[chopper]

Um, if I may, manifold vacuum is caused by the pistons pulling air into the engine past the restriction of the throttle plates. Most passenger car engines (ie, non-high performance with little overlap) at idle will pull approximately 18-20 inches of manifold vacuum. Higher performance engines with high-overlap camshafts pull less manifold vacuum at idle due to the overlap. That is, with both valves open while the piston is moving downward in the bore, the cylinder is pulling less air through the intake than it would if the exhaust valve were closed; hence less manifold vacuum. My car, for example, idles at about 10.5 inches of manifold vacuum at 900 rpm.