Experimental results obtained in previous work dealing with the topic “heat-resistant Al-Si foundry alloys” are usually not interpreted in consideration of the interdependencies of their respective main alloying elements. However, the knowledge of mutual interdependencies provides an opportunity of producing alloys with maximized mechanical properties preferably cost efficiently. In the course of this work, three main alloying elements (Cu, Ni and Mg) were defined, interacting and influencing the high-temperature strength of Al-Si cast alloys by various strengthening mechanisms. Systematic compositional variations illustrate the significant hardening effect of secondary precipitates such as Al2Cu and Mg2Si. It is also shown that the elevated-temperature strength is increased by the addition of Ni, albeit only to a certain extent, depending on the fraction of eutectic phase in the particular alloy. The alloys are considered as coarse two-phase systems, where a hardening effect is caused by load transfer to the harder phase, which requires a certain contiguity of the latter. Additionally, the mechanical properties of two commercial heat-resistant alloys were determined, analyzing the single effect of elevated test and ageing temperatures as well as their combined influence on strength and opposing both alloys concerning their high-temperature performance. As the physical properties play a major role in determining the life time of certain motor components, this work aims to quantify the effect of Si, Cu and Ni on both the thermal conductivity and the thermal expansion coefficient of Al-Si cast alloys. The characteristic of the investigated alloys is assumed to resemble complex poly-phase composites. Generally, the thermal behavior of such heterogeneous materials lies between that of its components and mainly depends on the volume fractions of the particular phases. Analyzing the experimentally obtained data revealed a significant influence of the main alloying elements on the thermal conductivity and the thermal expansion coefficient. The results are selectively discussed on a systematic basis of thermodynamic calculations and compared to theoretical models for the thermal conductivity and thermal expansion of heterogeneous solids. A comparison of the investigated alloys concerning an ideal combination of their mechanical and physical properties was made, using the results of this work as well as the information from the temperature distribution of a real in service part as a function of thermal conductivity.