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Trying to understand compression readings & compressioon ratio

Discussion in 'Technical Q&A' started by THonda, Jul 1, 2011.

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  1. THonda

    THonda Formula Junior

    Jul 7, 2004
    724
    Long Island, NY
    Full Name:
    TH
    #1 THonda, Jul 1, 2011
    Trying to understand compression readings & compressioon ratio

    --------------------------------------------------------------------------------

    I understand the CS to have a compression ratio of 11.2 : 1 so does that mean a compression test would yeild about 165psi in a closed system.

    As part of my major, I noted I had a compression reading of 135~140 across the board. In readings the past threads it apppears the reading should be in the 200's. I did not have a leak-down test performed.

    The dealer noted that in a compression test they are looking for dramatic changes in cylinder compression and the actual reading has little relevance.

    What do you guys think?

    Thanks

    T
     
  2. greyboxer

    greyboxer F1 World Champ

    Dec 8, 2004
    12,705
    South East
    Full Name:
    Jimmie
    #2 greyboxer, Jul 1, 2011
    No two people measure it the same way on the same day with the same atmospheric conditions

    Therefore spread is more important than absolute numbers
     
  3. Maniak

    Maniak Rookie

    Jan 2, 2009
    47
    Amsterdam
    Full Name:
    Marc
    #3 Maniak, Jul 3, 2011
    The 165 psi cannot get higher because the compression ratio indicates that pressure. As far as basic knowledge serves me, it is that simple.
     
  4. PFSEX

    PFSEX Formula Junior

    Jun 30, 2006
    843
    Las Vegas
    Full Name:
    John Ratto
    #4 PFSEX, Jul 3, 2011
    Maybe 165 would be the max if you only had compression stroke???
     
  5. beast

    beast F1 World Champ

    May 31, 2003
    11,479
    Lewisville, TX
    Full Name:
    Rob Guess
    #5 beast, Jul 3, 2011
    Last edited: Jul 3, 2011
    There are to many variables that you have not covered here..

    Are you testing with all spark plugs in or out?? Engine warm or cold?? Throttle open or closed?? Fully charged battery and a starter motor that is cranking the motor over properly?? compression tested that is not leaking at the spark plug hole and of the proper spark plug length??

    Also there is altitude and air density that can alter compression readings.

    If you are getting 135 - 140 across all holes I would not be worried. The readings are within 10% of the highest reading. If you are still concerned then go with the leak down test on a warmed up engine. If any hole has more than 10% leakage then look for where it is leaking.
     

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