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Have you ever wondered how large an aircraft engine can be? Imagine an engine with ten times the displacement of your family car. This engineering marvel exists—the Lycoming XR-7755, a true behemoth in aviation history.

The Lycoming XR-7755 holds the distinction of being the largest piston aircraft engine ever constructed in the United States. Its staggering specifications speak volumes: 36 cylinders arranged in four banks, with a total displacement of 7,750 cubic inches (approximately 127 liters). To put this in perspective, Lycoming's current largest production engine measures just 720 cubic inches—making the XR-7755 more than ten times larger.

This mechanical giant generated an astonishing 5,000 horsepower, earning its reputation as the "Hulk" of aviation. The engine's physical dimensions were equally impressive: approximately 10 feet long (3 meters), 5 feet in diameter (1.5 meters), and weighing nearly 3 tons—comparable to a small truck.

The XR-7755 was developed to power the Convair B-36 "Peacemaker," a strategic bomber designed to fly from U.S. soil to European targets without refueling. The Air Force demanded unprecedented power and reliability for this intercontinental mission.

Despite its technical achievements, the XR-7755 never entered full production. The B-36 ultimately adopted Pratt & Whitney's R-4360 "Wasp Major" engine—a 28-cylinder radial design that offered slightly less power but proved more maintainable and reliable for operational use.

Though never deployed, the XR-7755 represents the pinnacle of piston engine technology. Its development pushed mechanical boundaries and informed future aviation propulsion systems. The project demonstrated engineers' willingness to challenge conventional limits during an era of rapid aeronautical advancement.

Today, surviving XR-7755 specimens are displayed in select aviation museums as testaments to this transitional period between piston and jet propulsion. These rare artifacts preserve the memory of an ambitious engineering endeavor that helped shape modern aviation.