Overlap Kills Vacuum--Here's Why
Depicted here are two consecutively inducing cylinders of a V-8. The right-hand cylinder is halfway through the induction stroke and is moving down the bore at near its maximum speed. The left-hand cylinder is in the overlap period. Because a single-plane intake allows all cylinders to communicate, it is easier for the right-hand cylinder to draw in exhaust from the exhaust port of the left cylinder through the open valves. The smaller of the red circles is about equal to the overlap through the flow area of a 280-degree cam on a 108 LCA while the larger one is equal to a 300 race profile on a 108 LCA. The blue circle represents the through-flow area of a four-barrel cab at idle. The overlap area looks like a large hole in the intake, as far as generating vacuum is concerned.