That's probably flywheel HP under full throttle- - -full load at the rated RPM, without accessories such as the water pump, generator or other bolt-on devices. Pick an RPM, start applying load and increasing the throttle opening, until any further load drops the Revs. Measure the torque output, and calculate HP, taking into account ambient air temperature, altitude, barometric pressure, and relative humidity. I've run many hundreds of dyno tests that way.
Jerry
