wrote in message
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Well, I was looking at the DeWalt DPH38003800 model, which claims 4
GPM. It has a CAT 67DX, and the manual for that says 3.9 GPM. The
DeWalt manual says the water source should be at least 5 GPM. So it
basically wants a guarantee of always having more than 4 GPM coming
in. I sent an email to DeWalt support to see what they say, but I get
the impression the pumps really want plenty of water coming in.

What's missing in you comment is the pump inlet pressure requirements. You
have an infinite source of water (city supply) and it seems to me that as
long as you have a positive inlet supply pressure to the pump it will put
out the specified flow. You'll only lose flow output for one of the
following two reasons: if the inlet is restricted to the point that
cavitation occurs or if the output line line loss (restriction) results in a
pump discharge pressure high enough to activate the pressure relief valve.
There is no mistaken which of the two events is occurring---cavitation will
result in a pulsating or sporadic flow rate and most likely will be
accompanied by very loud noises from the pump. Cavitation, if allowed to
continue, will in many cases damage/destroy the pump. If you activate the
relief valve, the loss in flow can be any where from a small reduction to
almost nothing coming out of the system.
MLD