I understand what air density is.I also understand that closing the throttle blades restricts airflow thru the carb.I was refering to the density of the A/Fmixture.The richness of the mixture.Engine displacement never changes.Rpm and load does.The faster an engine turns the faster the air will travel thru the venturi and the more fuel will flow thru the main jet,making the mixture richer(denser)and allowing increased cylinder pressure and therefore power output.Engine speed is determined by throttle position,engine displacement remains constant.