When the perforation erosion option in "Perforations" is checked in StimPlan, it applies a set of correlations to estimate the discharge coefficient based on the proppant concentration. (For more information about the discharge coefficient, please refer to the StimPlan help file "Perforation Erosion" and "Perforations/Frac-Initiation Depth".) If the proppant concentration is zero, different correlations give different initial discharge coefficients. The initial discharge coefficient ranges from about 0.64 to 0.87 when the perforation erosion option is checked. However, without the perforation erosion option checked, the discharge coefficient is a constant of 0.75. That explains why the perforation friction estimation is different at no proppant with/without the perforation erosion considered.