8 3/4 Open 489 (2.71) to Sure-Grip 3.91

[CoalCracker]

Hello guys,  it's been over a year since I walked away from my '72 Charger.  Got burnt out and took a break. 
I'm back now full of piss -n- vinegar, ready to continue.  I have a 489 open rear that I wanted to change to Sure-grip and 3.91 gears.
I've been told that the center sections swap out.  If I were to buy one from Summit, what else should I be aware of? I guess the gears aren't part of the deal.  Anyone give me a step-by-step of this project?  Can it be done by me in my garage?  Thanks for any input, this site
has been a Godsend.

[dodge freak]

I would not do it. You need the right tools to adjust the gears right or they make alot of noise and may break. I know its not cheap but its is worth it. Call a good trans shop if you know of one or take it to a rear end shop. I took my rear end out and took it to D T S here in Michigan back in 97, I have about 40,000 miles on my richman 3.91's and it is still great, just changed the oil once. Cost me $750 but I am still happy. Don't buy the stuff from Summit cause the shop might get it cheaper and they don't like getting parts that way unless they say its ok. You don't have to take it out but cause I did they hot tanked it and painted it black for free.

[CoalCracker]

I have heard how challenging setting gears up is.  Stuff about shims, etc.  I just didn't know if it applied to what I had in mind.  I was hoping to just open the case, pull out the center, put in a new one and be done.  Wishful thinking, I know.  Biting off more than I can chew would just burn me out again.  Thanks for the suggestion.

[dodge freak]

Don't give up, even 3.55 would be much better than what you have now. 2.71 are no good, 3.21 maybe but not 2.71. 3.91 would make it feel a 1000 lbs lighter. You might not like it at 75 mph for more than 10-15 mins but I have done it for a hour a few times and everything seems fine. Your gas milage will go way down on the freeway but it would be great to drive if your motor has good valve springs and a better cam. I just have a 220 @ .050 and can still get over a 100 mph. I am afraid if you do it the gears might not last more than a few months so you would save nothing.

[TylerCharger69]

I used to originally have a 2:76 open rear end and the 489 case.   I switched the internals with a 3:91  and it's great for 1/4 mile, but highwayability.....downright sucks!!!  That was a Richmond gear setup by the way. A guy in Arizona sold me a rear out of a Ford Mach 1....which I believe sports a 3:00 gear which isn't much but is actually a little more when put under a Mopar, and I like that much better!!!  I didn't really lose much top speed on the car, and it does allright on the take off.   The spring perches had to be slightly moved outward, but other than that, and a brake junction block...it bolted right up!!!   And....I had to have a driveshaft fabricated for it too,  but I prefer the driveshaft with a dampner in it anyway.    My point is....3:91  may be a bit too much  if you do any expressway driving...I'd go with the 3:55 sure grip myself.

[Chryco Psycho]

if you can buy a complete center section with gears etc , the whole center section can simply be swapped in

[CoalCracker]

if you can buy a complete center section with gears etc , the whole center section can simply be swapped in

Ahah!!!  You've solved my confusion Chryco.  I was thinking/using the terms "differential"  and "center section"  interchangeably.  Apparently they aren't. 
Well I ain't giving up then, the search for a CENTER SECTION begins.  I believe you gave me a how-to a few years ago on the old site.  Undo the rear cover, pull the axles, out comes the whole enchilada.  Am I missing anything?

[TylerCharger69]

489 case...there is no rear cover.....entire ring, pinion, bearings...everything  drops out from the front pumpkin.  From the driveline side.  Are you thinking Dana???

[CoalCracker]

rear cover = rearend cover (on the front side)
Didn't mean to confuse anyone.  I have to work on my language skills.

[BlueSS454]

I had 4.10's in my 70 SS Chevelle for 4 years before I changed them out to 3.73's.  I used to run it from NJ down to Maryland all the time via I-95 with the 4.10's never had a problem.  The 454 in that car likes to run around 3400-3800 RPM's, and it got 15 mpg so long as I kept my foot out of it.  I put the 3.73's in it because the 4.10's whined too much for my liking and it only got worse as time went on.
Also bear in mind that the taller the tire you have on the car, the gear ratio will be reduced.  Every inch taller a tire is over factory specs reduces the gear ratio by .10.  The 3.73's in my Chevelle are in actuallity closer to a 3.55 due to the tires I have on it.
The 8 3/4 rear in my Charger has 2.76 gears in it, they are about to get tossed for 3.91's.  The gears are not really hard to setup, just very time consuming.  It took me 7 hours to do the 12 bolt in my Chevelle from start to finish.

[TylerCharger69]

I went from 2:76  to 3:91  in mine too....but 4200 rpm  on the freeway  was a bit much for my liking   and as far as the Chevelle goes....wouldn't that be 4:11  and not 4:10???   Just thinking  GM  and Mopar differences?

[Bandit72]

I went from 2:76  to 3:91  in mine too....but 4200 rpm  on the freeway  was a bit much for my liking   and as far as the Chevelle goes....wouldn't that be 4:11  and not 4:10???   Just thinking  GM  and Mopar differences?

why would dodge 3.91's run so much faster than chevy's 4.10's? my buddy has a 3/4 chevy pickup with 4.10's and it only runs 3700-3900 on the freeway....

later 

[TylerCharger69]

ya got me...I'm just taking an educational guess....but the transmission ratios  are probably a bit different between the two

[BlueSS454]

and as far as the Chevelle goes....wouldn't that be 4:11  and not 4:10???   Just thinking  GM  and Mopar differences?

Nope, GM never made a 4.11, 41 teeth on the ring ring, 10 on the pinion. 

[TylerCharger69]

Do what?....I had an Olds 442,  Nova, Camaro, and a GS....all with 4"11  gears!!! 

[BlueSS454]

The 4.11 gear ratio is a common mistake as far as GM vehicles go.  I have never seen a set of GM gears that are 4.11.  I still have the set I took out of my SS and they are 4.10's.  Aftermarket gears are another story, but factory GM gears were 4.10's and only came in high performance cars such as a 70 LS6, COPO cars in 1969, etc.  Another little tidbit is that positraction was required when a 4.10 ratio was ordered.

[TylerCharger69]

Check your Summit   or Jeg's catalogs....You'll see the 4:11 is a very popular ratio.    The number of teeth is not what makes the ratio...I mean  it is.....but   that's not how the ratio is measured...I mean...it is...but   oh hell....you know what I mean....But seriously...look in any performance catalog...the 4:11  is one of the most popular!!!   I had a pontiac   which sported  4:56  gears.....Yeah...I got rid of that rear  real quick.   That was back in my chevy days  before I wisened  up  and went Mopar....lol....j/k   Or should I say  until I was able to afford Mopar.   BUT...I do use a GM charging system on my Mopar though....I'm not much on external regulators.....and...they are cheaper!!!

[Goldfinger71]

I actually had a 4:11 gear in a 71 RR I had years ago.......talk about take off! ..talk about bad gas mileage! :   ...talk about HIGH rpm's on the highway!

[Bandit72]

The 4.11 gear ratio is a common mistake as far as GM vehicles go.  I have never seen a set of GM gears that are 4.11.  I still have the set I took out of my SS and they are 4.10's.  Aftermarket gears are another story, but factory GM gears were 4.10's and only came in high performance cars such as a 70 LS6, COPO cars in 1969, etc.  Another little tidbit is that positraction was required when a 4.10 ratio was ordered.

you could get it in the 3/4 trucks as a one wheel wonder....

[Sublime/Sixpack]

Not sure if you're going for a new or used center section, but if you enjoy looking through wrecking yards you just may luck out and find a early 1/2 ton '70's(Pre'1975) Dodge van or pickup with the 3.91 geared suregrip center section you're looking for.  This happened to me a few years ago at a Pick'n'Pull lot.  It was in good shape and was cheap!  Its now in my Super Bee.  Good luck.

[deputycrawford]

     I have an idea, keep both pigs, (center sections). I have my 3.23 sure grip for the road trip from Toledo, Ohio to the Mopar nationals and my 4.10 pig for the rest of the year. It only takes about an hour to change the whole thing.  The 8 3/4 rear end is easy to work on. You can purchase a complete pig from Many places. Just buy one mopar magazine and you will find quite a few advertisements. I believe Chrysler ratio options were 2.76, 3.23, 3.55, then jumped to 3.91 track pack and the 4.10 super track pack. The dana's usually came with the 3.54 and optioned with the 4.10. Even the Hemi Automatic cars came with the 8 3/4, although you could option a Dana. Dana's only came standard on the 4 speed cars. I believe thats how it went.

     Oh, and my 4.10 gears are turning 28 inch tall drag radials. Im 3500 at around 70 MPH.
My .250 degree @ .050 cam necesitates the 4.10 and a 9 1/2 inch converter. It still drives wonderful on the street.

[SeattleCharger]

  Need help identifying my gears.   TK73 and I jacked up the car in back, when spinning left wheel forward, right wheel turned forward the same, when spinning the wheel backwards, other wheel spun backwards the same.
   
            Also, ten rotations of the driveshaft was equal to the rear wheel turning about four inches past three full revolutions.   Anyone know what all this means? 

    He thought the rear looked like a 8 3/4,  would a pic help?    thanks, Nate

[mikepmcs]

You have a sure grip rear if both tires are spinning in the same direction.  That means you are already ahead of the game.
A pic would help but here's a site for you to check out and it will most likely explain it just fine.

http://www.moparts.com/Tech/Archive/axle.html

If you do have a sure grip, which i think you do. jack the car up, mark a line on the driveshaft, make one complete rotation of the tire and count the number of times the driveshaft goes around.   ex: 1 tire revolution and 4  revolutions of the driveshaft will tell you you have approx, 3.91 or 4.10 gear ratio.
that's the way i understand it anyways.  if i'm wrong, someone will correct me.
good luck and post up what you find out.

v/r
Mike

[RD]

You have a sure grip rear if both tires are spinning in the same direction.  That means you are already ahead of the game.
A pic would help but here's a site for you to check out and it will most likely explain it just fine.

http://www.moparts.com/Tech/Archive/axle.html

If you do have a sure grip, which i think you do. jack the car up, mark a line on the driveshaft, make one complete rotation of the tire and count the number of times the driveshaft goes around.   ex: 1 tire revolution and 4  revolutions of the driveshaft will tell you you have approx, 3.91 or 4.10 gear ratio.
that's the way i understand it anyways.  if i'm wrong, someone will correct me.
good luck and post up what you find out.

v/r
Mike

you are right, but if he has a one wheel wonder, you will have to do two rotations of the wheel and count the rotations of the driveshaft to determine the correct gear ratio.

[SeattleCharger]

thx, the pic on that link looks like I have an 8 3/4.   So from our spinning wheels, maybe about one wheel rotation equaled barely less than 3 drive shaft turns,
  2 wheel turns equaled barely under 6 drive shaft turns, assuming I am doing the math right from our first test (ten driveshaft to barely over 3 wheel turns)
    He was guessing that it was a 3.23 maybe?  This would be about in the middle of the road for gear options, I would be happy with that. if that is right.
  I am psyched I have sure grip, never knew til now.