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Compression Test???

Discussion in 'Intermediate CJ-5/6/7/8' started by Chuck, Oct 18, 2004.

  1. Oct 18, 2004
    Chuck

    Chuck Sponsor

    Southshore Ma
    Joined:
    Sep 19, 2004
    Messages:
    1,394
    I will be doing a compression test on my 79 /I6 and would like to know the acceptable range per cylinder?
     
  2. Oct 18, 2004
    jpflat2a

    jpflat2a what's that noise?

    Riverside CA
    Joined:
    Jul 30, 2003
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    8,021
    my 81 manual shows 120lbs-150lbs acceptable with 30lbs being max varitation between cylinders
    should be close for the 1979 model as well
     
  3. Oct 18, 2004
    Chuck

    Chuck Sponsor

    Southshore Ma
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    Thanks Jim.
     
  4. Oct 19, 2004
    Chuck

    Chuck Sponsor

    Southshore Ma
    Joined:
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    Jim, after your helpful answer I researched it a bit more and also came up with 150psi specifications with 135psi-165psi being in the normal range. Thanks.
     
  5. Oct 19, 2004
    72CJ5er

    72CJ5er Renegade Member

    Hampton, TN
    Joined:
    Mar 31, 2004
    Messages:
    151
    I read somewhere that there was some kind of conversion where you put in a thicker head gasket and lowered the pressure to around the 110 - 120 mark. I dont recall why but I do remember reading this. mine was 125 average with 10 +- on all cylinders
     
  6. Oct 19, 2004
    timgr

    timgr Jeepin' Nerd 2020 Sponsor 2019 Sponsor

    Medford Mass USA
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    Aug 10, 2003
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    21,075
    You can calculate what the head gasket thickness is from the compression ratio, combustion chamber volume, bore, stroke, and deck clearance. Use the relationship P1*V1 = P2*V2, and include the atmospheric pressure. I'd be happy to do if for you, but it's a quadratic in gasket thickness and I don't have time to follow through right now.

    258 combustion chamber volume: wedge head 64cc, quench head 68.5cc
    258 deck clearance: 0.110" wedge head, 0.069" quench head.
    258 bore, stroke: 3.75", 3.895"

    The spec for my engine is 150psi, max variation 20psi between cylinders, and I measured 160/160/160/150/155/152psi. This is within spec (on the high side - lucky me!)
     

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