Pressure and temperature levels within a modern internal combustion engine cylinder have been pushing to the limits of traditional materials and design. These operative conditions are due to the stringent emission and fuel economy standards that are forcing automotive engineers to develop engines with much higher power densities. Thus, downsized, turbocharged engines are an important technology to meet the future demands on transport efficiency. It is well known that within downsized turbocharged gasoline engines, thermal management becomes a vital issue for durability and combustion stability. In order to contribute to the understanding of engine thermal management, a conjugate heat transfer analysis of a downsized gasoline piston engine has been performed. The intent was to study the design possibilities afforded by the use of the Selective Laser Melting (SLM) additive manufacturing process. Thus, the study here considers the original aluminum (aluminum alloy 2618A T6) piston with added cooling galleries and weight-saving lattice structures that can be achieved using SLM. An oil cooling gallery was introduced near the piston crown to allow a temperature reduction on the top land and a more homogeneous temperature distribution across the crown. Better temperature control should allow the combustion process to be less sensitive to knocking and pre-ignition. In addition, a shift in the top ring groove due to better cooling will help to reduce crevice volume thereby reducing engine emissions. The ultimate aim is to show that this new additive manufacturing technique applied to piston design could be used to enable further downsizing for fuel economy by increasing engine compression ratio and boost pressure with improved combustion stability and phasing.