Rear-End Help

[RDF]

I need some help with how to tell what I've got in my car for a rear end.  If I jack up the rear or the car and spin either tire, the other tire doesn't move.....I know when it's a limited slip rear end, if you rotate 1 tire, the other goes in the opposite direction, but I've never seen 1 tire move and not the other.  Usually with a posi rear end, both tires move in the same direction, right?

2nd, I am trying got figure out a rough estimate of my gear ratio, so I mark the driveshaft and a tire.  1 full rotation of the tire and my driveshaft rotated a full 1 & 1/4 turn  .  I'm assuming that means it's got some really, really ugly gears in it, which explains why I can't even roast the tires off the line.

Any help is greatly appreciated!!

[FJ5_440]

I would say you have an open differential, or less likely but possibly a sure grip that is so worn out that it no longer "sure grips".  

I think you need to try again to check your gear ratio.  I don't believe they made a 1.25 gear set.  Since it is probably not a sure grip, you will need to keep the opposite wheel from turning, then turn the driveshaft and count how many revolutions of the driveshaft it takes to make the wheel go around 1 time.  If it is like 2 and 3/4 you have a 2.76 gear.  If it is like 3 1/2 you have a 3.55 gear etc. Whatever you do, be careful under the car and make sure you support it so it won't fall on you.  We need all the Mopar guys we can get.

[RDF]

Well I rotated the rear tires about 10 times, and each time I watched the other tire and it didn't spin.  I know it sounds hard to believe, but on average the driveshaft rotated about 1 & 1/2 turns per 1 rotation of the tire.

Sure Grip?  I've heard of it, but where does it stand in the land of rear-ends?  Better than limited slip? Posi?  As you can tell, I'm not a rear-end guy. (No side comments on that last sentence please.... )

[MEK-Dangerfield]

With an open rear(insert comments here), if you spin one tire, the other spins in the opposite direction. A sure-grip rear will have both tires spinning in the same direction. I don't know what to tell you RDF.  Sorry.


  Mike

[RaptorF229]

limited slip and sure grip are the same thing

[cowboy]

-
And, - I think, - if only one wheels is turning, you have to dobbel up the turns of the driveshaft to figure out the ratio...

That'l give you about 3.00...

[Black340]

 

[GoodysGotaCuda]

Hey robert, more than likely you have an open diff. Jack one tire off the ground and try and turn it, if does then its open.

As far as the gear ratio, you could count the d/s turns, or you can also use an online calculator using RPM at a certain speed, and tire height.

And a sure grip is the same as a limited slip, posi, trac-lok, etc. 'Posi' is just GM's way of saying Limited Slip. Therefore it should be banned from the board. lol

-Mike

[bordin34]

You probably have 2.91 gears.

[RDF]

Ok, so if it is an open diff.  is that bad?  What if I want to throw some mid 3's for gears in there, is that possible?  SHOULD I change rear-ends before I get my 440 put in there? 

[73Chally]

Personally if you're going to be hooking up a 440, I would absolutely put in a sure grip, and either 3.55 or 3.91 gears.  That's what I have in mine.  I don't see much fun in doing single wheel burnouts.  You should be able to find a 3rd member ready to install for 600-800.

[360 'CUDA]

Go with goodys sugestion and only jack up one of the tires leaving the other one on the ground. Spin the free wheel by first putting the valve stem at 12 oclock. Like before, count how many times the U joint turns but now spin the wheel two comeplete times around making sure the valve stem is at exactly 12.

I believe you will find that the U joint has turned just a touch over two and a half times.  That means you have 2:56 gears. (probably not stock for your car)

The only time you check the rear gear by spinning the tire around once is if you have the Sure Grip and clearly you do not.  I bet you have the rear section out of a sixties four door car with a 741 case. The case your 'CUDA should have come with is a 489 and most of these had 3:23 gears

Or, you do have the 489/3:23 combo and your spins haven't been accurate because both wheels were in the air

[FJ5_440]

To Reiterate, while you probably have an open differential with something at or below 2.94 gears, I have seen an extremely worn "Posi" exhibit the one wheel turns behavior.  The only way to know for sure, is to pull it and see whats inside.  What casting number is on the nose of the rear?  If it was a 741, I would replace it with a  742 casting equipped with sure grip and 3.55 or 3.23 gears.  If it is a 489 or 742 casting I would pull it and see if it has a sure grip that has worn out and can be rebuilt, or add the sure grip and 3.23.or 3.55 gears.

[RDF]

I need some help with what I should look for in a rear end.  I'm thinking of my 440 when I do this, currently I have a 360 in the car now but I need something beefier when the new motor goes in.  I know there are a lot of "it depends on how you want it" comments, but I've got no clue where to even begin this search.  I think I need a whole new rear end, from the case to the gears and axles.  I'd like to have mid 3's for gears and a rear-end that won't smoke just 1 tire at a time.

ANY help would be greatly appreciated.

 

[73Chally]

I need some help with what I should look for in a rear end.  I'm thinking of my 440 when I do this, currently I have a 360 in the car now but I need something beefier when the new motor goes in.  I know there are a lot of "it depends on how you want it" comments, but I've got no clue where to even begin this search.  I think I need a whole new rear end, from the case to the gears and axles.  I'd like to have mid 3's for gears and a rear-end that won't smoke just 1 tire at a time.

ANY help would be greatly appreciated.

 

What rear end do you currently have?  Are there any existing problems (slipping, grinding, loose axles, etc)?  If it's a stock 8-3/4 and a stock 440 build, then in my opinion the only thing you need to look for is a 3rd member with 3.55s and a sure grip, probably around $600-$800 for a complete setup.