Adding a 2nd machine

Going into year 2 and want a backup machine on the truck. I run a 8gpm on a twin as main machine, Bought a gx390 cheap and want to add a 5gpm pump mainly for when we have slow supply or when I don’t need so much water on decks or for my ground guy when I’m spraying roofs. I run a udor now and want another, question is if I get another 8 gpm and run 2 separate unloaders with run quick connects. I would have 1 set for 8 gpm 1500 psi. 2nd bypassed down to 5 gpm what would the pressure be coming out of it be? 2500? I would want at least enough for concrete. Price difference is 250 ish between gear drive 5 and the 8. Mainly do house washes now and want to try and get some commercial flat work in this year. I’ve seen the hp=gpm equation is their way to figure out psi if your bypassing a bunch of water, will the pressure go up?


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Few things, Why would you need an unloader set at 1500#? Set it at what the pump is rated for and change your tips to get whatever pressure you want.
Unloaders are for setting pressure not flow. I read this as you want to set the unloader to restrict flow?

I’m a little confused.

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I’m not sure what you’re asking but just set up your second machine independently. Plumb it with it’s own feed and use it when you need it.

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Unloaders are for bypassing water when your of your gun. So I want less gpm from the bigger pump but m y question is does it increase the pressure. So if I set a unloader to bypass 3 gallons of water while on the gun with appropriate tip will I see over that 1500 psi that the horsepower equation gives me with 11 hp-8 gpm pump

If you set an unloader to bypass any more than a trickle you’re going to wear it out in a heartbeat. Buy the right size pump for your motor and set the unloader properly. Plumb the machines independently and be done with it.


Isn’t it the other way around? Set the unloader for max flow with just a trickle coming out the bypass side when your on the trigger. Then to adjust the psi you get at the gun, use meg tips with bigger or smaller orifices

I think i get what your trying to say…

one, don’t connect 2 washers with different pumps together.

Two, if the pump is rated for 8 gpm, then use it for 8 gpm, don’t have the unloader bypass half the water constantly to achieve 5 gpm through the hose… just get a 5 gpm pump and set unloader for max flow (trickle out the unloader when on the trigger). Better for the unloader that way.

If this answers the question cool, if not then I’m not sure what your asking

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? It’s 1 pump And 1 engine switching between unloaders can I get this combo to give me 2 different performance characteristics

  1. High volume - lo pressure —downstream houses volume - higher pressure- surface clean concrete

You can use the higher pressure pump to softwash houses too… I think you are over thinking it.

Just get a large orifice nozzle so your psi is low at the tip and it will draw soap through a downstream injector for house washing.

Man, I gotta tell you… That original post reads like a trainwreck. You say you bought a gx390 and want to add a 5gpm. Up to that point, it’s cool.

But then you say you want to get another 8gpm pump and run two different unloaders on it. On the 390? That won’t happen.

There’s way too much going on in there to decipher really. @DisplacedTexan and @DJPWS are giving solid advice. Run your machines independently, with properly set unloaders at the pumps’ gpm. Use your tips to drop the pressure.

These are the kind threads to avoid. The poster durant know what he wants, doesn’t understand pumps, and will intentioned answers are all across the board because no one understands the question. Bottom line: your back up should be equal to your main.

To answer your question, NO. Get the pump for 5-5.5 if you want to be able to clean concrete. That will still work fine for washing houses. In fact a lot of guys on here feel that’s the sweet spot for a house washing machine.