Hi Mark,

GPM and Head go in opposite directions which is why there is the minus sign in my formula, but they cannot be inversely proportional since the pump curves show a positive non-zero head with a GPM of 0. This is where the pump is spinning just to generate equivalent pressure against the head -- or another way of looking at it is that as much water is pumped towards the head as is leaked backwards so that the net GPM is 0. Your formulas don't work for that, unless I'm misinterpreting them. The formula I gave almost exactly matches all the pump curves (within 1 or sometimes 2 feet of head) for both the full IntelliFlow and the IntelliFlow 4x160 (except the latter appears to have a mistake labeling the 750 RPM which appears to really be 950 RPM).

Anyway, I'm going to call my pool contractor to see if they can do the work to replace my pump and I'll very likely go with the full IntelliFlow. I'll keep y'all posted as to my results after it gets installed.

Thanks,
Richard