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compression test

ksurfer2

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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!
 
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 ...
 
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.
 
I pull all plugs, pin the throttle to WOT and leave the fuel pump and ignition off.
 
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.
 
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
 
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
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then squirt some oil in and recheck
oil will seal the rings but will not change a bad valve :)
 
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.
 
I'd do all as above but first I cold check my valve clearances .
 
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