 In the case of the V-1650, used from the P-51B on, two sets of power ratings can be quoted because these engines were fitted with two-stage, two-speed superchargers; the maximum power generated by the V-1650-3 was at the critical altitude of in "low blower" using of mercury (61" Hg) "boost".The "boost" is the pressure to which the air–fuel mixture is compressed before being fed through to the engine's cylinders (manifold pressure).Because air pressure and air density decrease with altitude, the efficiency of a piston engine drops because of the reduction in the amount of air that can be drawn into the engine; for example, the air density at is ⅓ of that at sea level, thus only ⅓ of the amount of air can be drawn into the cylinder and only ⅓ of the fuel can be burnt.