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Technical TH400 question

Discussion in 'The Hokey Ass Message Board' started by Compulsion, Jul 12, 2014.

  1. Compulsion
    Joined: Mar 11, 2009
    Posts: 29

    Compulsion
    Member
    from BC

    OK, just to be clear. On the TH400 the vacuum modulator controls the shifts at normal street speeds. The governor controls the shifts at wide open throttle (WOT). I have a 454 with a .600 lift roller cam and 4.88 gears. The vacuum modulator isn't hooked up. We use this thing just for fun, bracket racing. Bracket racing isn't real drag racing because its about doing the same time not going faster. So, we put it in drive and mash the throttle and let the trans do the shifting.
    My question is, are we doing any damage by leaving the vacuum modulator unhooked to any vacuum source?
    Thanks
     
  2. choppedtudor
    Joined: Nov 28, 2009
    Posts: 683

    choppedtudor
    Member

  3. Hollow65
    Joined: Jan 4, 2014
    Posts: 27

    Hollow65
    Member

    I don't think its going to hurt, but why not hook up the modulator? does it a manual valve body?
     
  4. Compulsion
    Joined: Mar 11, 2009
    Posts: 29

    Compulsion
    Member
    from BC

    Nope was my guess but its nice to have it affirmed.
     

  5. Compulsion
    Joined: Mar 11, 2009
    Posts: 29

    Compulsion
    Member
    from BC

    just lazy and no manual valve body
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 49,511

    squirrel
    Member

    The modulator and governor work together to control shift points. The governor tells the valve body how fast the car is moving. The modulator tells it how much load is on the engine.

    The way you have it now, the transmission thinks the throttle is wide open all the time, and will shift hard and late. Which is fine for a bracket racer, but not so nice on a street driven car.

    You won't hurt anything.
     
  7. Compulsion
    Joined: Mar 11, 2009
    Posts: 29

    Compulsion
    Member
    from BC

    Thanks for the info squirrel. Its never driven on the street, well, OK … just before we load her into the trailer a quick pass down the street is always called for simply to ensure it takes throttle (grin)
     
  8. Mr4Speed
    Joined: Nov 16, 2008
    Posts: 89

    Mr4Speed
    Member

    With the modulator unhooked you will have very high pressure in reverse wish could cause problems.
     
  9. saltflats
    Joined: Aug 14, 2007
    Posts: 10,904

    saltflats
    Member
    from Missouri

    The modulator controls pump pressure so you do stand a chance of premature pump failure.
     
  10. Hook up the modulator won't hurt to have an easy up shift back to the pits. And you might check into a different set of governor springs to change the rpm of governor shift point. Most stock 400's put in manual 1st. and mashed shift @ 4500 some thing like a vett shifts @ 6500. The governor allows upshift and the springs are a snap to switch around.MW
     
  11. Compulsion
    Joined: Mar 11, 2009
    Posts: 29

    Compulsion
    Member
    from BC

    It just might be a good idea to hook the modulator up to avoid the possibility of excess pressure in reverse and/or premature pump failure. Besides it would make the drive back to the pits a little more comfortable. My only concern is that with my luck I'll end up with a vacuum leak (grin). Oh, and I have decided to get a set of TCI governor weights and springs.
     
  12. squirrel
    Joined: Sep 23, 2004
    Posts: 49,511

    squirrel
    Member

    As mentioned, the high per Vettes had nice high shift points. I had a 74 with the high perf 350 that shifted at 5500 every time. Kickdown switch was connected on it, which does make a difference.
     

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