gears?

Discussion in 'The "X" bodies' started by partsrparts, Aug 25, 2015.

  1. partsrparts

    partsrparts Silver Level contributor

    just picked up a 8.5 open rear diff for my 76 skylark, was wondering what would be a good limited slip posi unit to get. Also need to decide what ratio gears to run. engine is a 68 350 4bl. mild build, about 300-325hp, turbo 400 with shift kit and 2500 stall converter. running the stock 2:56 gears right now that turn about 2250rpm at 65mph. leaves alot to be desired stoplight to stoplight. want it to be alot more fun off the light but also want to still be able to run down the highway easily. i know overdrive is the way to go, but its a new th400.

    thanks for any input
     
  2. jzuelly1

    jzuelly1 Jesse Zuelly IV

    You would probably need to stick with a 3.42 or so if you want any highway drive ability. If you go to much higher that thing will be well over 3,000 RPM at 65. Just my thhougt process. Also if you plan to run it on the highway a lot. A good trans cooler would be a wise investment to keep it going for a long time.
     
  3. TheSilverBuick

    TheSilverBuick In the Middle of No Where

    Agreed, 3.42 would be about as high as I'd go without an overdrive. Cruising at 3,000-3,200rpm doesn't bother me at all, but it does some. I'm running 3.42 gears in my Firebird with an overdrive, but there have been a few times where I accidently left it in 3rd out on the highway at 70mph without noticing. 3.08-3.23 are the next down gears that are decent. OE for my '77 Skylark was 3.23 and those gears were ran without an overdrive for 25 years or so with plenty of highway driving.
     
  4. skylarkgp

    skylarkgp Member

    My 76 was a 231/th350/2.56. It is currently a 231/700R4/3.42. I swapped in the 8.5 and threw in an auburn posi when I did the 700R4. I also accidentally left it in drive on a long trip and it wasn't an issue. I'm looking forward to the day that I can drive it again.
     
  5. Bigpig455

    Bigpig455 Fastest of the slow....

    Agree with all here - 3.23 - 3.42 is your sweet spot. I've got 3.90's on my 71 and that's tough to take anywhere but the track......
     
  6. partsrparts

    partsrparts Silver Level contributor

    sounds like 3:42 will be the way to go, installed trans cooler when i installed trans, what about a good posi unit? any input on that?

    thanks
     
  7. TheSilverBuick

    TheSilverBuick In the Middle of No Where


    I have 3.89 gears (9" rear) with a 26" tire, and for giggles this morning I ran in 4th gear a bit out on the highway going to work, and actually I didn't find it as tragic as I thought it would be. 65mph at 3,200 rpm, which is spin'in, but not as horrible as I thought it would be. 3,500rpm at 70mph, which is the highway speed limit here.

    Then threw it into OD for a nice 2,250rpm cruise speed at 70mph :)
     
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

  9. Bigpig455

    Bigpig455 Fastest of the slow....

    x 2 on the Eaton - I got talked into an Auburn and it's already starting to go away after 3-4 seasons.
     
  10. partsrparts

    partsrparts Silver Level contributor

    Thanks Larry & Bigpig! Looks like i"ll go with the eaton posi and still debating on 3:23 or 3:42's.

    The rearend is out of a 75 nova, open, but i just needed the housing and 28 spline axles. there are oil leaks between the axle tubes and the center housing, which seems to be quite normal, should this be welded or? I'm going to have it powdercoated before assembly.

    thanks!
     
  11. Bigpig455

    Bigpig455 Fastest of the slow....

    my 8.5 in my 73 "weeps" there too. I've never heard of an 8.5 housing having tube welding issues like a 12 bolt, and I'm not sure you could stop the seepage without a lot of effort - Coat it, enjoy it, and wipe it off every once in a while
     
  12. partsrparts

    partsrparts Silver Level contributor

    thanks, will do! off to the powdercoater!
     
  13. snga

    snga New Member

  14. wormwood

    wormwood Dare to be different

    never herd of em. make sure you get some opinions before you go with this
     
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

  16. TheSilverBuick

    TheSilverBuick In the Middle of No Where

    I have a Quick Performance 9" rear end in my '77 Skylark and many folks I know on Drag Week used them for their rear axles. I have no experience or aware of anyone that have used their GM 8.5" stuff, but if it's anything like their 9" stuff it should be good. They also have good customer service when buying and a few years after I got mine I had a question about a seal and they e-mailed me right back with the answer.

    [​IMG]

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  17. snga

    snga New Member

    Thanks for the input guys.

    Aaron
     

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