In normally aspirated engines the throttle sets the manifold pressure, and the manifold pressure will always be lower than the ambient pressure due to losses in the air induction system. For a given throttle setting the manifold pressure, and therefore the power, will decrease as altitude increases, because ambient pressure decreases with altitude, see black dotted line in fig. PO 8.1. Power will be increased if the manifold pressure is increased, since more air and hence more fuel can be burned per unit time. The power increases because more chemical energy is released per time unit. An increased manifold pressure can be obtained with turbochargers or superchargers. These units are called power augmentation devices.A supercharger is an internal centrifugal compressormechanically driven from the engine crankshaft, usually through a gear train.