Legislative and customer demands in terms of fuel consumption and emissions are an enormous challenge for the development of modern combustion engines. Downsizing in combination with turbocharging and direct injection is one way to increase efficiency and therefore meet the requirements. This results in a reduction of the displacement and thus the bore diameter. The emerging trends towards long-stroke engine design and hybridization make the use of small bore diameters in future gasoline engines a realistic scenario. The application of direct injection with small cylinder dimensions increases the probability of the interaction of liquid fuel with the cylinder walls, which may result in disadvantages concerning especially particulate emissions. This leads to the question which bore diameter is feasible without drawbacks concerning emissions as a result of wall wetting. In the previous project “GDI Boundary Bore” the feasibility of an SI engine with direct injection and small bore diameter (60 mm) was shown by the analysis of two different cylinder head concepts (3V and 4V). For the acquirement of deeper understanding of the mixture formation in such engines the laser induced exciplex fluorescence (LIEF) was applied on a transparent engine in a follow up project for the simultaneous visualization of the vapor and liquid phase. The optical investigation on the transparent single cylinder engine included various operating points with variation of the start of injection. Within the scope of the investigations on the 3V concept, it was possible to determine why a charge movement flap increasingly loses its influence with early injection. The optical investigations on the 4V concept showed that a 7-hole injector with a lower penetration depth has disadvantages compared to a 6-hole injector. Furthermore, it was possible to determine why a charge movement flap can have a negative effect on mixture formation, contrary to common opinion.