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Thread: 390 cylinder compression

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  1. #1
    Littleport22's Avatar
    Littleport22 is offline CHR Member Visit my Photo Gallery
    Join Date
    Sep 2005
    Little Rock
    Car Year, Make, Model: 41 Ford Pickup

    390 cylinder compression


    I think I have a leaking valve guide seal causing some smoke. I hope it's a valve and not the piston rings. Just to be sure I want to check the cylinder compression. Does anyone know what the cylinder compression should read on a 1964 390ci?

    Would leaking valve seals effect pressure? Would this test really eliminate the piston rings, or would low compression be read even on a leaking valve? How can I rule one or the other out? HELP?????

  2. #2
    76GMC1500 is offline CHR Member Visit my Photo Gallery
    Join Date
    Aug 2003

    I believe most stock V8's were upto 150 psi cranking pressure. Remove all spark plugs or the engine will not spin fast enough on the starter to get an accurate reading. Anything from 130ish-165 is acceptable if nominal is 150. A bad valve guide or leaking valve seal will not show on a compression test. A worn out engine with leaking rings will show low on all cylinders. Add a little oil through the spark plug hole. If compression is still low, it is a bad valve. If it comes up to 150 or so, it's bad rings.

  3. #3
    FFR428's Avatar
    FFR428 is offline CHR Member Visit my Photo Gallery
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    Mar 2004
    Car Year, Make, Model: 68 Cougar S code, 427 Tunnelport.

    64 390 comp 180 lbs. 10.1 compression.

  4. #4
    R Pope is offline CHR Member Visit my Photo Gallery
    Join Date
    Jun 2005

    A leakdown test is more revealing of engine problems than a compression test, you can hear where the air goes.

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