Specifications
[P-51D Mustang]
DCS
EAGLE DYNAMICS
33
rest. It compensates automatically for the difference in air density at different altitudes by gradually
opening the carburetor butterfly valve in a climb or smoothly closing it in a descent.
Carburetor air comes through a long carburetor air scoop directly under the engine. The aircraft’s
motion forces air at high speed (or rams it) directly into the carburetor. This is termed ram air.
Figure 16: Ramming Effect
If the scoop becomes obstructed by ice or other foreign matter, a door in the air duct opens
automatically to admit hot air from the engine compartment to the carburetor.
During normal operations, ram air is always used, but in the event of extreme icing or dust
conditions, the carburetor air controls on the left cockpit pedestal allow the pilot to select either
unrammed filtered or, in later model aircraft, unrammed hot air for operation. In order to obtain hot
air, the Hot Air control handle must be set to HOT and the Cold Air control handle set to UNRAMMED
FILTERED AIR. If the Cold Air control handle is set to RAM AIR, the hot air control will be ineffective.
Hot air should not be used above 12,000 feet. At high altitudes its use affects the carburetor’s
altitude compensation and may result in an overly lean fuel mixture.
War Emergency Power
In order to provide an extra boost to the engine in extreme situations, the throttle can be moved
past the gate stop by the quadrant to break the safety wire. The engine will then be opened up to its
absolute limit and will give approximately 6 in. of additional manifold pressure in excess of the
normal full throttle setting of 61 in. (with mixture control set to RUN or AUTO RICH and prop set for
3000 RPM.) This throttle reserve is called War Emergency Power (WEP) and should be used only in
extreme situations. If used for more than 5 minutes at a time, vital parts of the engine may be
damaged.
WEP provides no benefit at altitudes below 5,000 feet. The throttle alone provides more than enough
power to exceed the operating limits of the engine at these altitudes.