This article details a method for modelling the most critical parts of an injection system. It focuses on the most important component of the system, the injector itself. As a clear example of this methodology, the modelling of a first-generation common rail injection system is carried out using a commercial code. The proposed methodology for modelling the injection system is based on two types of characterization: a detailed dimensional characterization and a hydraulic characterization of the different internal parts of the injector. The dimensional characterization is based on the use of a fine detail measuring technique applied to all the constituents of the injector. These include the passages and internal lines of the injector, internal volumes, calibrated orifices, nozzle springs, clearances between moving sections of pistons, etc. The second type of characterization makes reference mainly to the hydraulic characterization of the nozzle and injector control orifices, which together with dimensional information makes it possible to determine the discharge coe cient. In this case, special emphasis is placed on the detection of critical cavitation conditions and repercussions of this on the flow. This is a typical phenomenon in control orifices and also in nozzles subject to strong pressure gradients. Once the model is obtained, it is tested and validated. Following this, the values of the experimental injected mass and rate of injection at different operating points are compared with the model results.