Reply To: compression 57 3 hp Johnson

[David Stanton]

Hey Aquasonic. Let me try another way of explaining. I am very familiar with Pascal’s law, and you are right in that the pressure generated against cylinder combustion surfaces is the same as that in the hose, but please understand that with a hose on the gauge and the checkvalve up at the gauge, a piston stroke is pushing all that air into a larger space, consisting of the volume of the upper combustion chamber + the volume of the hose, so that air can’t compress to as many pounds per square inch in the first place because it’s being squeezed into a bigger place. Exaggeration may help understand. Imagine if the hose was two inches in diameter and you are pushing one piston stroke of air into both it and the combustion chamber. Would you get much of a reading?

To use your words you used describing an added head gasket, the same applies to the added hoseby adding the volume of the hose “you have changed the characteristics of source of the compression by increasing the volume of compressible air”. The only difference with adding the hose is that while the air in the hose is not in the combustion chamber – it is still part of the equation, adding to the volume of air being compressed. So compression pounds will be less, just as adding a thicker headgasket does.

I know it can be confusing, but I have two each of both kinds of compression gauges, and the results are very repeatable. I make sure I don’t use one with a gauge-end checkvalve on small cylinder engines. If the checkvalve is at the engine end of the hose, the readings are accurate. When I test my gauges for accuracy by applying a continuous pressure with my air compressor, they all read alike
Dave