Automatic tranny issues

Discussion in '1955-1959' started by dorcutt, Nov 13, 2021.

  1. dorcutt

    dorcutt Member

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    Ok here goes. It’s been a while since I posted but looking for some feed back on a automatic transmission with a 235 engine. I have a 57 Chevy truck with the 235 and a t350 tranny. I up grade the rear end gears last winter to 3:38 and have driven a little this year and while it has improved the drive ability I’m still not happy with it. It is slow coming off the line and is sluggish, it will do 55 at apr 2300 rpm which is an improvement. Changing rear end gears dropped the RPM apr 4 to 5 hundred RPM.
    The 235 runs well and was rebuilt about a couple hundred miles ago and I installed one of Tom Langdon’s carburetors on it so I don’t believe it is a engine issue. I suspect it’s the gearing and transmission ratio issue.
    Did I screw up by putting the 3:38 in? While it improved the top end I now suffer on the low end.
    would I be better off at looking at a 200r4 tranny or the 700?
    I’m not looking for a hot rod and recognize there may be some limitations because of its age but believe it can do better then it is. Any hints or suggestions will be appreciated.
     
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  2. Bill Hanlon

    Bill Hanlon Member

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    Low gear on your TH350 is 2.52:1 while a 200R4 is 2.74:1 and the 700R4 is 3.06:1. So the 700R4 would give slightly better acceleration in low gear, but not much.
    2nd gear on your TH350 is 1.52:1 while a 200R4 is 1.57:1 and the 700R4 is 1.62:1. All three are so close you'd not know the difference.
    3rd gear is 1:1 in all three, so no difference at all.
    The 700R4 and 200R4 both have 4th gear, an overdrive, the 200R4 is 0.67:1 and the 700R4 is 0.70:1. Again, so close that there is no significant difference in the OD trannies, but a big difference vs. the TH350.

    In my opinion, the only way a transmission swap is going to give you better acceleration is to swap back to your original 3.90:1 rear end ratio and swap in either OD transmission to keep your 55 MPH RPM at 1818.
     
    Last edited: Nov 13, 2021
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  3. dorcutt

    dorcutt Member

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    Thanks Bill, I was hoping you would weigh in. I’m not totally surprised with your conclusion as that’s what I thought it was. Tom’s Langdon’s information implys the 200 might be a better fit but it seems to me I’ve read their have been some issues going to an OD tranny with the 235 engine. My ordinal gears were 4:11s and I’m not sure I want to go back.
     
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  4. Bill Hanlon

    Bill Hanlon Member

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    My numbers above were based on a 3.90:1 rear end.

    The 3.38:1 with a 1:1 high gear gives you 55 mph at 2300 rpm. The 4.11 with a OD high gear of 0.70 would give you 55 mph at around 1960 rpm. Depending on the load in the bed, hills, etc, the OD transmission could be shifting down into 3rd quite often,
     
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  5. Bill Hanlon

    Bill Hanlon Member

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    Thought more about your statement "issues going to an OD tranny with the 235 engine". Following is all hypothetical and may be full of bu||$#i+ It is based on my limited knowledge of aerodynamic drag (and therefore horsepower needed to overcome it) increasing much more rapidly than speed (actually drag increases as velocity squared).

    If it takes X horsepower to push your (not very streamlined) truck down the road at a certain speed (say 55 MPH) and your engine is not developing that much horsepower at the lower RPM required of that speed in high (OD) gear, the drivers response would would be to press on the gas pedal. The transmission's response would be to downshift to 3rd gear. Maybe this is why the automatic OD trannies are not recommended for low horsepower/high load combinations???
     
    Last edited: Nov 15, 2021
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  6. dorcutt

    dorcutt Member

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    Thanks Bill, stupid question, would it make any sense to go to a manual transmission with the 3:38s in the rear end? What transmission would you consider? I’m not stuck on an automatic, I got the truck that way.
     
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  7. Bill Hanlon

    Bill Hanlon Member

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    I'd keep your TH350
     
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  8. dorcutt

    dorcutt Member

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    Thanks Bill. Will consider it. Just exploring options at this point.
     
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