I tried to make it simple. There is a lot more involved,the piston on the downstroke creates vacuum,the exhaust manifolds are tuned to create a vacuum to help each suceeding fired cly gases to be drawn out. In the presense of a vacuum atmospheric pressure can push air in,to a point.When an engine is running at 3,000 rpm. An intake valve is only open for a few thousandths of a second,a strong vacuum is needed to draw in enough air.With atmospheric pressure no air will move,the intake manifold-air chamber is at atmospheric pressure already,with the engine off.You need six or more times the volume of air than atmospheric pressure can provide.If you use a vacuum gauge on an engine,atmospheric pressure is read as "0" depending on engine,speed,ect,you may get a reading of 15 to 30 in of mercury above atmospheric pressure,created by vacuum.If you could open up a small hole in the intake manifold to atmospheric pressure, the engine will falter and run rough.Open up a big hole,eliminat! ing vacuum and the engine will die. Stephen
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