This paper presents an experimental and computational study with the aim of developing a fundamental understanding of the nozzle internal flow behavior in a prototype piezo-stack injector with a multi-orifice nozzle, which allows controlling the needle position at different percentages of its maximum lift. In order to characterize the flow behavior, several measurements of mass flow rate and spray momentum flux have been carried out at different rail pressures (up to 200 MPa). Furthermore, to develop a deep understanding of the internal flow behavior, computational fluid dynamic simulations have been carried out using the software OpenFOAM in order to determine the pressure drop through the nozzle seat and estimate the real needle lift at different conditions. The experimental results demonstrate the nozzle flow behavior and the commonly used nondimensional coefficients; an interesting reduction in the nozzle discharge coefficient at partial lifts (needle positions smaller than the maximum lift) is shown. The computational results allow the estimation of the pressure and velocity profiles in the needle seat at different needle positions. Afterwards, the pressure drop through the needle seat and the real needle lift is determined for each test condition. Thus, with the velocity profiles obtained, the variation in both the dynamic pressure and flow directionality at the orifice inlet could be determined. Thus the decrease of effective area and its dependency on the lift were justified since the calculation of these nondimensional coefficients assumes only static pressure upstream the hole conditions.