Silicon effect at a lower level around 3.5 wt% on the tensile, impact, and fracture properties of Al-based automotive alloys has been investigated through T6 heat treatment. The results demonstrate that tensile strength increases in the aged alloys mainly due to development of Al 2 Cu and Mg 2 Si precipitates inside the Al-matrix. Higher Si-added alloy shows higher strength through Si-rich precipitates and hinders the true strain at tensile behaviour. The alloys ductility decreases with treatment temperature in favour of GP zones, βʹʹ, βʹ, β precipitates formation plus Qʹʹ phase and finally increases for precipitates coarsening. Microstructural study confirms the coarse grain boundary and plate-like eutectic phase when Si is added. Fractography shows a small dimple structure from the Al dendrites and the crack initiates through brittle Si-rich particles.