8 3/4 rear, 489-vs-742 rear, which is better?
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Thread: 8 3/4 rear, 489-vs-742 rear, which is better?

  1. #1
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    8 3/4 rear, 489-vs-742 rear, which is better?

    I picked up a 489 and a 742 rear center section today, both are open, I need to build one, which one should I build? What are the pros-vs-cons of each?

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    The gear ratio selection is not a factor, I am set on a 3.73 gear. I am thinking of the Mancini aggressive style diff. Which is easier to set up myself, i want to do it myself this time?
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    I went through this a while back. All the research and questions I asked ended up pretty much the same way. Either one will stand up to reasonable abuse if built properly with good parts. The whole "you gotta have a 489" thing is a little overrated. Hell, I know a guy with a pretty hot car that runs a 741... go figure. I suppose if you look at it from an engineering standpoint, the 489 with the tapered pinion is a bit stronger and if you have a 489 takeout to build, with all other parts going into either case being about the same price, by all means choose the 489. You might offer yourself a little piece of mind and you'll be more "fashionable" with the 489 also. Just my 2c.

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    The 742 is a little easier to rebuild because you use shims to set pinion bearing preload. In the 489, unless you buy the replacement shim adjustable crush sleeve, it can be a little tricky.

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    If you go with the 489, get the solid pinion spacre instead of using the crush sleeve.
    66 Dart GT: BB project. Shooting for high 10's
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    What if I sell the 489 to finance the 742 build, how much do you think a 3.23 open geared 489 is worth?
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Quote Originally Posted by flyboy01
    What if I sell the 489 to finance the 742 build, how much do you think a 3.23 open geared 489 is worth?
    I'd try for $75-100
    66 Dart GT: BB project. Shooting for high 10's
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    The mopar performance aluminum case is a 742. Seems to me like it must be favorable over the 489 or it would be a 489 not a 742

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Is it me, or did the one on the right just get back from nam?

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?



    Its not paint, I think its pollen. I plan to hit them both with engine degreaser and power wash them next weekend.
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    I have a 742 and 489 also I'm in the process of building. I believe some people said the 742 is better for manual trans. I'm building the 489 because my 742 is complete and my 489 is just an empty case. Oh yea and bigger is better.
    68' Dart 69' 440, purple 590 cam, .30 over, holley 850, 727 man shift, 8 3/4 3.91 motive, dynamic 9.5, csr pump<br />99' 2500 dodge diesel

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Either will work fine.

    With the 742 you don't have to spend an extra $30 on the solid pinion spacer/crush sleeve eliminator.
    I'd do whichever cost less to have as a complete unit.

    As far as strength... ALL 8 3/4's reduce to the same diameter at the pinion yoke. Short of busted teeth, that's the other place that EVERY one I've had let go broke. Nothing like banging the limeter while the driveshaft is exiting the transmission.

    The cases you currently have is worth the value of the case, IMHO... $50 or so. If it has a preferred style yoke on it, maybe more. I see some real $$$ crazy fools asking $175-200+ for 489/742 open 3.23 cases and they've had them for YEARS!

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    I have 2 489 posi uits for my car. I would not give them up. I have a 3.91 and a 3.23. They should be great with my overdrive.
    Cary Snyder<br />5.7L Hemi Cuda

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    either will work, good choice with the 3.73s though.

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    3.73 will allow me to turn about 3000 rpm at 62 mph and still have enough gear to run fast in the 1/8.
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Quote Originally Posted by flyboy01
    The gear ratio selection is not a factor, I am set on a 3.73 gear. I am thinking of the Mancini aggressive style diff. Which is easier to set up myself, i want to do it myself this time?
    with fylboy01 everyone says use a 3.55 or 3.91 and I think right in the middle. 3.73 should be just right for me in my setup also.

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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Just noticed you said it was open. I mentioned the 2 piece side gear on the some/all 742 sure grips that can cause problems with aftermarket axles. I'll leave the reminder in case someone reading this may benefit.

  19. #18
    Senior Member guzzimike's Avatar
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    Hiya. I have heard that 489 are a bit stronger than 742.

    I have both. A 742 in 3.23 SG and a 489 in 3.91 SG. the last set is currently in my car.

    I wanted to get a 3.73 for my 742 but found out that this is not an available option. Only 3.55, 3.91, 4,10 and up.

    So I got the 489 in 3.91 with the added advantage that IF I ever wanted to get 3.55 or 3.73 fitted, I could do so readily..

    Are you totally confused now..?

  20. #19
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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    For those that may have not seen this, have a look at this comparison of different rears from Dr. Diff over on Moparts.

    http://www.moparts.com/Tech/Archive/axle/1.html


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    Re: 8 3/4 rear, 489-vs-742 rear, which is better?

    The 742/741 axles spline got us once. '70 slant auto valiant green bearing 489 sg 3.23 We went to the track,my brotherinlaw put in my 741 3.91's SG. At the line something looked like it broke. The car would spin the driveshaft and the wheels didn't turn. We called for the 3.23 set and swapped everthing back to get home. The axles bumpers kept the axles out of the inner spline just alittle. He stripped the axles splines on the end just enough to not engage. **(*) remove the bumpers when installing green bearings. The 489 didn't care.

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