Hi, Im wondering I have a 1986 Pontiac Trans Am with 305 V8 LG4 motor. Wondering how can I tell if I have possi rear end or not. I looked on the axel there is only 1 tag that says 2 HP which I looked up is the code for 2.73 Ratio and I have the RPO code of GU2 thats for it too. But it does not say if its posi or not? Oh and I have 700R4 Trainny and I dont have rear disk brakes.
Please let me know if you can answer this! thanks Jacob

 

According to my books, there is no 1986 2HP axle code. Also there is no 2.73 ratio listed that year.... the highest it shows is a 2.77.

The 2HP code shows for a 1987 2.73 rear axle but there is no notation as to whether or not it is limited-slip.... Someone else here may be able to add more.

 

your RPO codes will say "G80" if it's a posi.

however the only way to be absolutely sure is to pull the cover.

 

So only cars with G80 are posi, and if Im correct is that the posi with limited slip? And if so, Thats just one type of Posi made with the limited slip, I dont think its the only ones for 3rd generation F-bodys. I looked in my Haynes Book at it shows many Trans Ams that had posi, but it only goes up to year 1984. I have an 1986 so I am still not sure If I have a posi or not!

 

The GU2 - 2.73 / 2HP was used '86-'88.

An '86 GU2 with a posi would have a 2HT

 

All 'posi's are 'limited-slip.' They both are terms that mean the exact same thing. Positive Traction, and Limited-Slip both mean that when one wheel starts spinning power is applied to the other so that they both turn, basically.

The terms were actually coined by different companies. I think Positive Traction was coined by Mopar and Limited-slip was GM, although I could be backwards on that. Same thing in any event.

your car most likely does not have a posi unit, but if you see code G80 in the RPOs, then you did have it originally. If G80 is not included, then you didn't.

 

Ok, I dont have G80 in the RPO. So I guess no posi traction. Can you guys tell me the pros and cons from posi from normal?
I used to have a 87 Formula with Posi, or limited slip It also had 350 TPI and it was fast, burned out well too.
But I dont see a big Difference! Probably I just dont notice it!

 

One way you can find out, jack the back of the car up, turn one wheel, if the opposite wheel turns in the same direction, you have a posi, if the opposite wheel turns in the opposite direction i.e. you turn the wheel forward, the other turns backwards, you have no posi or the posi is shot.

 

There is nothing bad about having a posi. Get a good unit and have it installed by a reputable shop and it'll last a long, long time.

The pros would be: Increased traction and better burnouts

Every fbody ever build should have come with a posi I'm glad they included it in every V8 4th gen

 

Limited slip is a generic term describing the function of the rear. GM called it "Positraction", Mopar called it "Sure-grip", Ford called it "Traction-Loc"

 

http://www.hioutput.com/tech/ratio.html shows 2HP as an 87 rear with a 2.73 open gearset in stock form.

And that doesn't tell you if you have an LSD or not as there is no resistance on either wheel/tire to engage the clutches/springs in the differential.

 

The 3.70 9 bolt I have does that until I hold one of the wheels, then it breaks loose with a slight clunk and one wheel turns. With a posi unit, both wheels should turn in the same direction if one is turned, if the other wheels turns opposite of the way the wheel you are turning it isn't a posi unit.

The unit in my Mustang does the same thing, both wheels turn in the same direction, hold one wheel, slight clunk and the wheel turns until the other one is let go, then it turns in the same direction.

 

Unless hes got a gov-loc in that car. ( I hope not). I guess thats a form of a limited slip (until it grenades). The wheels will turn opposite directions while in the air, unless you get one turning REALLY fast....then it will lock up and turn the other one the same direction. Id rather have an open carrier in my rear end, as opposed to one of those pieces of crap.

 

I think Positraction is the generic term and "Limited Slip" is the GM term.. The Pontiac Firebird factory service manual calls posi axles "Limited Slip" axles.. There is absolutely no mention of positraction anywhere in the manual....

Yes.. Mopar is "Sure Grip" and Ford is "Traction-Loc".

 

No, that is backwards. Limited Slip is the generic term. Which is why you go to the parts store and buy tubes of K&W Limited Slip Additive, and it's for all brands of rears (and fronts...)

"Positraction" was GM's trade name for "limited slip" back in the 60s. It sort of stuck in the world at large, even after GM quit using it. Don't expect to find that word used in any owner's or service manual after about 67 or 68. After that, they went to using the generic "limited slip" terminology.

The thing about "both tires turning the same way" only applies if the drive shaft has more friction than the opposite wheel. And with a posi, both tires will turn the same way, only if the car is out of gear or not in park; if the drive shaft is held still by whichever condition, the wheels will eventually turn in opposite directions if enough torque is applied. Typical production differentials have breakaway torque specs in the 30-65 ft-lb range.

Most limited slip systems, notably excluding the Gov-Lock, consist of a set of clutches connecting the axle to the carrier. In other words, the axle is clutched to the carrier and thus the axle RPM is always forced to be the same as the carrier RPM (not zero RPM or twice the carrier RPM, as it is in stationary one-wheel-peel), unless there is sufficient traction to overcome the clutch friction. A 9-bolt is a standard clutch type unit as far as that goes. If it allows the wheels to turn in opposite directions, EVER, with less than 50 ft-lbs or so of torque, then it is worn out.

The Gov-Lock locks up at about 150 RPM of difference between the 2 axles.

If you want to know if your diff is equipped with limited slip, take off the cover and look. All the rest of that "spin one tire" crap is guesswork at best.

 

from my research and file searching(in my mind) i have understood that:

the good old 'eaton' style(with the springs and no gov-lok) posi units send power to the wheel with more traction

open diff sends power to the wheel with less traction

gov lok sends power to one or both(usually both) wheels when there is more than 100 RPM difference between the two axle shafts

thats what the governor does inside of it
detects RPM difference between the shafts

G80- gov lok or lim slip or posi

there were some others too like in the eaton and dana axles that gm used over the years, different names for the units in them though and most all in trucks

G80 is way less common than most ppl know or believe


of all the vehicles ive had and sene ive run into good old style posi only three times, and G80 crappy posi about 4 times.

and ive had a billion vehicles probably approaching 200 if i could recall them all, and plus ones ive checked out that my stepdad and friends and neighbors have come up with,

and im counting the G80's, that ive actually seen,
IE taken the cover off of
and in some cases removed to sell/swap/trade(- the grenade-loks i dont remove or keep though)
only had one third gen F body with g80 and it was the 85 trans am L69, it was 3.08 posi disc

i sold the whole axle for 30 bucks to a neighbor cuz i didnt want the gov crap or the 3.08 crap, id rather have a 342 or 373 open









good luck

 

G80 is the RPO code used by GM for all limited Slip rears on 3rd/4th gen Camaro/Firebirds. Additional note this was a extra cost option and was ONLY required on certain engine options. Gm only offered one type of limited slip with the 10-bolt and a different type with the 9-bolt on 3rd Gen cars..

 

My 83 Z28 has the J-65 disc brake option.Was the G-80 a separate option even then?The option code G-80 goes back a long way.My 73 C10 CST has a truck 12 bolt and G-80 is on the RPO list on the inside left fender.Pontiac called it Safe-T-trac up to 81.Did they continue with the 3rd Gens?

I've found a little more by searching on here and I also just received a Camaro "Whitebook" I bought off Ebay.The J65 4 wheel disc option required option G80 limited slip according to the Whitebook and the '83 dealer brochure.Haven't found my build sheet as of yet to check RPO's,driver's seat has been replaced so my luck it was in there.But crawling around under her revealed a couple of things;1)the aluminum tag that says requires limited slip additive is still there and 2)I believe the factory exhaust is too.