The numerical model of piston motion in free piston drivers is proposed including diaphragm ruptures. Numerical results are compared with experiments, and show excellent agreement. Diaphragm opening time is shown to have strong influence on the piston motion and pressure holding time for relatively compact compression tubes. Parametric studies are also conducted to investigate the influence of the opening time on the performance of free piston shock tunnels. The results show that the performance depends on the opening time and the length of compression tubes. The feasibility of tuned operations is adversely affected by the opening time, when the length is small. Once the tuned operation is achieved, then the maximum pressure holding time depends on the length of compression tubes and the minimum diameters of downstream sections. Influence of the opening time on the maximum pressure holding time is shown to be weak.