War emergency power

War Emergency Power (WEP) is an American term for a throttle setting on some World War II military aircraft engines. For use in emergency situations, it produced more than 100% of the engine's normal rated power for a limited amount of time, often about five minutes. Similar systems used by non-US forces are now often referred to as WEP as well, although they may not have been at the time, as with the German Luftwaffe's Notleistung systems.

Maximum normal power would be limited by a mechanical stop, for instance a wire across the throttle lever slot, but a more forceful push would break the wire, allowing extra power. In normal service, the P-51H Mustang was rated at 1,380 hp, but WEP would deliver up to 2,218 hp. In the P-51D Mustang, the model most produced and used during World War II, the WEP increased the HP from 1490 to 1720. The Vought F4U Corsair, not originally equipped for WEP, later boasted a power increase of up to 410 hp (17%) when WEP was engaged. Several methods were used to boost engine power by manufacturers, including water injection and methanol-water injection. Some earlier engines simply allowed the throttle to open wider than normal, allowing more air to flow through the intake. All WEP methods result in greater-than-usual stresses on the engine, and correspond to a reduced engine lifetime. For some airplanes, such as the P-51D, use of WEP required that the engine be inspected for damage before returning to the air. 5 hours use of WEP on the P51D required a complete tear down inspection.

British and Commonwealth aircraft could increase power by increasing the supercharger boost pressure. This modification was common by the summer of 1940, with the widespread availability of 100 octane fuel. Raising supercharger boost pressure from 6 lb to 12 lb increased the Merlin III engine rating to 1310 hp, an increase of over 250 hp. Pilots had to log the use of emergency boost and were advised not to use it for more than 5 minutes continuously.

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