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BOP 350 questions

Discussion in 'The "Juice Box"' started by Gulfgears, Oct 19, 2020.

  1. Gulfgears

    Gulfgears Gulfgears

    Sooner than I may think, I may have to ditch my 300sp transmission.
    After considering my options and driving habits, I think a 350 may work out best for me. I have seen all the posts on 200r4, but don't feel up to all the cable problems and other stuff.

    My questions are, how complicated is the shift cable hookup to the carb, (running a AFB), and what is the modulator looking thing on the end of the transmission with vacuum line running to it?

    Driveshaft and mounting points are the same, correct?
     
    Gulfgears, Oct 19, 2020
    #1
  2. Matt69olds

    Matt69olds Well-Known Member

    You don’t HAVE to have the kickdown cable connected. Unlike a 2004R, a TH350 kickdown cable has zero influence on line pressure and shift points.
    All the kickdown cable does is downshift the trans at WOT. There are aftermarket brackets/adapters to hook up the cable to most carbs. As long as the cable travel maxes out at full throttle, the trans will downshift automatically, and depending on valve body/governor calibration will automatically upshift around 70mph.

    The vacuum modulator need full time manifold vacuum. Don’t use a section of rubber hose, I have seen many times where the hose gets soft from the heat of from the exhaust and collapse. Use a piece of brake line tubing with short sections of rubber hose to make the connections at either end.
     
    Matt69olds, Oct 20, 2020
    #2
  3. Matt69olds

    Matt69olds Well-Known Member

    And to answer your other question, the driveshaft and trans crossmember should fit. You may need to drill the holes in the frame (the TH350 trans didn’t exist prior to 69, I don’t know what car your working on). It’s about the easiest swap there is.

    Unless this is a 100% factory car, and that factory image is important, no way in hell would I ever waste the time fixing a ST300/Jetaway trans. The added 1st gear ratio will make the car feel like it has an extra 50 hp.
     
    Matt69olds, Oct 20, 2020
    #3
  4. BrunoD

    BrunoD Looking for Fast Eddie

    If I remember right,68 had the TH350 and TH400.Bruno.
     
    BrunoD, Oct 20, 2020
    #4
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Sorry Matt, but that is not true. Read any book on the THM350 and you will see that the detent cable modifies pressures via the detent regulator valve and influences shift points. It does more than actuate a WOT kick down. Despite this, the internet is full of opinions and anecdotal evidence on this. Can you run without the cable? Yes, but it will make a difference. The cable does more than actuate full throttle kick downs, it actually makes part throttle kick downs possible. Look at any troubleshooting chart for the THM350, and shift point problems will point to the detent system among other factors. The THM400, running without the detent system will make the transmission short shift at WOT. That may or may not be the case with the THM350, but it will influence shift points, and of course, kick downs. The vacuum modulator (engine load), governor (road speed), and detent system (throttle position), all influence shift points and feel.

    Can you compensate by playing with the governor weights and modulator? Yes, but it still isn't the same. From the 71 Chassis manual,

    THM350Detent.JPG
    THM350Detent2.JPG
    THM350TS.jpg
     
    Last edited: Oct 20, 2020
    LARRY70GS, Oct 20, 2020
    #5
  6. Matt69olds

    Matt69olds Well-Known Member

    You are technically correct. I should revise my statement, the TH350 will upshift with the kickdown cable unhooked, and with no effect on line pressure. However, to get the upshifts “stretched” out, the cable need to be functional. In other words, the trans will “short-shift” unless it’s connected.

    I doubt you could modify the governor to get the shift points high enough at full throttle without screwing up the part throttle shifts. And the vacuum modulator can move shift points up or down 2-4 mph at best.
     
    Matt69olds, Oct 20, 2020
    #6
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Agreed.
     
    LARRY70GS, Oct 21, 2020
    #7

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