The research conducted and explained in this paper aims to explore and understand the influence of flow regime (laminar, transition or turbulent) inside diesel injector nozzles. For this purpose, an experimental study based on mass flow rate and momentum flux measurements on three convergent nozzles has been carried out. The combination of both types of measurement has been helpful to obtain information about the nature of the flow and its consequences on important variables, such as injection effective velocity and effective area of nozzle outlet orifices. As a main result of the investigation, and depending of the flow regime, a differentiated behaviour has been observed which was clearly reflected in the non-dimensional flow parameters defined and used through the study.