8.5 10 Bolt Rear End Questions

Discussion in 'Got gears?' started by Christopher Spouse Drew, Jun 26, 2018.

  1. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    Hey guys, I have a 71 Skylark with the 8.5 Open Rear End. I read on a previous post they are reliable and stronger than chevy 12 bolts. My question is what upgrades have you done to them to make them handle power better? Have you converted them into posi rear end? How? I've never worked on a rear end before so im very green to the subject, Im restoring my car, just got a 455 and TH400 that im gonna rebuild I want to make the car very reliable and have longevity without having to give up power(im shooting for 400hp to the wheels). Any comments or advice will help, thanks!
     
  2. BrianTrick

    BrianTrick Brian Trick

    The 8.5” is a very stout rear. You can take what you have,and rebuild it with a posi unit,and the gear ratio suited to what you want to do with the car. The factory 28-spline axle shafts will be fine for your power output,and are good in a car like yours,running as quick as 11:70’s with a regular foot brake automatic. If you were to run a trans-brake,slicks,more power,or something that would put more blunt force to the rear,I would upgrade it with 30-spline Moser axles. Then it will take anything you throw at it.
     
  3. gstewart

    gstewart Well-Known Member

    I added an Eaton posi back in 2010.
     
  4. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    Okay that helped out a lot, thanks!
     
  5. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    how do you like it? Do you feel the difference?
     
  6. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    and gear ratio, can someone give me the formula on how to get a ratio right for a specific car? I understand you count the teeth of both and divide the amount by the other, but why 3.73:1 or 3.48:1? How do you decide what gear ratio to go with?
     
    Last edited: Jun 26, 2018
  7. Bens99gtp

    Bens99gtp Well-Known Member

    Not that I would recommend anyone doing this, but our car has been running low 11s and high 10s for years on stk axles, both footbrake and transfer brake, car and driver is 3850, 9"Hoosier slicks, upper 1.4s in the 60ft time. I have twisted a few axles and keep a close eye on them.........but it does show the strength in them.
     
  8. Bens99gtp

    Bens99gtp Well-Known Member

    I don't know if there is a right formula. What would be right for 1/4 mule won't be right for the 1/8th or on the street. This will vasting depend also on the motor. Some motors don't make power over 5500, some 6500, some 7500. They will have a different right gear for their combo.


    But I think most street car's use between 3.40s and 3.70s and work out well, but again, tire size and what rpm your comfortable turning going down the road at what speed will effect this. If you want to drive your car at 75mph a lot and have short 14"tires the 3.70s will be turning a lot of rpm
     
  9. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    Okay im understanding, I guess i really need to get a complete list of what my motor, wheel package, weight, etc will be and make all these different systems work.
     
  10. Bens99gtp

    Bens99gtp Well-Known Member

    There you go, it all works as a system, one part can vastly effect others....like tire height. The fear ratio will also effect your speedo reading so once you figure out tire height and ratio wanted don't forget that last piece
     
  11. monzaz

    monzaz Jim

    ONLY in racing does th e gear ratio really make a difference.
    As far as on the street YOU the driver pick your ratio to make the car perform in the area you drive it most.
    So if you never leave town or do not ever get on the freeway... you can go nuts with steep gears for all the torque you want... But if you want the car to have overall good drive ability stay in the mid to lower 3 series gears ... If you are wanting a state to state cruiser 2 series and very low 3 series gears are what you want.

    NOW other factors in a ratio decision will be your engine mods cam size and what you set it up for revving or torque or high speed running? transmission 3 speed 4 speed manual or overdrive trans???

    IT really is up to you... iut is your car. You need to sit back and figure out what your car will be doing to make you happy and add the gears that will enhance that experience for you.

    IF you were drag racing or Bonneville flats racing we could figure out what ratio will work best with your combo ...But being you have a 400HP 455 ANY ratio will work in your application. Not hard to get 400HP from a big block Buick.

    Jim
    J D Race
     
  12. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    Yeah I want the best of all those driving scenarios with the primary being driving on local street and highway(I go on both almost everyday), next being on the track, and lastly taking a road trip maybe every year or two across a few states(wishful thinking). Ill be running a TH400, my style of driving on the street is like the track, I see that green light turn and I slam on it haha but i want longevity and reliability. I dont want to fix this car every week after i drive it.
     
  13. BrianTrick

    BrianTrick Brian Trick

    I would suggest something in the 3:08,3:23,3:42 range. If you have AC,and stock tire size,then 3:08 or 3:23.
     
    techg8 likes this.
  14. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    C
    Can you explain why A/C can effect a rear end? Im going to try to fit the biggest wheel and tire package under the stock tubs.
     
  15. 12lives

    12lives Control the controllable, let the rest go

    You don't want the engine spinning higher rpms (highway speeds) and spinning the AC compressor too fast too long.
     
  16. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    But wont a good aftermarket a/c compressor be able to handle that?
     
  17. BrianTrick

    BrianTrick Brian Trick

    An aftermarket compressor might be different,but the originals will disengage at a certain rpm,leaving you cruising down the highway with no AC. You also need to realize that the speed limits are much higher than they were back then.
     
  18. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

    Yeah I understand, Im restoring my a/c system right now and im going with a reliable aftermarket a/c compressor.
     
  19. Christopher Spouse Drew

    Christopher Spouse Drew Well-Known Member

  20. monzaz

    monzaz Jim

    Yep and when you get a noisy gear set-up do not ask us for help...lol. Jim
     

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