Psi and gpm

Hey my name is jason, I am currently researching and planning my pressure washing career …i purchased a simpson 3200psi/2.8gpm “professional” pressure washer that I thought was pretty good machine to get started…then I really started looking into it and holy shit my pressure washer really is Not “professional” lol i have read alot about rebuilding and or putting different pumps on to achieve more gpm…maybe I am reading way too much into this but my question is how do you elevate your gpm without also elevate your psi ?..if you are pushing more water thru the hose would it not also push out at higher psi…i am pretty sure I have read or misread somewhere that you can add a larger pump to current motor that will allow higher gpm but remain the same psi…

You bought a home owner machine. Sell it and buy the right equipment if you want to be a professional. Just being honest and straight with you.

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p.s.i to me is useless. Ignore it. More g.p.m.'s equal more water to utilize what you want to do with it.

This argument is as old as downstream vs. Xjet. Get ready .

Yeah I see that…i saw it for $100 only used once…i looked up price for it new and jumped on it…

I have gathered gpm is more important than psi…i am just trying to figure out how the 2 aren’t directly correlated…only makes sense too me

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How many horsepower is the motor? And what brand motor?

You can push higher gpm, but it will result in lower psi, since the motor HP will be the limiting factor. If you know what you have for a motor, and how many GPM you want to upgrade to, you can figure out your max psi with a formula.

And you’ll need to change out your nozzles for a set rated for the higher gpm. You should have a set of low pressure (larger orifice) nozzles for house washing, anyway.

But your best bet is probably just to flip the machine on CraigsList for $300+, and invest in a proper belt or gear driven machine.

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I found a honda gx390 13hp…in pretty good condition for $275 on craigslist…does anyone know my options on gpm for that Motor ?

probably 4 g.p.m is the highest i’d go.

You could go with 5.5 @ 2500. That’s what my machine is.

Here’s the ratio for determining the consistent running HP of an engine to pump ratio:

Desired PSI X Desired GPM X .0007 = HP Required

A 3500 PSI X 5.5 GPM X.0007 = 13.45 (This is a great pump for residential work. BUT running an 13.5 HP engine at that all the time is very hard on the engine.

Best of luck!

I ran across this explanation a while ago…Cleaning Units (CU)
The true test of a pressure washer is measured in Cleaning Units. Imagine two streams of water under the same amount of pressure. The first stream contains twice the water as the second. As a result, it will clean twice as fast.

Calculate the CU by multiplying the PSI times the GPM. (PSI x GPM = CU)

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They are correlated. As you indicated before, higher gpm through the same size nozzle means higher psi. But pushing more water at a higher psi requires more horsepower.

The point about CU (cleaning units) is helpful in the sense of understanding the required power. But it isn’t all that helpful in respect to cleaning houses. 3.5gpm at 4000 psi will be nowhere near as useful as 5.5gpm at 2500 psi.

If you know anything about electrical theory, a similar principle exists with respects to Volts, Amps, and Watts.

Watts would be analogous to HP (power required/consumed), Volts to PSI, and Amps to GPM.

120 volts at 1 amp does me no good for running the pump on my WFP setup. But 12 volts at 10 amps is plenty of power. Both would consume the same watts (our limiting factor). But lowering the voltage (PSI) means I can accomplish the same amount of work (amps/gpm).

Sorry for confusing the matter further :open_mouth: