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Crawl Ratio with Auto Tranny

Discussion in 'OffRoad Design' started by Mudzer, Apr 29, 2002.

  1. Mudzer

    Mudzer 1/2 ton status Author

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    Stephen,

    I have talked with several people and I have a person who disagrees about the Torque Converter Multiplication of roughly 2 - 2.5. Can you enlighten me on this subject? I am changing over to an auto transmission from manual and just want to compare apples to apples.

    One thing I will say about having a manual tranny. The truck seems more "jumpy". I view video of my truck wheeling in boulder piles and rock gardens in Disney and the truck seems to lunge and roll back alot. I think this adds to the breakage of my 1/2 ton front shafts (all factory stuff) and the fact that I have broken a ring gear. I would not consider my driving style as "pedal to the metal" type driving. I think an Auto tranny will put less stress on my parts and allow me to concentrate on watching my spotter.
     
  2. zakk

    zakk 1/2 ton status

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    before the stall speed of a converter, there is slippage. this slippage is what give the approx. 2 to 1 reduction in crawl ration.

    There is a certain amount of "give" in a tranny coming up from low rpm's. just like slipping the clutch to start in 2nd if you will. as soon as the converter is locked up (over the stall speed) you are back to a "normal" crawl ratio.

    or that is the way it was explained to me/forums/images/icons/smile.gif
     
  3. morphed86k10

    morphed86k10 1/2 ton status

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    I know exactly what you are refering to when you say your truck is "jumpy." In my 63:1 with my old truck and SM465, the truck was quite jumpy. It's because the manual tranny is directly tied to the motor, at at such high gear reduction, the gear lash is amplified. With an auto the converter more loosely connects the motor to the driveline, so its generally smoother.
     
  4. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    A lot of the lunging would be helped by dropping the ratio of the manual combo, they seem to get pretty smooth when you get over 100:1.
    BUT, autos alway seem smoother to me, even when I was running the stock drivetrain in my K5 with 33's (3.73 gears) it worked well in slow stuff. General driving technique with the auto is to drive with one foot on the brake and one on the gas so between the two you can keep it under control pretty easily.
    The torque converter gear reduction ratio probably does vary a bit with the type of converter and other factors I'm sure, I think the 2:1 guideline is just that, a guideline to make some kind of comparison between a manual and auto rig.
    The big thing an auto trans will do for you is disconnect the travel speed from the gear reduction. With a manual, your travel speed is set and you just deal with it, with the auto, you can vary travel speed and keep the torque converter locked up by riding the brakes to keep you slow. It separates two important parts of crawling so you can control them individually.

    Best thing to do is look at what everyone is running and liking and work toward that, then tune it to your style. For example, we know that a TH350 with the Doubler and some 4.56's works pretty well with 35-38" tires in general conditions. There are a ton of variables but it's hard to go wrong with mechanical crawl ratios of 50:1 or deeper.
     
  5. schneggie

    schneggie 1/2 ton status

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    Just adding my input here- I have the doubler hooked up to a TH350 w/4.56's and 35's, I put in a 1800 stall converter per Steve recomendation and I love it. It seems to work very nicely all around.
     

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