How do I tell from outward appearances of rearend, what rearend I have in my 89 IROC. The rear is a junkyard unit actually from a 92 with Disc brakes. I assume it to be since there is "92 3.23" is written on it in yellow crayon.

THANKS,
Craig

 

im not much on 10 bolts but i was told the ratio is stamped on the outer edge of the gear.
dunno if it was still done in 92 but on some older rears the RPO code will be stamped on the bottom of the pas side axle tube.

 

only way to know for sure is to pull the cover and take a look as far as posi or not and gear ratio. if all you want to know is what kind of rear it is count the cover bolts

 

THANKS for the info guys. I'm trying to learn!

Does anybody know if there is a tech article/FAQ/thread either on the thirdgen board or somewhere on the internet that gives the basics of Chevrolet/GM rearends that would give an overview of how to identify rearends, types, types of posi and general info about them? Not so much how they work, but more just to educate about types, what type of posi it uses, what series carrier, ETC.

If there is not one, I know there a handful of names that U always see responding to questions in this area. I sure wish one of U guys would write up an overview similar to the type article that Willie wrote up about thirdgen cooling systems in the cooling section.

https://www.thirdgen.org/techbb2/sho...hreadid=170903

I know I would sure appreciate it and I'm sure it would help others as well!:hail:

 

Another real quick way to tell the ratio, without pulling the cover. Jack up the rearend of vehicle, and mark the rear u-joint with a chalk line, and the area where the pinion goes in to the housing with a chalk line also. That way when you look at the two chalk lines they line up horizantally for reference. Then with two small pieces of masking tape, mark your rear tire at the 12:00 o clock position. Do the same for the fender lip , so they also line up. As a friend spins the tire one complete revolution, from 12:00 to 12:00 , you count the revolutions the driveshaft makes. (example, 3 and 1/4 driveshaft revs= 3.23, 3 and 1/2= 3.42.etc.) This is an old trick Ive used in the boneyard to identify rears, without removing the covers. If the tires spin the same direction, youve stumbled across a posi, or limited slip....Tom

 

Brutalform, Is a posi and a limited slip the same or different? If so what is the differences? Thanks in advance.

 

I never gave it much thought, I only know that it leaves two black skidmarks when i step on the gas.

 

"Positraction" or posi is just a fancy name for limited slip. They are one-and-the-same.

There's a catch to the tire-turning trick. Both tires have to go one full turn, and in the same direction. Or one tire must stay stationary and the other must go two turns.

 

Why not use the calculations at https://www.thirdgen.org/newdesign/t...ulations.shtml to see what you have??

I used them and according to them I have a 3.73.

All I did was drive down the road and with it in 3rd I checked the speed at every 500 RPM's.

In case you where wondering my tire radius is 12.5" and at

1,500 = 28 MPH
2,000 = 39 MPH
2.500 = 51 MPH
3,000 = 60 MPH

I also did one test in 4 th (Over Drive) and it was

2.500 = 70 MPH

I did the test at so many speeds to remove any errors in the results.

 

To correct my above post I have a 3.76 not a 3.73. Also to see if it is posi stomp the throttle. If you leave 2 marks it's a posi. You can also tell by power breaking it and see if both tires spin.

 

You don't have a 3.76. You can belive that.

The tach isn't accurate enough to split hairs like that.

The "2 tire marks" thing is not dependable.

The only way to know for sure what's in that case, is to take off the cover and look. You'll get to see what kind of condition it's really in, while you're at it; and since it's a junkyard rear and you have no clue when the fluid was last changed, you'll get to catch up on the overdue maintenance.

 

What makes you say that?? Keeping in mind the rear end in my car is not the original rear end that came in a stock 87 GTA. The one I have has drum breaks and I would guess it came out of a truck.

 

it woudlent be a 3.76 gm never made a gear like that as far as i kno it could be a 3.73 but i didnt think they ever putthose in fromt he factory btu that dosent mean it hasent been changed

 

If it has a torque arm mount, and fits under your car, I doubt it came out of a truck. I also doubt it has 3.76

To the original poster, do NOT think that turning and counting is a good way to tell the ratio of a rear. The ONLY WAY to tell positively what gears are in a rear, and if it is a limited slip, is to take the cover off and count. Look at it and see if there is a posi in there. Dont waste your time speculating about it.

 

Well I just ran the #'s again and with the info about the tack off. It is only like a few MPH difference both ways. Like if I say 1,400 RPM or 1,600 RPM. I have a crappy digital tack and I was trying to get the readings in about the middle of where it made the next light come on. All and all pretty close.

I also agree the only way to know 100% is to remove the cover.

As far as the 3.76 or 3.73 that is less than 1 MPH so yes I could say it is more than likely a 3.73 and not a 3.76. I am just so damn used to Mopar stuff and 3.76 is a standard Mopar gear I was thinking 3.76 all along.

Any of you with a 3.73?? if so can you tell me if my RPM and speed #'s look correct.

 

PS: I dont want you guys to think I am being some kind of wise a$$.. I am really just trying to learn. It has been about 10 years since I built my last car and it was not a Chevy.. I have always wanted to build a TPI car from the first time I seen one but I had 2 kids and money was tight for about the last 10 years.

 

I have 3.73s, in fact they came stock in my car; but my tach and speedo aren't accurate enough to be bothered with what they say. I just go by what the parts themselves are. And I know for certain that's what's in there, because I bought a set of them and installed them in it not too long ago.

The reason I don't think it's a 3.76, no matter what brand of sheet metal it's wrapped in, in just pure numbers. Ratios are pretty simple actually. There's a whole number of teeth on the ring (no fractions of teeth, at least you hope not!!), and a whole number of teeth on the pinion (no fractions there either); you just count and divide. 4th grade arithmetic. Pinion gears for ratios near 3.73 nearly all have 9, 10, 11, or 12 teeth; and then a number on the ring that makes the ratio. Additionally, one or both of the numbers of teeth, are almost always a prime number (11 or 13 on the ring, or 37, 41, 43 on the pinion). 3.73 for example is 11 and 41. It's pretty easy to see that there's no whole number of teeth on the ring that will produce 3.76 with a pinion tooth count of around 9 to 12. A Chrysler rear I worked on recently had 3.55s, for example; tooth count was 11 and 39.

However all that may be, the factory usually stamps the ratio on the edge of the ring; which saves one the trouble of even having to count. So while you're catching up on the overdue routine maintenance, you might want to just look for the stamping. On my 3.73s for example there's "41:11" stamped on the ring.

 

Thanks for this info.