The engine will use the maximum amount of fuel at maximum flow and minimum head pressure. In other words, a 3" intake hose drawing from pump level and discharging a 3" open head at pump level.

Any time you increase head pressure (pumping water higher or closing a valve partially off to slow flow) and reduce flow it will actually use less gas because enginge load is reduced.

Sounds counter intuitive but that is the way it works with centrifugal pumps. Liquid or air (centrifugal fans like in vacuum cleaner or shop vac).

If you doubt that close an output valve on a centrifugal water pump and make it go to stall head pressure and listen to the engine gain RPM's because the load is reduced. Hold your hand over a shop vac intake or outlet and hear the electric motor gain RPS's because the motor speed increases because motor load is reduced.

When you stop flow water just goes round and round the impeller case. The power consumed is only what it takes for the impellers to create the stall pressure. As the outlet opens and flow starts the impellers "grab hold" of new water forcing more resistance against the impellers. It is a combination of flow and pressure that causes the pump to resist the shaft turning (requiring horsepower), but the flow is the biggest component on a centrifugal type pump (but not on others like a positive displacement gear type pump). Maximum flow equal maximum horsepower requirement equal maximum fuel usage. Reducing flow (by pumping water higher or pulling a higher lift or closing a valve partially or fully) reduces load on the engine because less work is being done.

Fuel usage is going to be tied to load.

At least that is the way it seems to work to me but I'm no engineer. That's just what the big boys told me.

Last edited by snrub; 05/04/17 02:07 PM.
John

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