I have a 99 Z28 and I am getting 410 gears installed with a short throw. I just got a call from the mechanic saying that I do not have posi. Can this really be true???? I thought all Z28's had posi. It is also a 6 speed with the 342 gears.

 

It did when it was new.

 

Jack up the car from the rear, turn the wheel with your hand (brake off). If the other wheel turns in the same direction you have LSD, if it turns in the opposite direction its an open diff.

 

Here's an even easier way. Floor it/Dump the clutch. If you see 2 burnout marks it's a posi. All Z's/SS's/WS6's/Trans Ams/Firehawks should have a posi with the 3.42 gears. It could be worn out if you only see one burnout mark.

 

that is 100% untrue. I had this big arguement with the dealers when i had my rear replaced 2 times. If u put my TA up on a lift and spin the wheels, THEY GO IN OPPOSITE DIRECTIONS. BUT, If u spin the dryshaft both wheels spin the same way. And according to my burnouts i am leaving 2 marks. I 2 thought the same way Mudge, but it is untrue in my car and maybe in others. Just sharing my experiences.

 

this is a common problem, many people dont understand the inner workings of the rear ends in these cars. let me explain.

You have a 99 Z28. this means you have a torsen differential. a torsen is not a limited slip differential. traditionaly limited slip diffs are refered to as posi's, as Posi-traction was the corporate name Chevy gave to them and it just became the standard practice to call them all that. (ford called em trac-lock, and pontiac and chrysler also had different names for them that I cant think of right now)

a limited slip and a torsen work in completely different ways. and this confuses lots of people with torsens, making them think they have an open diff.

In a limited slip differential, there is some type of friction surface that locks the two axles together, but allows them to slip around corners.

the torsen however, uses gears instead of friction surfaces to bias the torque to both axles. it acts as an open differential most of the time. in order for it to "lock up" you MUST have BOTH axles loaded sufficiently. this is the reason that a lot of people complain they have one wheel peel, and think the rear end is messed up. it also means that if you jack up the rear of the car the tires will not turn in the same direction, but they will act like an open diff. this is almost definetly what your mechanic is seeing, and not being experienced with torsens, he thinks its an open diff.

as for it being worn out, torsens dotn wear out like limited slips do, as there are no clutches to wear. thats one of the bennefits of them, unless you break it, it will bias torque the same for its entire service life.

hope that helps

 

My 99 Z28 has one of these torsen diffs too. It acts like an open diff in the air, but always leaves 2 tire marks of equal length .

For a more in depth description I went to the FAQ at torsen.com. Lotsa info including an 8 page paper about torsen diffs.

 

Wonder what the torque rating is like on your posi then, because my open diff does exactly that.

Torsen gear drive/worm gear unit is an ATB, auto-torque biasing diff, other details above The mechanic was probably looking for a clutch pack.

 

Ya know ... if the mechanic couldn't figure that out, I would look for someone else to do your gear swap.

 

Exactly

 

on my 1999 z28 the option code is "G80-Locking differential". Is that a torsen or limited slip? Never checked it while up on hoist but do leave "posi" burn outs.

 

All 99 up Z28s got torsens. the SS's could get a limited slip auburn as an option

 

Boy I sure wish this were true. Ask anybody that tries to drag race a Torsen a lot. They absolutely will quit working. I haven't taken mine apart yet but I think that the mating surfaces must galled up or something so that it won't posi any more. 1 wheel peel in the burnout box with wet tires isn't cool. It worked good for about 18,000 miles and near 250 passes. Now I use an Eaton HD clutch type posi.

 

My mechanic did figure it out once he actually looked at it. He explained to me about the worm gears and showed me and said that I have a torsen differential. I should be getting it back any time now. I cant wait!!! He said that it wont even feel like the same car.

 

I just talked to Reider racign yesterday to order my centersection, we talked about the similarities and differences between the torsen and the truetrac (which works the same way) according to them the torsens have a tendency to chip the worm gears. I dont count that as "wearing out" thats simply breaking in my book.