BfB is correct, there is no difference between the 19# and 21# injectors (I think they are actually 23#), they're the same Bosch injector. The flow rating, which is given by Ford at a fuel pressure of 44 psi (or somewhere around that), was changed based on the change from return style systems to returnless. In 2002 the electrical connection changed when Bosch switched from the EV-1 style to the EV-6. Other than electrically, they retain the same capacity and spray cone pattern. As others have pointed out, the flow rating is completely contingent upon fuel pressure. However it is not linear due to physics , you cannot double fuel pressure and expect a 21# injector to behave like a 42# injector. Yes, fuel will increase but it will not come close to doubling.