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Technical 904 Transmission Lockup vs Non Lockup

Discussion in 'The Hokey Ass Message Board' started by Mopar Tony, Dec 7, 2019.

  1. Mopar Tony
    Joined: Jun 11, 2019
    Posts: 240

    Mopar Tony
    Member
    from SE Iowa

    I was originally planning on putting a 5 speed in my 52 Plymouth but now I'm looking at a automatic for a nicer cruise. I am building this car into a sled plan on making it a good cruiser with some power (around 400 horse). I have a 904 lockup transmission and when I searched it there are all kinds of threads about how to make it a non lock up. Why is that? I would assume the lockup trans woud help with fuel mileage even if marginal. If the lock up stall RPMs aren't at cruising speed I don't know why it is a problem. Any elaboration on which is better and why, the lock up trans has a better gear ratio so I'd like to use it.
     
    Last edited: Dec 7, 2019
  2. The fuel consumption reduction is secondary to the reason that lock up couplers are to prevent overheating of the fluid and transmission through the slippage of the stator and turbine. Heat kills transmissions, lock up couplers extend their lives.
     
    wackdaddy and Mopar Tony like this.
  3. Mopar Tony
    Joined: Jun 11, 2019
    Posts: 240

    Mopar Tony
    Member
    from SE Iowa

    So if the lock out is a beneficial then what is the purpose of locking it out so that is non lock up. It seems they all want the non luck up for some reason.
     
  4. VOETOM
    Joined: Aug 6, 2006
    Posts: 258

    VOETOM
    Member
    from MO

    Tony, I'd go for lockup if you do not hot rod it.
    This book will provide some data about both types and the gear ratio changes in the 904 and 998 and 999s.
     

    Attached Files:

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  5. It's because they don't know any better or don't know how it works, can't figure it out and take the easy way.
     
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  6. Mopar Tony
    Joined: Jun 11, 2019
    Posts: 240

    Mopar Tony
    Member
    from SE Iowa

    What do mean hot rod it? I won't be drag racing it but I do have a heavy foot.
     
  7. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 904

    Elcohaulic
    Member

    So many times I put big cams and quick gears in cars and wished I would of went for the cruiser set up.. This one I have now (69 elcamino) I am leaving alone, stock 1978 454 with only 10,000 original miles/th400/2:87. I love it, it just needs a pair of Porters..
     
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  8. daliant
    Joined: Nov 25, 2009
    Posts: 683

    daliant
    Member

    The problem with the mopar lock up transmission is that it locks the converter at a pre-determined speed (depending on rear gear ratio) at let's say 38 mph regardless of load, so for everyday driving this is fine but for spirited driving where the 2-3 upshift happens above the lock up speed the converter will lock when the 2-3 shift occurs. There's no easy way to adjust the speed the lock up occurs, it's done hydraulically in the valve body so you would have to play with the spring tension on the lock up valve to adjust it.
     
    Mark Yac likes this.
  9. Hnstray
    Joined: Aug 23, 2009
    Posts: 11,425

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    Respectfully, I suggest you have the horse before the cart. Lock up converters were most definitely to improve fuel economy, as the manufacturers used several methods to achieve CAFE (Corporate Average Fuel Economy) as the financial penalties for not doing so were burdensome. A secondary benefit is, as you state. a minimizing of heat buildup and extended transmission life.

    Ray
     
  10. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,459

    greybeard360
    Member

  11. garyf
    Joined: Aug 11, 2006
    Posts: 212

    garyf
    Member

    The early lock up trans had a lot of bugs ,beginning in about 1978. If you are looking for more performance than fuel economy look for a year 904 with part throttle kickdown and no lock up
     
  12. Mopar Tony
    Joined: Jun 11, 2019
    Posts: 240

    Mopar Tony
    Member
    from SE Iowa

    I have heard of putting in a stiffer spring so that the lock up occurs much later which I think could help performance.
     
  13. Mopar Tony
    Joined: Jun 11, 2019
    Posts: 240

    Mopar Tony
    Member
    from SE Iowa

    Very good read indeed. So basically I could get one of these kits and put it in when I putting in my shift kit and intern it would raise the lockup point which would help with bogging and ultimately the lockup wouldn't be bad but potentially useful.
     
    Hnstray likes this.

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