Rear End Woes

[BBillyC]

Hey guys, I am driving the bird now, but have never had the rear end apart. I know that it is not a posi or sure trak unit in the 489 because when I raise it up and turn the rear wheel the other spins in the opposite direction. When I set one of them down and turn it, two turns of the drive shaft = about 1 turn of the wheel. When I take it to 60mph the tach reads 3000 rpm in 4th gear. I may be wrong but that seems a bit high for a ho-hum differential.

I am planning to redo the rear (with larger tires but don't know what will fit in the wheel well) next year and would like to go with a 391 gear set. However I still need to put in a posi-trak/suregrip. I have been reading that a Dana 60 is the way to go (according to some readers) and I believe the 489 is just as good if built right.

You guys have gotten me through this rebuild of the entire car and this will be the last step, so I am again asking for your most valuable opinions.

 

[Big John]

With an open rear and one tire on the ground, 2 turns of the driveshaft giving 1 turn of the wheel adds up to about 4:1. Closest is 3.91:1, so you may have what you want already. Add a sure-grip unit and you'll have what you want.

The 8 3/4 rear is plenty strong for most uses. The Dana is much stronger... but a lot heavier and expensive. The 8 3/4 also has the ability to change the pumpkin out in less then an hours work. You can have that 3.91 for around home and if you wanted to drive the car across country you could change to a 2.71 pretty easy.

 

[wannarunner]

Just a heads up, Sure grip is chrysler, positrak is GM (Goverment Moters) You can't go wrong with a Dana, they are big $$$$$ though so unless you have one available thats in good shape from a friend or some other good deal, break out the cash. You are correct in that the 489 in good repair/set-up right is an excellent unit. I am finding that 391's are a good compromise between performance/streetability/RPM range, but you need to match gear ratio with your Motor/Cam/tire dia/driving habit etc. even the gear set in your tranny, if your that anal. of course this is only my $.02 and your milage may vary. (pun intended) I would wait for some of the Gurus to reply also......

 

[wannarunner]

....In the time it took me to reply, Big John had already summed it up.....

 

[Basketcase]

hey Wanna...good to see you posting!
A Chrysle 8 3/4 is hard to beat unless you're putting some major HP under your hood.
The 3.91 will give plenty of performance for regular use, but will hurt the mpg some.
I have 15x8 magnums on mine with 27560s, which fit nicely without any rubbing.

 

[BBillyC]

Wow you guys are quick. When I bought the car it was advertised as having 3.9.1 gear set and you've just confirmed that. I guess I need to get a sure-grip to fit and install it. I see some on e-bay so I will go for the clutch type rather than the cone type. How can you tell I don't know much about rear ends. I've never rebuilt one. My experience goes to replacing a bearing on the right side of my 68 charger back in 1971 which didn't work too well. I guess I didn't set the adjuster properly and the axle came out at 70mph and tore the side of the car up flipped in front of me as I was skidding and jammed itself through the radiator.....a big ouch.

Rebuilding with the new unit sounds like a pain in the A$$ with all this crush sleave stuff. Probably a good time though to put on sealed bearings though since my last experience.

Pic attached is rust killing in the rear window area earlier this year

 

[BBillyC]

Hey Basket...how about a picture of that set-up on your car. Thanks

 

[Basketcase]

how's this?

 

[Basketcase]

ow! quit twisting my arm :jester:

 

[Basketcase]

.

 

[Basketcase]

.

 

[BBillyC]

Very, very nice. I am hoping to squeeze every bit of rubber under the birds tail that I can without having to tub it out.

 

[SomeCarGuy]

Maybe the clutches are burned up in yours. I can't recall seeing a 3.91 that wasn't a SG.

 

[BBillyC]

I was told it was a sure grip but a sure grip wouldn't let the opposite tire spin in reverse, or so I'm told. For the pro's out there the question would be: If the clutches (or cone) are burned out would the opposite tire spin in reverse?

 

[Big John]

I was told it was a sure grip but a sure grip wouldn't let the opposite tire spin in reverse, or so I'm told. For the pro's out there the question would be: If the clutches (or cone) are burned out would the opposite tire spin in reverse?

I don't believe it will.

BTW, I have come across open 3.91 rears. Quite often they were in mid-sixties half ton trucks.