Water piston internal combustion engine is a very simple propulsive engine invented in the 1970's to be used in different applications. The water piston engine consists simply of L shape tube immersed in water where the water column inside the tube acts as a piston. In the present study, two propulsive units from this engine were compared. The two units are identical in their dimensions except the exit port where one is curved and the other one is sharp. The effect of this shape on the engine thrust, fuel consumption, power and number of effective firing was investigated. The effect of combustion chamber size on engine performance is also considered for the two units in this study.