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Leakdown Testing

Discussion in 'The Hokey Ass Message Board' started by RatRoy, Nov 24, 2010.

  1. RatRoy
    Joined: Jul 9, 2008
    Posts: 371

    RatRoy
    Member

    Anybody have any experience with preforming a cylinder leakdown test on a cold motor?,,i.e. a motor on the shop stand...How much difference with the leak percentage between hot motor and cold motor ? :confused:
     
  2. NWRacing
    Joined: Aug 29, 2010
    Posts: 124

    NWRacing
    Member

    Is it a new build or one thats been run
     
  3. The motor has to be at least broken in. A leakdown test on a new motor just confirms that the rings and valves won't be completely seated in yet.

    It has been my experience that it works best with a warm motor, but then that is because most of the time I am trying to diagnose why a motor has gone down in the field, and most of the time they are still warm.

    Good Luck.
     
  4. RatRoy
    Joined: Jul 9, 2008
    Posts: 371

    RatRoy
    Member

    The motor I want to test is my old motor that I replaced with a new motor. Just trying to determine the condition of my old motor.:)
     
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  5. NWRacing
    Joined: Aug 29, 2010
    Posts: 124

    NWRacing
    Member

    For a street motor I would like to see less than 15 % and more importantly a variance of less than 2-3% among all cylinders. If I had 17% on all cylinders I would take that over a spread of say 7 % to 15%. Cold with dry cylinder walls you may run 4 or 5 % more, you can listen to the air and get an idea of where you losses are.
     
  6. Beef Stew
    Joined: Oct 9, 2008
    Posts: 1,253

    Beef Stew
    Member
    from So Cal

    i do my leak down test at tdc and bdc. i've found that it can be good at tdc but bad at bdc so i check both.
     
  7. RatRoy
    Joined: Jul 9, 2008
    Posts: 371

    RatRoy
    Member

    Thanks for the input.....:):):)
     
  8. RidgeRunner
    Joined: Feb 9, 2007
    Posts: 826

    RidgeRunner
    Member
    from Western MA


    Made it a habit when checking for water leaks in diesels.

    Something I hadn't thought much about with gas engine compression checks until now and should have.

    Great tip, thanks for sharing and waking me up.

    Ed
     
  9. JOECOOL
    Joined: Jan 13, 2004
    Posts: 2,756

    JOECOOL
    Member

    I wouldn't worry so much about the % as I would where it was leaking ,Intake ,exhaust ,crankcase , and which cyl.
     
  10. NWRacing
    Joined: Aug 29, 2010
    Posts: 124

    NWRacing
    Member


    I agree the % is just a indicator of overall condition and a reference point from cyl to cyl. I have leak downed some motors with 2 ring slipper pistons that would leak down close to 20% but were dead even on all eight cyls. static leak down % is not always an indication of dynamic compression loss. When the piston is in motion the dwell of the compression stroke is too short to allow any significant loss. A hot compression test of this engine showed each cyl with 180 psi with no measurable variance in between cyls.
    As JC said the important thing is where the loss is. if you are leaking from a valve keep in mind that a motor being turned by hand will not always close the valves with enough inertia to seat them properly, if I find one leaking I will usually remove the rocker arm and tap the tip with a brass hammer(it will popwhen you do this) if it seals it is good if not you may have a problem in the valve or seat.
     

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