Effect of Courant number
 

I am using the density solver to model the blowdown of a high pressure tank. The results change dramatically (the pressure in the tank versus time) if I change the Courant number limit in the Solver window. I have monitored the time step (Monitor, Statistics window) and it does not change.
I do not understand how the Courant number affects the results if the time step is not changing. Does anyone know how the Courant number works?