The science of dust collection falls into the mechanical engineering field called fluid dynamics. As an engineer I studied fluid dynamics in college and sadly much of the “wisdom” on the internet is incorrect. I have a white paper I wrote that I’m happy to share with anyone who asks. The short version is this. The smallest opening the air much flow through and the suction pressure of the collector determine for the most part the actual CFM. ?Big mains connected to a saw with a small opening inside under the blade will do little to increase the CFM. The way to increase CFM to McRae’s suction pressure at the machine. Bigger duct does this but only to a point as low cfm in a big duct has effectively no resistance.?

I have an Oneida 5HP variable speed dust collector. ?It automatically increases the suction when the cfm is low to maximize. I tested the cfm at each machine before and after the variable speed controller and I approximately doubled the CFM at each machine. My Felder KF700SP has a port on the side that is about 5”. But inside the machine the flex hose is about 4” and the cast iron part under the blade has an opening that’s equivalent to about a 3” duct. ?I can run 5” mains or 10” mains to the machine and get almost exactly the same CFM. The way to increase cfm is more suction pressure. The 5HP Oneida I use can go up to 25” water gauge suction. Non variable speed 5HP collectors are more like 14-15”.?