires fairly often, so don't trust that "it left 2 tire marks" method.



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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

ede, I'm not doubting what you witnessed, but from what 82camaro just said, I have to change my opinion. I remember some old trucks (class 1 and 2) my father used to service that had that type of LSD. Too bad I was too young and nieve to pay attention and understand how/why it functioned like that. So yes, you're right. But I don't think GM used this type of LSD in 10-bolts (especially 3rd gens)?

[This message has been edited by MRZ28HO (edited December 24, 2000).]

 

i've been thinking about this all afternoon. i don't remember now if it acted like "they" say an open rear would or if it didn't act like a posi rear would. i know it puzzled me because i knew it was in fact a posi. the one didn't have oil in it so i figure that may account for the way it acted, but the other i had saw had a posi carrier that left 2 black mark burn outs every time. there is always the chance it worked but not 100% correctly and i never saw or knew the problem, but it's still in the car and working fine.

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ICON Motorsports
1st & 3rd

 

I run a stock 85 3.42 posi unit and the wheels turned opposit directions when we installed it, I asked my mechanic about it and he said dont worry she is going to cook both of em! He was right. Before I was running an open 3.42 and I could mess around with the weight transfer to make it work like a posi. I beleive if you have even weight on both of the rear tires is when it works.

 

The wheels on my '85 also turn in opposoite directions when jacked up, just like an open diff would. However, it's an Eaton Locker (or was until it broke) and it works on the principle of engaging the clutches when one wheel spins 100 rpm faster than the other, or something like that. GM Positraction works by letting the clutches slip when you turn.

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1985 Yellow IROC-Z 5.0 HO, T-5 3.73 Posi, 4 wheel disks
PFCM pads, GTA wheels, Koni yellows, B&M Ripper, Dynomax

 

one of mine was an eaton the other was an alburn. this always confused me when i saw it happen and i knew my posi was working. that's why i said the best method is to pull the cover and look.

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ICON Motorsports
1st & 3rd

 

Odd, because my 9-bolt (kimosabi what kinda diff it is) and my '94 Z28's 10-bolt (heard they were auburns?) spin the tires in the same direction when off the ground. Only my OEM '84 Z28's 10-bolt spin opposite, and it's open. But yes, opening the cover is the ONLY way to find out if it is a posi and what kinda gears.