I have a HF dust collector I have highly modified. What I did was took it
apart, threw away everything but the impeller and put a cyclone inline and
ported the dust outside the building into a catchment area.

If I were to get another HF DC and parallel the current impeller with the
one I currently have I wonder how much I would pickup in CFM? I could get
a Delta, whatever, impeller and motor and improve my CFM and static
pressure but about 30-35%. I wonder if the parallel arrangement would not
give me on the order of 60+% increase.

I currently am running 4" PVC with 4" flex drops to about 7 blast gates.
Max run is about 30 feet (including the drop). The exhaust is also 4" PVC
and its run is about 30 feet, with little or no back pressure.

Thanks

Deb