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Originally Posted by twistedmosaic
No, they're right, the piston moving down produces a low pressure area that is filled by the atmospheric pressure pushing air in. It seems like a trivial distinction, but it is helpful to know an NA car recieves air at 14PSI when you're doing turbo or supercharger calculations (at a very very crude level, 14psi = twice as much air = twice as much horsepower, allowing for proper support mods.)
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Sorry, but for the record I will just have to disagree with you on this one. I see the point you are trying to make, but in a real world situation, there is "14psi" of normal atmospheric pressure at all times at sea level. so a moving piston would create a vacuum based on the principal of air flow through a restriction (i.e. the open valve) If you were to actually connect a vacuum/pressure gauge to the cylinder you would see the fluctuations of vacuum (and pressure when on compression stroke). We did this experiment many years ago in our physics class when I was getting my degree in automotive technology. The question we had was based on a naturally aspirated engine, turning at normal cranking RPM of a standard V-8 engine. The solution was that since the movement of air at atmospheric pressure was too slow, a vacuum HAD to be introduced by the piston, otherwise the cylinder could not fill up fast enough to produce proper combustion. (based on volume of air that can be moved at atmospheric pressure, v/s volume of air that can be moved under vacuum)