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Originally Posted by Deltona Couple
OK, lets see if I can clairify my opinion:
What I am saying is that in a NA engine, where no movement of the piston is present, and the intake valve is open, we have a ballance of pressure on both sides(14.7 psi at sea level). when the piston moves downward, it creates a "negative pressure" or vacuum in laymans terms. This difference in pressure causes the outside air to be drawn in. So in MY opinion, it is the creation of "negative pressure" that causes the air to move, not just the static pressure of atmospheric pressure that moves itself. This leads me to the hypothosis that it is the VACUUM that is the action, and the air movement is the reaction.
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If you were getting a negative reading, you weren't measuring absolute pressure. If you had been, there wouldn't be any negative pressure (there's no such thing), there would have just been some positive pressure lower than atmospheric. This isn't an action/reaction situation, it's a definition of terms. The piston creates a low pressure area that the atmospheric pressure fills. I'm not trying to tell you you don't undnerstand an engine--For most applications, your understanding is close enough to correct as to not matter. What I'm saying is that if it gets down to the brass tacks of the definition, like it did on this test, it's the pressure that does it.