I dont know where the  HP number came from the old singleengine number I remember was similar to the HP of a small aircraft engine  You omitted the M would have eight engines rather than the number of engines in the RX  Only four engines would be needed for vertical landing when flying with the aerodynamics of a brick is relevant but all eight would usually be used at less than full power during vertical flight when the dual redundancy in each engine pod is required  The intent of the design is for a single engine in a pod at full power to be able to handle the load if one engine fails  When in horizontal flight full power from all engines would not normally be required  Also the Rotapower engines have an ancestry stemming from marine engines which may be somewhat different from Mazdas  Comparison with anything else is indeed difficult