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  •  » Odd behavior during compression test (to me at least)

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    9/14/2014 10:28 AM  #1

    Big Tag
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    Registered: 8/13/2013
    Posts: 61

    Odd behavior during compression test (to me at least)

    Ok, so did a compression test on the 302 stoked to 331 today (still having trouble, pretty sure it's the carb running too rich due to lower vacuum... duh! Carb calibration kit gets in Tuesday)

    And I saw some behavior that seemed odd to me, maybe (hopefully???) it's normal.  

    Cyl. 1-3 and 5-7 all had 90-100 psi on the first pump, then 160 or so as a max.

    However, 4 and 8 did somethign different.  Half the time they did the same, some times they would initall hit as high as 130 and go as high as 170-180 for a max.  I also noticed that with all the plugs removed and cranking, plenty of gas would come out of the sprak plug holes on 4 and 8, and none on the others.  Seems a little too convienent that both cylinders in the same position, and the end position at that, would do the exact same, different thing.  Anyone else see this before?

    And a secondary point, those readings seem ok?  As mentioned, 331... mild-aggressive  Edelbrock cam (not their base, 2nd from the base), ~11:1 compression ratio

    BT

     

    9/14/2014 11:20 AM  #2

    GPatrick
    Senior 24-7 FYI Addict

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    Registered: 4/01/2013
    Posts: 1,132

    Re: Odd behavior during compression test (to me at least)

    Cylinders may be washed with fuel.  Did you have the choke wide open and the throttle wide open for your tests?  

     

    9/14/2014 1:51 PM  #3

    Big Tag
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    Registered: 8/13/2013
    Posts: 61

    Re: Odd behavior during compression test (to me at least)

    I did have a head-slap moment afterwards - throttle closed and choke closed (engine cold - no start) but I rechecked some of the cyl. after correcting the situation ont he end and got the same readings (prob. too late).  

    But even so - why the fuel spraying out of 4 and 8 but not any of the others?  I fifure this is also why 4 and 8 read differently.

       •Thread Starter
     

    9/14/2014 3:59 PM  #4

    BobE
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    From: White Plains, NY
    Registered: 4/04/2013
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    Re: Odd behavior during compression test (to me at least)

    Regarding 4 & 8; maybe the car was on a slight incline towards the rear and the gas run to the rear cylinders (?)
    Note that the gas going to these cylinders could have created a seal on the rings, giving a higher reading in those cylinders.  (One method to check if low compression is caused by the rings or the valves is to put some oil in the cylinder and re-test.  If pressure goes up, the problem is the rings.)
    Also, how radical of a camshaft will effect the pressure seen by the compression test.  Typically what you're looking for is consistency of the pressure readings w/no more than a 10% variation between cylinders.

    65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.
     

    9/14/2014 5:27 PM  #5

    MS
    Forum Originator

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    From: Canyon Lake, Texas
    Registered: 1/24/2013
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    Re: Odd behavior during compression test (to me at least)

    I spin the engine for eight revolutions and read the gauge.  No worry about the first couple of pumps.  You just want to know the capability of the cylinder system.  Differences in first pump can have differences due to different runner length on the intake,how much gas or oil is in that cylinder etc.

    If you have real concerns about the engine condition, do a leakdown test rather than a compression test.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    9/14/2014 6:53 PM  #6

    barnett468
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    Registered: 9/15/2014
    Posts: 770

    Re: Odd behavior during compression test (to me at least)

    Big Tag wrote:

    Ok, so did a compression test on the 302 stoked to 331 today (still having trouble, pretty sure it's the carb running too rich due to lower vacuum... duh! Carb calibration kit gets in Tuesday)

    And I saw some behavior that seemed odd to me, maybe (hopefully???) it's normal.  

    Cyl. 1-3 and 5-7 all had 90-100 psi on the first pump, then 160 or so as a max.

    However, 4 and 8 did somethign different.  Half the time they did the same, some times they would initall hit as high as 130 and go as high as 170-180 for a max.  I also noticed that with all the plugs removed and cranking, plenty of gas would come out of the sprak plug holes on 4 and 8, and none on the others.  Seems a little too convienent that both cylinders in the same position, and the end position at that, would do the exact same, different thing.  Anyone else see this before?

    And a secondary point, those readings seem ok?  As mentioned, 331... mild-aggressive  Edelbrock cam (not their base, 2nd from the base), ~11:1 compression ratio

    BT

    hello;

    why are you doing the test?

    how was it runnung befire?

    what color are your plugs?

    exactly what carb do you have?

    if its a holley, set the gas level so it is just below the inspection holes while it is runnimg.

    disconnect dist vac hose and plug.

    what is your timimg at idle?

    at 1600 rpm?

    what is max timing?

    what rpm is max timing reached?

    what octane are you using?
    .
     

     

    9/15/2014 5:28 AM  #7

    jkordzi
    I FYI 24-7

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    From: Sequim, WA
    Registered: 12/28/2013
    Posts: 927

    Re: Odd behavior during compression test (to me at least)

    Big Tag wrote:

    Ok, so did a compression test on the 302 stoked to 331 today (still having trouble, pretty sure it's the carb running too rich due to lower vacuum... duh! Carb calibration kit gets in Tuesday)

    And I saw some behavior that seemed odd to me, maybe (hopefully???) it's normal.  

    Cyl. 1-3 and 5-7 all had 90-100 psi on the first pump, then 160 or so as a max.

    However, 4 and 8 did somethign different.  Half the time they did the same, some times they would initall hit as high as 130 and go as high as 170-180 for a max.  I also noticed that with all the plugs removed and cranking, plenty of gas would come out of the sprak plug holes on 4 and 8, and none on the others.  Seems a little too convienent that both cylinders in the same position, and the end position at that, would do the exact same, different thing.  Anyone else see this before?

    And a secondary point, those readings seem ok?  As mentioned, 331... mild-aggressive  Edelbrock cam (not their base, 2nd from the base), ~11:1 compression ratio

    BT

    I don't think this is the problem, but I'd make sure all the valves are adjusted properly as well.

    Cheap, Fast, Good:  Pick Any Two
     

     

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