RPM and flow are only a gauge of how much air the motor is pumping. BSFC determines the amount of fuel an engine will require to pump that air.
The number of factors that go into BSFC (Brake Specific Fuel Consumption) are all of the internal factors of the motor PLUS the external load placed on it. One of those factors is friction. Anyone knows that a six cylinder engine has only about 3/4 the internal friction of a V-8. And, with a lighter load on the engine since it's hauling around less weight, less friction in all the bearings from flywheel to axles, plus the reduced road friction ... well, HP for HP, the OHC is going to do it for less fuel.
