Numerical simulations of internal nozzle flow that include transient needle valve motion offer the potential to better predict spray penetration, mixing and liquid breakup. For example, the level of gas initially inside the sac and holes, as well as the rate of needle movement, influence the initial fuel delivery rate and spray development, thereby affecting ignition position and combustion. In this study, needle movement and gas exchange inside operating transparent fuel injectors are imaged at high speed, and CFD simulations with fine resolution (2- micrometers) in the needle-seat area are performed to understand the impact of needle movement and initial gas in the sac on ramp-up in rate of injection. The injector bodies and sac geometries are replicas of the Engine Combustion Network Spray A and Spray D injectors. Imaging shows that gas is ingested into the injector at the beginning of needle movement, an unexpected results given the high injection pressure above the needle valve. Finite element analysis simulations accounting for the elastic properties of the metal seat and needle are performed to explain this result. As forces on the needle and seat are relieved at the beginning of injection, the sac volume enlarges while contact between sealing surfaces remains. Needle and nozzle wall measurements confirm that the needle tip may move roughly 5-10 micrometers before the passage opens at the needle seat to allow flow and pressurization of the sac. Measured needle movement from an experiment (optical or x-ray) must be corrected to achieve a different “needle gap” profile for simulations with no elasticity. This elasticity-corrected profile should be used for CFD simulations, otherwise, early and incorrect spray development will be predicted. Simulations with the corrected needle-lift profile and gas initially within the sac show that the mass flow rate at the start of injection includes cycling in flow rate caused primarily by sac pressure fluctuations, which are recommended for future Lagrangian CFD simulations.