I have a rather detailed diffuser that is time consuming in large assemblies. (18" diameter rolled tube, 100 plus quantity of 1.25" diameter holes).  I would like to replace it with a Porous medium because there are multiples in the Actual assembly to test.  However, when I isolate the Diffuser to get a pressure drop and use that information in the Porous Medium, the overall result in simulation with porous medium is 18% different.  I have varied the Porosity (from 0.1 to 0.7), and the table values (20 to 200%, as well as using mass flow rate and volume flow rate) and there is almost no change in the Porous Medium effect. What am I doing wrong?

One of the trial data sets: