Sean, I have a Challenger 2/0.33 HP with a 30in sand filter. The two speed motor is non-standard so I have not been able to locate a flow rate graph for it. This pool is over-designed for low head - I'm running about 2PSI of pressure at the filter at low speed. I'm planning on hooking up a flex pipe to a return and measuring the GPM by timing the fill of a couple of 5 gal buckets. Interestingly there is always some air in the pump when running at low speed. Sand filter does not have an air bleed valve - I've been told it's unnecessary. Thanks for the info -