Hi Andy, well i was blocking off one side to try and find what shape produces the best result, its harder to know that if the other ports bleeding air down it.
What i did find was that youd be able to generate quite good figures just down one side, say for arguments sake 100cfm , then youd modify the other side and get another 100cfm so youd think youd have 200cfm in total.
Unfortunately it seems theres perhaps some turbulence that chokes off that desirable extra flow because its always been lower when you test with both sides open, which as you say is how it operates.
Its probably the impact of the air interfering with each flow and bottling up somewhat.
I generally close off the port simply to get the best possible flow down each side and keep them balanced as closely as i can, that being the other reason for doing them that way.
Now, if i had a nice Centroid cnc machining centre........