Hi everyone, i just have a simple question, were all rearends put in formula firebirds posi units? i ask because i own a formula and am not sure as to the origin of my rear end. If i go to brake torque it, it will only peel the right wheel, but on loose or slippery surfaces it feels like it peels em both (it fishtails) Thanks, Derek

 

No they were not all posi.

I'd guess, with the low-ball TBI motor, your car's odds of having come from the factory with a posi, are somewhere between 1 in 1000 and 1 in 1,000,000. Which of course doesn't say anything about what's in it now; no matter which way it came.

Only way to know for sure is to take off the cover and look. All else (especially the "two stripe" method over the Internet) is a waste of time and produces nothing of value.

 

thanks for the info, i probably will take the cover off ne way just to re do the gasket and possibly put a new pinion seal in it along with new axle shaft bearings. As for a posi check, until i get around to redoing the gaskets and stuff i think i will put the rear end in the air n spin the wheels with my hands, and if they rotate in the same direction its gotta be a posi, and that my unit is just wore out.

 

That won't prove anything; won't even make a very good guess.

Best thing to do is to forget about it until you have the cover off to change the fluid (which is no doubt overdue... am I wrong?)

 

ya it is overdue, my old man owned it bfor me n he nvr changed it, car has 103k on it, which is y i want to do my lil wheel spin test. it will prove the axle to b a posi or open, bc on open cars if i spin one wheel clock wise the other will spin counter clock wise. but on posi cars turing one wheel clockwise turns the other clock wise, ive done this on a few other cars, which is why i want to do it to mine. After the test, if i find it to b a posi unit, i will change the fluid and put some fresh stuff in along with fresh friction modifier. -does the cover have to come off to just change the fluid?

 

The "test" is a waste of time. Don't bother.

Your interpretation of the behavior of a posi is exactly BACKWARDS. A posi such as would be in these cars from the factory consists of a clutch system that connects the 2 axles together; and requires some significant amount of torque to cause them to do anything but turn TOGETHER. That torque would be anywhere from 35 ft-lbs to 70 ft-lbs for the various ones used in these cars. If you have the car in park or in gear if it's a stick shift (drive shaft locked still) and you turn one wheel one way oand the other wheel turns the other way, that is positive proof that it's NOT a posi.

Some other posi units will still allow the wheels to turn in 2 different directions under no load; the Eaton Gov-Lock (which came in S trucks and often find their way into these cars), and the Zexel Torsen such as is used in LS1 F cars, will appear to be non-posi because the wheels can turn in opposite directions.

Misinterpretation is as much a problem with these "tests" as the "tests" themselves. Since you don't know what you're doing, all you'll end up with is the wrong answer and a bunch of wasted time.

The presence or absence of parts inside the housing is the only true "test" for the presence or absence of the parts inside the housing.

Change the fluid. Buy a thing of posi additive before you start. If it turns out it's a posi, put it in. If it's not, take it back.

The cover doesn't "have to" come off to change the fluid; you can get most of the old fluid out by siphoning it. However, it's as much work to do that, as it is to pop the cover off. Takes about 3 minutes total.

Here's what you're looking for when you get the cover off. This is a stock posi I have laying around. If yours has the springs and stuff inside it like this, then it's a posi. If all you see is a shaft about ¾" in diameter and 4 small gears, then it's not.

 

RB, where did the 'logic' come from that suggested:
1) spinning a wheel when the rear is off the ground to determing if it's a posi or not
2) counting the number of times the driveshaft rotates vs. the number of times the tire rotated for rear-end ratios
3) the one-wheel peel = open diff vs. two wheel peel = posi

I'm agreeing with you and everything, but where do these ideas come from, if they're wrong? Got any more 'logical' assumptions people fall prey to? I'm wondering if I've fallen for more them my fair share of them over the years. It makes sense, despite how messy and stinky it is, to simply unbolt the cover and look. Heck, I should probably do that just to see what condition my rear is in.

 

1. Some posi rears will act like open rears, even though they're posi. At one time that was not true: virtually all posi rears used clutches. Since there are 2 popular and plentiful models of posi for the rear in these cars that will fail the posi test, the only logical conclusion is that the test is no longer valid. Too many "false negatives" for posi.

2. That works reliably, IF AND ONLY IF the person performing the test, can be ABSOLUTELY CERTAIN that the 2 wheels are rotating EXACTLY EQUALLY. If one tire is allowed to rotate at a different rate than the other, which is almost unavoidable with an open rear, the test will not producs accurate results. In particular if the tire that's being counted for its "one complete revolution" rotates more slowly than the other, then the driveshaft turns count will be too high, and one will over-estimate the gear ratio; and if the other tires rotates more slowly, then the gear ratio will be under-estimated. This is EXTREMELY difficult to gauge by eye. Therefore, the logical conclusion is, that although the "theory" of the test is accurate, the execution of it in the real world is fundamentally flawed, and the results are not trustworthy.

3. That is entirely dependent on the traction that the 2 tires happen to get during the test. It's possible for an open rear to spin both tires, quite easy in fact. Therefore it's not reliable, as a test. Too many "false positives" for posi.

As far as other "assumptions" that don't square with reality, you don't want to get me started on that!!!

 

So, to sum up what you said, it all looks good in theory, but reality tends to muddy the actual results up a lot. Sounds like my budget. No, really, I'd like to know some of the other assumptions we as people can get stuck in. If you don't want to hijack this thread, you could always start a sticky or something in the FAQ - I'd read it for sure. That or PM me. I'm always willing to learn something from the old hands.

 

Now that's funny right there.
The only problem with the "test" is that sometimes a clutch type limited slip needs to be loaded on both wheels for it to expand the clutch pack and work. An example is if you go out 4 wheeling with a factory clutch type LSD and get a wheel in the air, the one on the ground will not recieve any power. A trick is that you can apply a little brake and the resistance of that will help expand the clutch pack to engage the other wheel.
As for peeling out, if the traction is equal for both surfaces you will leave two stripes even in an open diff.
It won't guarantee you don't have one, but I find the easiest way to identify a good posi is to pull off the side of the road, with one tire on the pavement, the other in the dirt/gravel etc.... Give it enough throttle and see if both sides catch. Or nail it around a corner and see if only the inside wheel gets incinerated. If it's posi it will ge a bit more exciting
And after you're done with that.........pull the cover.

 

The Army teaches (EDIT: taught, I've been out for five years) us (for HMMWV's, fun trucks, those are) that if you get a wheel up in the air, to do just what you said, to apply some brake pressure so the wheel in the air quits turning, and you can haul yourself out of just about darn near anything. Chances are, if you could drive into it, you can drive out of it. However, if you're truly retarded, some include a self-recovery winch for those extra-sticky problems. Deuce and a halfs were really good about that, too - and they're only rated for something pitiful like 175 HP, if my memory is correct. The new Stewart and Stevens-designed LMTV's and MTV's are even better. Traction is simply not an issue for those - just press a button and your tires deflate to something stupid-low like 10 PSI, and you can drive it through sand. CTIS is really cool. Driver's dream, mechanics worst mightmare. (everything is electronic - even the transmission shifter is just a bunch of BUTTONS!)

But yeah, I'll have to schedule yanking my cover for the TTA so I can change the fluid and see what my posi actually looks like. and count teeth, but unless it isn't the factory rear, I should find the BW 3.27 gears in there.

 

How many different posi units did they install in the Formula's during the 7 year run?
I know that the 10bolt 3.23 rear end that was in my 88 Formula was posi and both wheels turned the same direction when lifted in the air. My 9 bolt 3.27 rear in my 86 Trans Am did the same.

Now I have a 98 LS1 rear and it is posi but looks like an open rear with raised off of the ground. I am not sure if this posi unit was ever used in the older cars.