Directly from our manual.
Quote:
The Packard Merlin V-1650-7 used in this P-51D was a potent engine and a good performer at high altitudes. The two-stage supercharger did a good job of maintaining power up to moderately high altitudes and its operation was normally automatic. The maximum allowable manifold pressure for this aircraft is 67", which is considered "War Emergency Power" or "Combat Power". This setting was to be used for only five minutes at a time. The normal maximum power for takeoff is 61" at 3000 RPM. There is no War Emergency Power switch on this aircraft. Just use the throttle to set the appropriate manifold pressure.