Advanced technologies such as direct injection DI, turbocharging and variable valve timing, have lead to a significant evolution of the gasoline engine with positive effects on driving pleasure, fuel consumption and emissions. Today's developments are primarily focused on the implementation of improved full load characteristics for driving performance and fuel consumption reduction with stoichiometric operation, following the downsizing approach in combination with turbocharging and high specific power. The requirements of a relatively small cylinder displacement with high specific power and a wide flexibility of DI injection specifications lead to competing development targets and additionally to a high number of degrees of freedom during optimization. In order to successfully approach an optimum solution, FEV has evolved an advanced development methodology, which is based on the combination of simulation, optical diagnostics and engine thermodynamics testing. The first two methods can be integrated into the very early phase of development and are performed in parallel and interactively with the design process. In this paper this development methodology is described with the main focus on the optical diagnostics applied to assess the mixture formation process for an exemplary turbocharged DI SI engine. Optical diagnostics are performed on a flexible motored engine FloTec (Flexible Laser Optical Test Engine Concept), which is capable to be operated with metal flowboxes instead of prototype cylinder heads. Several optical techniques are applied. Particle Image Velocimetry PIV is used to investigate the charge motion intensity and stability. The second technique is high speed imaging of the injected fuel droplets by Mie scattering to investigate the interaction of charge motion and injection, and the resulting spray-liner and spray-piston impingement. The third is Laser Induced Fluorescence LIF in order to analyze the fuel vapor distribution with respect to mixture homogenization and cyclic stability of the mixture formation process. In combination with the FloTec engine these optical diagnostics a detailed analysis of the mixture formation quality in a very early stage of development prior to the first prototype engine investigations is feasible, and thus allow pre-optimization of the injection system specifications with respect to mixture preparation for subsequent engine testing.