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Proper Compression Testing

Posted by BLACK'98DSM, Sep 23, 2019

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  1. BLACK'98DSM

    BLACK'98DSM Proven Member

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    Is it required to open the throttle when compression testing?

    If you don't, will the numbers be too high or too low?
     

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  2. Vegas smith

    Vegas smith Proven Member

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    I've never actually opened the throttle on any of my tests but it's the recommended method.
     

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  3. BLACK'98DSM

    BLACK'98DSM Proven Member

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    Neither have I, that's why I'm asking LOL
     

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  4. silvreclips

    silvreclips Proven Member

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    I think overall you will get very similar numbers. But it may take more cranks to get it there
     

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  5. tk106

    tk106 Supporting Member

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    Open the throttle, you'll get there faster.
     

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  6. ec17pse

    ec17pse Freelancer

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    When i did a comp test i just cranked the engine and the TB was closed, it will always take a few cycles to rise anyway, never tried it open on the TB so i cannot say if its a good or bad thing to do
     

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  7. stillrunning

    stillrunning Proven Member

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    You are trying to fill the cylinder with air which will be compressed by the piston.
    Best way to achieve that is to eliminate the restriction create by the throttle blade and let the cylinder breath as naturally/easily as possible.
     

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  8. rabenne

    rabenne Proven Member

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    Both ways work so who cares.The old rule of thumb is to block the throttle fully open. That is also what it says in "the books". For the reasons stated above.

    Do either one, I think the more important thing is to do the same procedure on each cylinder to minimize differences you introduce into the test. Typically you are supposed to crank the engine through the same number of compression strokes while testing each cylinder. This is a more accurate measure of cylinder compression balance when compared with today's standard advice "crank it until it stops going up".

    If it takes 10 compression strokes to reach 175 psi on the gauge for cylinder 1 and 5 compression strokes on cylinder 2, you should probably repeat the test after dropping a bit of oil in the cylinders (wet compression test).

    That all assumes consistent cranking speed as well. If possible, hook up a strong battery charger to ensure the battery isnt getting weak by the last cylinder.

    Best of luck!
     

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  9. BLACK'98DSM

    BLACK'98DSM Proven Member

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    I've already compression tested. I was just wondering why people said to leave the throttle open.

    Thanks again guys!
     

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