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Compression Testing Issue

Think of it this way. lets say a bicycle tire takes 35 psi to fill. It will not take long to fill using a air compressor creating say 85 PSI and if not careful tube will explode and possibly tire. Now take a 31x10.50 pick up tire. It will fill to 35 PSI but will take a very long time to fill. This example if filling both tires that are flat. Keeping in mind 35 PSI is the fill pressure. So the end result is both tires end up with 35 PSI. This is why length of hose should not have or make a difference in finally reading of compression in cylinder for both reading should be the same using same gauge but different hoses. So it comes down to this that a old time mechanic told me once, " You can not blame a bad job on your tools."
 
Another way to look at it. Although not directly related; hydraulic pressure vis air pressure. Consider the location of an oil pressure gauge; same principal except dealing with a "column" of fluid instead of air. Would a gauge located 24" away from the port connection than a gauge located 12" away from the port connection?o_O

A more tangible reason for the difference, IMHO, would be that there was a leak or blockage somewhere in the hose connection. The accuracy of the gauge should be confirmed just because it is important to know whether or not one is working with an accurate tool. I would suggest that you get your hands on another hose known to produce accurate results and re-test to verify the integrity of the 18" flex hose/connection used in the first test.;)
 
I would suspect the quality of the hose in this problem

Brian
 
dolt said:
Consider the location of an oil pressure gauge; same principal except dealing with a "column" of fluid instead of air. Would a gauge located 24" away from the port connection read less pressure than a gauge located 12" away from the port connection?o_O
Click to expand...

Too late to edit. I think everyone here is smart enough to read between the lines but I need to clean up my previous post.:rolleyes:
 
dolt said:
Another way to look at it. Although not directly related; hydraulic pressure vis air pressure. Consider the location of an oil pressure gauge; same principal except dealing with a "column" of fluid instead of air. Would a gauge located 24" away from the port connection than a gauge located 12" away from the port connection?o_O
Click to expand...
dolt;
I get what you are saying, but in my mind there is a BIG difference in your example and the case of a compression test using a flex hose.
First, the oil pressure situation is steady state more or less; ie no flow. A compression test is a dynamic changing compression/expansion situation.
Second, and more important, air is compressible, whereas oil (as with any other liquid) is essentially incompressible.
As I see it during the compression stroke the volume of air above the piston as well as the volume in the hose is being compressed until it overcomes the gauge release valve (depending on the pressure already on the gauge), whereupon some flows into the gauge and raises the pressure.
Then on the piston downstroke the same volume of air above the piston INCLUDING THAT IN THE HOSE, up to the release valve, is expanding again down to 0 psig. This would be the same as putting the gauge right in the spark plug hole (if that was possible), and increasing the head chamber volume by the same amount as the volume of the hose. In other words the hose volume is acting as a part of the head squish volume, and the net effect is the observed CCR is lower than it should be, just as if the head chamber was bigger than it actually is.
The situation would be different if the gauge release check valve was at the base of the hose, say right at the spark plug hole hypothetically. In this case once compressed air has passed the valve on the compression stroke, it will stay there in the hose and gauge (still compressed) on the decompression stroke. CCR readings would be accurate in this case I believe.
I don't know how else to describe it.

I just had another idea.
Instead of endless dialog about it, how about if someone out there who has a tester equipped with both types of adapters, runs a test on his/her bike both ways, and let us know what you find out?
 
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Come on guys, I qualified my post recognizing that the comparison between hydraulic and air pressure was illustrative of a principal and not a direct comparison. While it is true that air is compressible, it is until it isn't. Not sure how volume has anything to do with measuring pressure. Make the line pumping oil to the pressure gauge larger to pump more volume but the gauge will read the same pressure.;)

CCP is what it is and the distance between the spark plug hole and the gauge does not change CCP.:confused:
 
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Sorry dolt; I did not mean to put you into a corner! :)
As an aside, could I ask a favor?
Would you mind running my specifics through your calculator and see what you get for SCR, CCR, and CCP, just for comparison?
103 build, flat tops, 255 cams, .045 gaskets, assume 85 cc chambers.
Thanks!
 
Too much over thinking, no need to re-invent the wheel.
Pull plugs, thread in compression gauge hose (length is a non issue) open throttle, crank engine several revolutions and read results.

DONE. :)
 
Hose length did not make any difference, I checked my Snap On set 3 different length hoses No change :)
 
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