Main duct size info from Bill Pentz site
Here's an except from Bill Pentz site posted above. While I can't say how relevant this info is, I am merely presenting it.
Main Duct: If you want the most possible CFM you need the biggest duct you can get that keeps the air velocity ample to keep the dust moving instead of clogging your ducting. Most hobbyist vendors sell 4" dust collection duct and flex hose as their standard. This works well to collect chips, but strangles the airflow needed for good fine dust collection. A typical 2 hp hobbyist dust collector with a 12" diameter impeller moves a maximum of about 1200 CFM, but a 4" duct airflow drops that airflow to only about 450 CFM. It takes a 5 hp motor turning a 16.5" diameter impeller to force a 4" duct to carry 800 CFM. Most prefer a less expensive solution. In addition to maintaining that airflow, our blowers must keep the duct airspeed high enough so we don't get plugging or piles of wood dust. Dust piles in ducts pose a fire hazard and ruin both blowers and filters when they break loose and go slamming around. Air engineers found designing for a duct speed of 4000 FPM keeps the ducting clear. Because FPM=CFM/duct size, a little math shows we need 6" diameter ducting to move 800 CFM at 4000 FPM. Most hobbyists need all 6" hoods, 6" main ducting lines, and 6" down drops.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 01-19-2012 at 06:35 PM.