Lycoming had not been successful in designing a high power engine. They had started with an attempt to make a hyper engine that led to the 1,200 hp (890 kW) O-1230, but by the time the engine was ready new aircraft designs were all calling for more power. They tried again by "twinning" the engine to produce the H block H-2470, which saw some interest in the Vultee XP-54 Swoose Goose project. Work on the H-2470 ended when the XP-54 was cancelled.
In one final attempt, Lycoming decided to go all out and build the largest engine in the world. They put together a team under the direction of VP of Engineering Clarence Wiegman at their main Williamsport factory in the summer of 1943 and started work.
The resulting design used four banks of nine cylinders arranged around a central crankshaft to form a four-row radial engine. Unlike most multi-row radials, which splay the cylinders to allow cooling air to reach them, the 7755 was water cooled and so the cylinder heads were in-line under a cooling jacket. Contrast this with the Junkers Jumo 222, which looked similar from the outside but ran on a V-style cycle instead of a radial. The 7755 was ten feet (3 m) long, five feet (1.5 m) in diameter, and weighed 6,050 pounds (2740 kg). At full power it was to produce 5,000 hp (3.7 MW) at 2,600 rpm, maintaining that with a turbocharger to a critical altitude that was apparently never published.
Each cylinder bank had a single overhead cam powering the poppet valves. The camshaft included two sets of cams, one for full takeoff power, and another for economical cruise. The pilot could select between the two settings, which would shift the camshaft along its axis to bring the other set of cams over the valve stems. Interestingly the design mounted some of the accessories on the "front side" of the camshafts, namely two magnetos and four distributors. The seventh camshaft was not used in this fashion, its location on the front of the engine was used to feed oil to the propeller reduction gearing.
The original 7755-1 design drove a single propeller, but even on the largest aircraft the propeller needed to absorb the power would have been ridiculously large. This led to a minor redesign that produced the 7755-3, using a new propeller gearing system driving two shafts to power a set of contra-rotating props. The propeller reduction gearing also had two speed settings to allow for a greater range of operating power than adjustable props alone could deliver. Another minor modification resulted in the 7755-5, the only change being the replacement of carburetors with a new fuel injection system.
The engine first started testing at 5,000 hp (3.7 MW) in 1944 with the XR-7755-3, but demonstrated terrible reliability problems. A second example was provided, as planned, to the Army Air Force at Wright Field in 1946. However, by this time the Air Force had lost interest in new piston designs due to the introduction of jet engines, and the Lycoming delivery team was instructed to simply "dump it on the ground". This engine has since disappeared. Luckily the original test engine was later delivered to the Smithsonian Institution, where it was recently restored.
Studebaker was also working on a very large engine, a 24 cylinder H-type. The H-9350 (8.0 x 7.75 = 9349.4 cuin/153.21 L) was being designed to deliver over 5,000 hp. It was probably not built.