Al,
You're probably right.  I looked at some of my other wp pictures and it looks like the 20B impeller may be thicker (larger blades) as well.  I'll have to check that tonight.  That could make some significant difference.  Some day, when I have some free time and nothing else to do, I would like to do a comparison test on the two.  However, I don't see that happening anytime soon. 

Mark

 

Mark;

 

My opinion would be that the .17 difference in diameter is negligible compared to other differences in the design (shape of housing exit flow, clearances, etc.)

 

Al