Got 2 650 CFM Dust Collectors. &the old Sears Craftsman Whole Shop Dust Collection System , that Sears sold 20 years or so ago. It's got the 2 1/2 " clear pipe with blast gates, etc. If I run all 2 1/2 Inch pipe around the shop, and then connect that 2 1/2" pipe system to 4 inch flexible dust collector hose where it connects to the tool; what might the reduction in CFM be ? This is assuming that I would be running 1 machine at a time. Would I need to run Both Dust Collectors , or would 1 be good enough ? Shop is 25 x 30. 3 Wood Lathes, 6 inch Jointer, Table Saw, Grinder, 13 Inch Jet Planer, Band Saw, and the combo Sander.

You might want to consider that not only does the 'flex pipe' reduce the effectiveness by creating internal turbulence, but also factor in any bends, the length of 'flex pipe', and the fact that when you go from 2 1/2 to 4 inch the static pressure is reduced in a direct ratio of the volume of the two pipes. I am sure that you know that sawdust moves with lower pressure than lathe shavings. On line there are several forums that will enable you to do the math for a successful conclusion. When all else fails, just give it a try. If it works, then all is good(and you don't have to do the math)

Zookeeper

If you can curve any bends instead of a sharp 90* bend you will help yourself considerably. The larger you can make the radius the better.