Flow rate imbalances among the nozzle holes are responsible for undesirable effects, as hole to hole differences, that worsen the regular fuel spray development. Multi hole injectors for common rail-equipped small diesel engines have been investigated by 3D-CFD modeling, under ballistic needle motion; simulation campaigns have been devoted to highlight, on one side, how the actual nozzle layout influences the fuel flow pattern upstream the nozzles; on the other side, the role of actual operating conditions of the injector has been studied. Relating to geometrical layout, different factors define the actual injector tip features; here, the effects of specific details have been taken into account; in the modeling of the actual nozzle layout, the effects of hole key-shape, inlet edge radius and hole diameter have been investigated, pointing out their influence on the fuel flow development. Then, regarding the modeling of injector actual operation, needle eccentricity and injector axis inclination have been considered, as well. Once the modeling details have been described, the fuel flow features are shown and discussed, both for VCO and Micro-Sac nozzle types, addressing the role of the different driving factors on the flow development.