Arguably, the best piston aircraft engine ever built, the “Merlin” powered all Royal Air Force (RAF) front-line aircraft during World War II. In 1942, the P-51 “Mustang’s” engine was replaced with the “Merlin,” greatly increasing its capabilities and making it one of the best fighter aircraft in history. Early models of the “Merlin” produced 900 hp and later models over 1,200 hp.