Register now to get rid of these ads!

1951... Buick rear end gear

Discussion in 'The Hokey Ass Message Board' started by Yaril, May 8, 2007.

  1. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    Your opinion is requested,

    So now that i have a 79 monte carlo rear end (it was the right width), its time to buy gears to match my straight 8 motor and 700R4 transmission. I have done all the calcs based on my 215/75R15 tires = 27.7 diameter. The 700R4 has a final gear of 0.7:1 so my final gear ration is multiplied by that number:

    3.08x0.7=2.15 final gear = 57mph@1500rpm ; 76mph@2000rpm ; 95mph@2500rpm ; 115mph@3000rpm

    3.23x0.7=2.26 final gear = 55mph@1500rpm ; 73mph@2000rpm ; 91mph@2500rpm ; 109mph@3000rpm

    3.42x0.7=2.39 final gear = 52mph@1500rpm ; 69mph@2000rpm ; 86mph@2500rpm ; 103mph@3000rpm

    3.73x0.7=2.61 final gear = 47mph@1500rpm ; 63mph@2000rpm ; 79mph@2500rpm ; 95mph@3000rpm

    So based on the above mphs and rpms and my 56 year old motor that runs strong with just over 100k miles, what gear should i buy? I dont care to race, i want longevity and highway speeds. My concern is mostly that the motor will work properly at highway speeds with low rpms.

    Thanks,
    Yaril
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 45,641

    squirrel
    Member

    3.73 would probably be best. 128 or 152 hp doesn't go far, you need as much gear as you can live with to get the car moving.

    and 2500 rpm at 80 mph is a reasonable speed to run the old motor at.
     
  3. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    that is exactly what i was worried about. i know the less rpm and higher speed the better, but could my motor push it? thats why you recommend the 3.73, i may run out of power to even get to those speeds with the other gears? wish there was a sure way to know.
     
  4. Orn
    Joined: Jul 17, 2005
    Posts: 1,043

    Orn
    Member

    What is the ratio for the OG axle? 4.10? With your 27.7” tires it gives you
    4.10:1.0 = 70mph@3571rpm
    Vs.
    3.73:0.7 = 70mph@2274rpm

    An improvement with 1300rpm to help the old str8 to stay healthy.
    I vote for 3.73:cool:

    Good luck with whatever you choose.
    Per
     
    Register now to get rid of these ads!

  5. Scotch
    Joined: May 4, 2001
    Posts: 1,488

    Scotch
    Member

    You're right on target. See my PM and also know I chose 3.73s to back up my Straight 8. It's not all together yet, but your math matches mine.

    ~Scotch~
     
  6. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    thanks for the feedback! i honestly measured 7 82' and newer camaros/firebirds and they were all 4" too wide. again, maybe the 50 has a wider rear than the 51, could be possible.


    seems to me that 3.73 is the way to go. if i go any lower on the gears my motor may not be able to turn the gears to the desired speed.


    btw this is crazy for me, cause this is the first time i ever got so technical on this car, so i thank you all for your help.
     
  7. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

  8. old dirt tracker
    Joined: Sep 20, 2006
    Posts: 1,003

    old dirt tracker
    Member
    from phoenix

    dont worry about getting the car moving with a 300 to 1 low gear, with a 300 axle you would have a 9 to 1 low gear.with a 3.73 you will have around 11 to 1 low gear. you say your concern is low rpms. 3.73 gives you the most rpms. i would not want to run the moter 2500. interesting that no one has a running car here to really help you out. my vote is for the 3.42
     
  9. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    can you please explain that concept of 11:1 and 9:1? my concern was that the motor would be running sluggish, if that makes any sense. i of course want the fastest mph with the lowest rpm, but as Squirrel "128 or 152 hp doesn't go far". so now im torn between the 3.42 and 3.73.
     
  10. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    bump for other opions, looking like the majority votes for 3.73 gears.
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 45,641

    squirrel
    Member

    the 9:1 and 11:1 thing is the overall ratio in first gear, transmisison first (3.06:1) times the rearend ratio.

    question: how do you plan to drive the car? will it mostly be used in town, or mostly on the freeway at 85 mph, or at 70mph, or a combination, or what? that should determine what you want to do. If acceleration is not important to you, and you plan on cruising at 80 all the time, then go with 3.42s. If you will spend most of your time in the car driving around town, and occasionally get on the highway for a short jaunt to the next town, then running 4.10s would probably be fine.
     
  12. Just for information, in -53 the str8 buicks had 3,9:1 with stick, and 3,6:1 with dynaflow, thare was a 3,2:1 optional, but only for the V8 models!
    I only have a -53 book, but should be about the same in -51 on the special model!
     
  13. squirrel
    Joined: Sep 23, 2004
    Posts: 45,641

    squirrel
    Member

    they did not have overdrive available though, did they? generally cars with OD had steeper rearend gears
     
  14. No there were no overdrive availible as far as i know!
    I just posted it as a reference of what the 263 likes to get the car moving!
     
  15. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    my goal with this set-up is to have a great long distance car, highway speeds are priority. i do plan to drive it daily which is more in town, but i do not care to jump light to light. i am a very patient driver and want to drive this car to carshows that are out of the state.

    if you feel that the 3.42 will work with my horsepower i would rather go with it.

    this is for referrence from the same post i did on buicks.net:

    "I would opt for the 3.73 . I have a 3.42 in my straight 8 without an overdrive and would worry that you will be in the power circuit of the carb . 2500 RPM is sustainable with this engine with no problem. I have driven my 40 over 104,000 miles and I run the interstates at 70-75 all day long. It is a super convertible. My 40 has the stock rear end with the center pig from a 55 Century. ( The center sections interchange from 40 to 55)It is also running the stock tranny"
     
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 45,641

    squirrel
    Member

    Sounds like a tough decision! glad it's you making it, not me :)

    fwiw, my kid's 70 camaro has about the same tire size, with 2.73 gears, and a powerglide, behind a stock 6 cyl rated at 155 hp. It weighs less than a Buick, is probably more aerodynamic, and cruises great at 75. it's damn slow off the line, though.

    2.61 is real close to 2.73
     
  17. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    wow, thanks for making my desicion even tougher! i was actually hoping you or scotch would have jumped in and said... no brainer go with ...blablabal...

    well i know its between 3.42 or 3.73

    700r4 gears are 1st-3.06 ; 2nd-1.63 ; 3rd-1 ; 4th-0.7
     
  18. old dirt tracker
    Joined: Sep 20, 2006
    Posts: 1,003

    old dirt tracker
    Member
    from phoenix

    the whole deal about 700r4 is kind of misunderstood. by 78-81 before 700r4 most cars had 2.56-2.73 rear axles trying to get mileage. now cars with 700r4 still have the same final ratio in od. the idea in a 700r4 was not overdirve it was to add driveability but getting the car rolling easier with the low gear in the rear and trans. you cant tell the difference between a 81 camaro th350 and a 82 camaro700r4 once you get in high gear but you sure can tell the difference in the way they pull off from a dead stop.
     
  19. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    all right i got my numbers! and there are a bunch of them for anyone who care to follow them and try to help me out...

    stock 40 series buick with syncromesh and 3.9 rear end gear
    :
    1st - 2.67 x 3.9 = 10.41
    2nd - 1.66 x 3.9 = 6.47
    3rd - 1 x 3.9 = 3.9 53mph @ 2500rpm & 63mph @ 3000rpm
    i drove it to daytona one year at 65mph stock (800 mile roundtrip).

    700r4 with 3.42 rear end gear:
    1st - 3.06 x 3.42 = 10.47
    2nd - 1.63 x 3.42 = 5.57
    3rd - 1 x 3.42 = 3.42
    4th - 0.7 x 3.42 = 2.39 86mph @ 2500rpm & 103mph @ 3000rpm

    700r4 with 3.73 rear end gear:
    1st - 3.06 x 3.73 = 11.41
    2nd - 1.63 x 3.73 = 6.08
    3rd - 1 x 3.73 = 3.73
    4th - 0.7 x 3.73 = 2.61 79mph @ 2500rpm & 95mph @ 3000rpm

    now i did shift pattern simulations to see how much drop in rpms there would be under normal driving conditions... more numbers...

    stock 40 series buick with syncromesh and 3.9 rear end gear:
    1st gear - 2000rpm @ 15mph shift-
    2nd gear - 1200rpm @ 15mph accel- 3000rpm @ 35 mph shift-
    3rd gear - 1800rpm @ 35mph accel- 3000rpm @ 63mph

    700r4 with 3.42 rear end gear:
    1st gear - 2500rpm @ 20mph shift-
    2nd gear - 1400rpm @ 20mph accel- 2500rpm @ 37 mph shift-
    3rd gear - 1700rpm @ 37mph accel- 2500rpm @ 60mph shift-
    4th gear - 1800rpm @ 60mph accel- 3000rpm @ 103mph

    700r4 with 3.73 rear end gear:

    1st gear - 2500rpm @ 18mph shift-
    2nd gear - 1300rpm @ 18mph accel- 2500rpm @ 34 mph shift-
    3rd gear - 1600rpm @ 34mph accel- 2500rpm @ 55mph shift-
    4th gear - 1800rpm @ 55mph accel- 3000rpm @ 95mph

    thanks!
    Yaril
     
  20. Scotch
    Joined: May 4, 2001
    Posts: 1,488

    Scotch
    Member

    700r4 with 3.73 rear end gear:
    1st - 3.06 x 3.73 = 11.41
    2nd - 1.63 x 3.73 = 6.08
    3rd - 1 x 3.73 = 3.73
    4th - 0.7 x 3.73 = 2.61 79mph @ 2500rpm & 95mph @ 3000rpm



    Doesn't that seem comfy to you? It sure does to me. Let that S-8 lay on the torque curve at about 2,000 rpm in o/d, and ease on down the freeway at 75...


    ~Scotch~
     
  21. Yaril
    Joined: Mar 13, 2006
    Posts: 990

    Yaril
    Member

    your right. that should be comfy. im just picky and most of my friends have 350s with turbo 350s geared at 240, so they like to roll at 85-90mph. i opt to be original at a price of course.

    thanks to everyone that helped me out. done, i am going with the 3.73.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.