time i e the time during which the pintle is off its seat may be expressed as in equation 1 as mentioned earlier this open time is noted a and hereinafter referred to as the open time variable when the variability of the opening delay is very low the term od can even be omitted for comparison purposes for some injector designs as explained earlier turning now to fig3 the fuel quantity q is plotted vs the open time variable a representative of the time period during which the injector valve group is open a homogeneous behavior can be observed in this plot which illustrates that injector open times to deliver a given fuel mass are fairly constant the present method relies on this finding in other words although a conventional flow vs pw graph exhibits significant part to part variations and non negligible variations exist in terms of od and cr open times a are quite similar between injectors to deliver a same fuel amount the graph of fig3 can be determined in the same conditions as the graph of fig1 indeed for each of the pairs fuel mass pw of the curves shown in fig1 it is also possible to determine the od and cr related to the pw and then to compute the corresponding open time variable a furthermore one can elaborate a master performance function flow vs open time.