Macrosegregation and porosity formation have been investigated by both a numerical model and by transient directional solidification experiments. The macrosegregation pattern, the theoretical and apparent densities are presented as a function of the casting length. X-ray fluorescence spectrometry was used to determine the macrosegregation profiles. The measurement of microporosity was performed using pyknometry analysis. The local composition along an Al-9wt%Si-3wt%Cu casting length was used as an input parameter for simulations of microporosity evolution. The results have demonstrated that the presence of Si in the alloy composition has inhibited the inverse copper segregation, which is a typical result of directionally solidified Al-Cu castings. The numerically simulated trend is in good conformity with the experimental scatter.