Role of Courant number in Density solver
 

I am using the implicit density solver to analyze the blowdown of a high pressure tank into the atmosphere (supersonic flow). I do not understand the effect of the Courant number on the time step.
Fluent uses the Courant number to control the time step if the eigenvalues become too small. I tracked the time step using the statistics option and did not see any changes in the time step, but Co has a large effect of the result.
The strange result is that a larger Co gives more stable results that I think are better.
How can a larger Co help?