A naturally-aspirated, Miller cycle, Spark-Ignition (SI) engine that controls output with variable intake valve closure is compared to a conventionally-throttled engine using computer simulation. Based on First and Second Law analyses, the two load control strategies are compared in detail through one thermodynamic cycle at light load conditions and over a wide range of loads at 2000 rpm. The Miller Cycle engine can use late intake valve closure (LIVC) to control indicated output down to 35% of the maximum, but requires supplemental throttling at lighter loads. The First Law analysis shows that the Miller cycle increases indicated thermal efficiency at light loads by as much as 6.3%, primarily due to reductions in pumping and compression work while heat transfer losses are comparable. The Second Law analysis shows that the throttling process in the conventional engine destroys up to 3% of the available energy in the fuel, and that the phasing of the heat transfer losses is more costly to its work producing potential. Overall, the availability analysis recognizes that the higher pressure in the LIVC intake manifold leads to a notable thermomechanical advantage, which the throttled engine has to overcome by consuming more chemical availability to achieve the same load.