An extremly intricate valve system for virtually no gain in power but a slightly lower engine weight of 1925 lb (strangly enough). The Hercules power output was 1375 hp from a 2360 cu in engine. The closest I could find readily was the Curtis Wright R-2600 which put out about 1500 hp at a weight of 2054 lb. The valve timing on a sleeve valve engine was more involved than assembling an old Swiss watch although the poppet valve engine had timing gears and a set of cam rings with lobes to lift the cam followers. I think that the close proximity of the Hercules spark plugs was one reason for the reduced power output of this type, even though the port openings on a sleeve valve engine was bigger than on an equivalent poppet valve engine.