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Torque converter question for trucks running doublers.

Discussion in 'The Garage' started by sageman, Jun 17, 2003.

  1. sageman

    sageman Registered Member

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    Just had my Th350 rebuilt and the tranny shop told me to use a Th400 converter instead... basically the converter would lock up sooner. Anyone else heard, seen, or know of this to be true?
     
  2. gravdigr

    gravdigr 1/2 ton status

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    TC only locks at highway speeds, usually in drive 3. You do not want the torque converter to lock sooner while crawling because that defeats the purpose of the torque converter...to convert torque. At low speeds the TC multiplies torque from the engine 2:1 before it goes through the tranny...this is a good thing. Pre 1979 TH350 and TH400 used the same TC as true locking TC did not appera for the TH trannies until 1980.
     
  3. Fubeca

    Fubeca 1/2 ton status

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    Thm350s and 400s tconverters don't lock up. I think he is talking about stall speed. Stall speed is the rpms the motor can spin before the converter starts putting power to the tranny. A vehicle with really low gears can use a low stall speed where high gears need a lot of engine speed to get the machine movin.
    If you use a tconverter with a stall speed too low it will seem really doggy off the line in 2H. I don't know what the difference btw the 400 and 350 tc is.
     
  4. zakk

    zakk 1/2 ton status

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    a HD stock unit would be fine. somewhere in the 1200-1500 RPM range is stock, IIRC

    i just run an off the shelf TC and its fine with the doubler. I do wish I had either more power (Crate 350) or lower gears (3.1 WMS 203 Range Box) as I still need to feather the throttle to get over the really big stuff. I'd like to idle over it...
     
  5. gravdigr

    gravdigr 1/2 ton status

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    Torque converters on all TH350 made after 1980, known as the th350c, are required by law to lock up. They actually have a type of clutch and pressure plate built into the TC and a system to lock it in 3rd gear. Basically making a solid connection between the flexplate and transmission. This was done to inprove gas consumption although the industry states it only provides about a 3% increase.

    EDIT: my mistake, not ALL made after 1980...diesels still had non lockup converters until the mid 80s
     
  6. Topdown

    Topdown 1/2 ton status

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    As I understand it a TC with a 2000 stall means that at 2000 rpm the TC is as "locked" as its going to get, not that you have to have 2k rpm before you start to move, otherwise a stock TC would be roughly 800 (off idle) instead of 1500 or so.

    -Ryan
     
  7. bigjbear

    bigjbear 1 ton status Staff Member Moderator

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    A TC w/o lock-up clutch will always have some slip at crusing speed. No matter what the stall speed is.
     

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