Well pump manufacturers publish "efficiency curves" for well pumps, such as this one (the curve that is most sharply convex down, whose Y axis is read from the "% EFF" scale on the right hand side):

Example efficiency curve

The well pumps are usually sold with a name that reflects the flow rate that yields peak efficiency. So this one is called a "33GS" because it is most efficient at 33 GPM.

These pump curves typically presume a motor speed of 3450 RPM. How might I expect the curve to shift or stretch if the motor speed changes? I'm particularly interested in the range from about 2800 RPM to 3600 RPM.

  • $\begingroup$ 33GS is a range of pumps suitable for different heads with motors varying from 1 to 10 HP. Each pump having a different Curve $\endgroup$
    – Brad
    Mar 13, 2021 at 5:40
  • $\begingroup$ The % efficiency of the pump head itself is shown as a function of flow rate. There is one % efficiency curve shown for each pump head, regardless of the attached motor. A similar curve—based only on the pump head, independent of the HP of the motor—is shown is other product literature. $\endgroup$ Mar 13, 2021 at 7:11
  • $\begingroup$ I don't think it's a straightforward question, so I'd recommend you call a Goulds distributor with your question. $\endgroup$
    – J. Ari
    Mar 14, 2021 at 22:23
  • $\begingroup$ I don't have time to dig into this right now, I don't see a straightforward way to solve this via pump affinity laws, maybe you can go via the specific speed of the pumpe - However I think the best course of action would be to get the data from the manufacturer. $\endgroup$
    – mart
    Apr 12, 2021 at 7:43
  • $\begingroup$ Are the impellers of all the pumps in the 33GS series mutually geometrically similar? And if so, does each pump in the series have a different impeller diameter? $\endgroup$ Apr 12, 2021 at 10:38

1 Answer 1


It is not possible to answer your question as you are missing a vital component. "Head"

The flow rate depends on the head as well as the speed of the pump. So it will have a given flow rate at a certain speed and head.

  • $\begingroup$ However disregarding your reference to efficiency curves, if I was to assume you actually had 33gpm discharge at 3450 rpm then reducing speed to 2800 would deliver approximately 26.8gpm and 34.4gpm for 3600 rpm $\endgroup$
    – Brad
    Mar 13, 2021 at 6:19
  • $\begingroup$ Use theengineeringmindset.com/pump-calculations for rule of thumb calculations. These will not give you exact results like working from curves (if you have all the values) will, but I would think that they will be close enough for your use. $\endgroup$
    – Brad
    Mar 13, 2021 at 6:27
  • 1
    $\begingroup$ I am not asking how much flow I will get. I am asking how the efficiency curve, which shows energy efficiency of the pump as a function of flow rate, changes with motor speed. $\endgroup$ Mar 13, 2021 at 7:07

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