A methodology which uses simultaneously 3D calculations and advanced experimental tools has been developed in order to characterize air-fuel mixing and combustion of gasoline direct injection engines at every stage of development. The analysis of Mitsubishi GDI engine has been carried out in order to validate this approach. The experimental tools used in this analysis underline the great cycle-to-cycle variability and show that the air-fuel ratio variations at spark plug correlate closely with the fluctuations of combustion starting and development. Despite this variability, average measurements are reproducible and in good agreement with 3D computational results obtained with KIVA-MB code. The common use of both kinds of tools allows to get a very fine understanding of Mitsubishi wall-guided concept.