In recent times, complying with increasingly stringent emission regulations has become ever more challenging than before. While an efficient after-treatment system, that includes a gasoline particulate filter, enables compliance with legislation requirements, lowering engine-out emissions by improving the combustion system must be considered as a crucial advantage for both pollutants emission control and performance. In this respect, high-performance enabling contents such as relatively large displacement, flow-capacity oriented intake ports and a limited stroke-to-bore ratio have significant drawbacks on the charge motion quality and, consequently, on mixture formation and homogeneity. As a countermeasure, fuel injection system components, as well as control strategies, need to be substantially improved. The increase of fuel injection pressure, coupled with optimized injection timing and splitting, has proven to be effective in reducing emissions, especially with regard to particulate matter. This paper provides results of an experimental study investigating the effect of different fuel injection strategies on engine-out emissions, with special emphasis on the influence of very high fuel injection pressures (up to 50 MPa) on particulate matter. A multi-hole inwardly-opening fuel injector fitted to a high specific power direct-injection spark ignition (DISI) single cylinder engine, was tested over a wide range of steady state operating conditions, including catalyst heating, part loads and rated power. A fast particle spectrometer was employed along with a PMP-compliant particle counter to fully characterize fuel injection pressure effect on particle number and size distribution. The results show that significant improvements can be achieved over the entire range of investigated engine operating conditions, especially in regard to particulate matter emissions.