Time-resolved mass injection rates of an outward opening piezo-actuated and a solenoid actuated multi-hole GDI injector were measured to investigate (1) the influence of both hardware and software settings and (2) the influence on the injection rates from a wide range of operational parameters and (3) discuss limitations and issues with this measurement technique. The varied operating parameters were fuel pressure, back-pressure, electrical pulse width, single/double injection and injection frequency. The varied hardware/software parameters were injector protrusion, upstream fuel pressure condition and the cut-off frequency of the software's low-pass filter. Signal quality was found to be dependent on both hardware and software settings, especially the cut-off frequency of the low-pass filter. Measurements with high signal quality were not possible for back-pressures lower than 0.5 MPa. For the smallest possible injections, the piezo-actuated injector was found to be superior since it can inject very small amounts of fuel very accurately with little fuel pressure dependency. For engine realistic back-pressures, the multi-hole injector was found to be back-pressure independent. The piezo injector however was found to be strongly influenced by the back-pressure. The dynamic range was found to be much higher for the piezo injector, and the non-linear flow area was much larger for the multi-hole injector. Both injectors were capable of double injections but the piezo can use shorter dwell times. Effects of upstream fuel pressure fluctuations, especially on the second injection of a double injection, must be carefully taken into account.