Might get better response in the "technical" forum. But before you go there, define what you're asking about, manifold vacuum? At the risk of oversimplifying, manifold vacuum will always drop on a normally aspirated engine when you open the throttle and then come back up as the engine stabilizes. Vacuum will be different at all RPM's and throttle settings. Reduced vacuum during acceleration and at WOT is what allows the power valve on a Holley carburator to funtion.