Im trying to figure out the CFM on a certain project. To sum it up I am taking a 4" PVC Pipe and it is going into a 3" pvc pipe (had to shrink size due to bends and turns). My question is that if a 4" Pipe @ 4500 FPM can withstand 390CFM how will it be affected shrinking it down. Will it speed up the FPM for the 3" and make more CFM? Here is what I have 3" @ 4500fpm= 220cfm 4" @ 4500fpm= 390cfm Will it increase velocity? If so, how can I calculate how much it increases to calculate the new CFM. How would it affect the outcome if I added another smaller inlet pipe in addition to the 4" to increase the amount of air COMING INTO the 3". Thanks guys/gals!