As boosted, direct injected gasoline engines become more prevalent in the automotive market, the boosting system architecture and efficiency are intimately entwined with the efficiency and performance of the engine. Single-stage as well as two-stage boosting systems, comprising of either two turbochargers or a supercharger in combination with a turbocharger, are potential configurations. When combining an internal combustion engine with boosting hardware, a mechanical, fluid-dynamic and thermodynamic coupling is created and the system as a whole will need to be treated as such. For the initial selection of the boost system, it is important to match all of the engine design features, such as the engine's compression ratio, valve profiles and intake and exhaust components as well as to adjust and optimize all engine controls' calibration parameters.1-D engine cycle simulations in combination with engine experimental testing were utilized to explore optimum engine configurations and calibration settings when using a variety of boosting systems. A total of five different engine and boosting configurations where configured for this project, including two 4-cylinder, 1.6 liter GDI gasoline engines with single stage boosting (turbocharging and supercharging) and three downsized, 3-cylinder, 1.2 liter GDI gasoline engines with two-stage boosting configurations (series-sequential twin-turbo, super-turbo and turbo-super). Design-of-Experiment routines were carried out to optimize both fixed engine hardware specifications, e.g. compression ratio, as well as variable parameters, e.g. intake and exhaust valve phasing, combustion phasing, for a given boost system architecture on the target engine.Fuel economy and performance comparison were conducted between the different engine and boost system architectures for steady-state operating conditions as well as for common vehicle drive cycles. For light load operating conditions and lightly loaded test cycles e.g. NEDC), the downsized engines with two-stage boosting systems, particularly the 3-cylinder engine with super-turbo configuration, offered the greatest fuel economy potentials. For mid and high load operating conditions and more highly loaded test cycles (e.g. US06), the 4-cylinder, supercharged arrangement offered best fuel economy potentials. For steady-state, high load operating conditions, the 4-cylinder, turbocharged arrangement yielded best fuel consumption values.