1. ksurfer2

    ksurfer2 Well-Known Member

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    I am going to do a compression test on my 440 tomorrow. A couple of questions.... Should ALL spark plugs be removed prior to testing? Is it necessary to shut off fuel to the carb prior to testing? If so, what is the best way to do so? Finally, what would a good range of pressure readings be?

    Thanks!
     
  2. 65-440

    65-440 Well-Known Member

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    All plugs don't need to be out, but it will spin over easier ... not necessary to mess with the fuel, won't shoot into the carb unless you hit the throttle ...
     
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    • 69 GTX

      69 GTX FBBO Gold Member FBBO Gold Member

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      You should do it at WOT, the butterflies will restrict air coming in, gauge should be 100-300 range, you'll probably be at 150-180 depending on pistons & year, smog motors 30psi less maybe. I typically like the leak down gauges which measure leak down. put 100psi on the regulator and what ever the other gauge reads that's your percentage 100/92 would be 8% leak down.
       
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      • Hemirunner

        Hemirunner Well-Known Member

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        I pull all plugs, pin the throttle to WOT and leave the fuel pump and ignition off.
         
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        • Don Frelier

          Don Frelier FBBO Gold Member FBBO Gold Member

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          Pull all the plugs, throttle wide open. I would clamp off the fuel line but not necessary since with the plugs out there is little suction. The readings can vary depending on the cam. They should all be over 125lb hopefully higher. All within a 15 lb range would be nice.
           
        • beanhead

          beanhead Well-Known Member

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          Yes agreed. Plugs out, throttle wired full open...now, it may suck, but it is better to get the engine hot first and then pull your plugs and do the test as soon as it's cool enough to work around. The normal procedure for testing a cold engine, is to put a little bit of oil in the cylinder first.. I usually end up getting oil places I don't want it when I've done that, so I just let the pipes cool off a little bit, wear gloves, and go to it! You want no more than 10% variation in psi between cylinders regardless of the numbers
           
        • dvw

          dvw Well-Known Member

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          Warm the motor up good. Why test in an enviroment where it's not going to matter (cold). Ring seal, valve lash will change with temperature. Compression numbers are subjective to engine speed. The faster it cranks (no plugs so no compression in the other cylinders) the higher the number. Late closing intake will make numbers low (big cam). So look for even numbers more than high numbers.
          Doug
          .
           
        • wyrmrider

          wyrmrider Well-Known Member

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          then squirt some oil in and recheck
          oil will seal the rings but will not change a bad valve :)
           
        • miller

          miller Well-Known Member

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          Something not mentioned, and something I was taught, and the ONLY way to get accurate compression reads...

          Yes, throttle wide open. Yes, all plugs out. Yeah, pull the coil wire, so it doesn't fire.
          For each cylinder, allow the cylinder to 'hit' the compression gauge three times. You'll notice, for each 'hit', the reading will go up slightly. So, three hits, will give the best accurate reading...all that cylinder has.
          Two cents worth.
           
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          • 1962polara

            1962polara Well-Known Member

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            I'd do all as above but first I cold check my valve clearances .
             
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