Oh, yeah...the Lycoming R-7755!
-7,755ci
-~5,000BHP
-0.64 hp/in³
-0.82 hp/lb
-10' x 5', over 6,000lbs
-36 cylinders, 4 rows, 9 cylinders per row, not rotated (9 straight banks, not "corncobbed" like the R-4360)
-Liquid cooled, turbosupercharged
-SOHC with 2 separate cams, selectable by the pilot, for economy cruise, and full takeoff power (variable valve timing!)
-XR-7755-2 had twin-output gear reduction box for contra-rotating props
-The reduction box itself had two speeds, since the was to much power to utilize by adjusting the props alone.
-Damn big azz engine.
It was delivered to the USAF, who (according to Wikipedia) told them to "dump it on the ground". It disappeared, only the single test engine was saved.
Nice, ey? Although you should check out the Russian "Zvezda M503": 7 x 6-row 42-cylinder diesel marine radial! Liquid cooled, 8,763ci. Not much of an aircraft engine (4,000BHP + 12,000lbs = 0.32 hp/lb! (vs the R-7755 @ 0.82hp/lb)