Chris, I'm not sure it is all that complicated. It's really that few actually test the airflow in their system. True, a 6" pipe works with a 15" impeller but it wastes a lot of capacity when coupled with a 5 hp motor. I can get 6000+ fpm through a 6" pipe in my system, but I get the same velocity in a 7" main and almost the same in the 8" line. Originally I thought that 30' from the cyclone I would reduce down to 6" as necessary to keep the cfm up. Using an anemometer at each port showed me the error in my "keep it simple approach" and I tore out the 6 and replaced it with 7" with no reduction in velocity. Through sheer brilliance in stupid planning, my Oakley edge sander sits 50' away and needs all of the 1500 cfm I bring to it. The original 6" made me wish for a 7.5hp system which would have been a failure with the same pipe. I run old machines that have mediocre DC with hood modifications so my needs are different than most, but if you choose the clearvue max for example and use 6" pipe you are wasting the upgrade. Dave