New environmental regulations have placed stringent requirements on emission levels from diesel and other types of engines. These requirements can only be met if engines can perform more efficiently, burning fuel with reduced emission levels, which requires components fabricated from materials that have the required set of mechanical, physical and environmental-resistance properties at elevated temperatures to ~200°ăC, in combination with low density and high stiffness. In this context, cast aluminum silicon (354 and 388) alloys are being evaluated, because recent developments at NASA have suggested that with compositional and process modifications, significant enhancement in high temperature strength can result. In this study, the microstructures of cast Al alloys 354 and 388, with and without NASA modification, and in the T5 and T6 conditions were characterized by TEM. In addition, the aging response and of these alloys was studied. Fracture studies of the 388 alloy were carried out as well. The results of the structures, compositions and morphology of second phase precipitates, and evolution and mechanisms of precipitation during aging will be reported.