Trying to go bigger with the engine I have


If a gearbox mounted to an 8gpm pump only requires 1400 rpms (ballpark), then the gx390 which runs at a max of I think 3600 rpms; should be able to work for that gearbox and pump, no ? Anyone ever actually try this ?


Someone is making an 8gpm off of a 390 but I think it’s only putting out 1400psi


I think you need 22hp for 8gpm


he would have 1800 psi at the pump


Yeah I’d have the volume, but piss poor pressure. That machine would only be good for house washes, and hardly anything else lol. Maybe I’ll just build one with a predator engine, idk.


Build it right or don’t build it at all. You will overwork the motor. predator is a good budget build. I have tons of time on mine.


You know what I think would make for a killer starter machine, that someone should invent?

A two speed gearbox or belt drive pressure washer. Have the option of switching a lever and going from 5gpm@3000, to 8gpm@1700


Throttle down a 8@3500 machine and walah


Yeah, but throttling down is bad for the motor, and you lose horsepower needed to hit the higher psi

And the point is to be able to run this configuration off the inexpensive gx390


Yes it is not a good idea I was just kidding with yah. But if you have an 8gpm unit you could use a dual gun unloader and reduce it down to 5 gpm just by changing the nozzle to a #6


I reworded my post to clarify my point. I think I was a bit unclear on the objective, lol


They do make 2 speed gear boxes used on trucks pto . But I guess that would not help cost wise


Someone had a pump mounted to a motorcycle awhile back. different gears gave different outputs


I for one think that would be a killer idea.


I don’t believe you can change the gallons per minute. Only the psi. Right @Innocentbystander

It doesn’t matter what speed the pump turns. It will produce the gallons per minute listed. But the psi changes with rpm.


That’s backwards


The pump had a displacement per revolution. The more rpms the more water. Pressure is controlled by restriction “nozzles “ and relief “ unloader “


Within reason. If you have the machine ideling then yes it will struggle to produce the gpm. If you have it at half the rated rpm you will get half the rated psi with the full gpm. I know it’s not backwards because on my predator build I was pushing 8gpm at 3200 psi when the pump is rated for 2500psi. I was running it at 4,000rpm instead of the rated 3,000.

Installed a tachometer set the rpm and now I’m at the pumps rated gpm and psi.

I know for a fact I can go out to my rig and set the motor at 1500rpm and still get 8gpm. Just would half half the psi. Then when I let off the trigger the motor would die or struggle to pump up before the unloader went into bypass mode.


You should do a bucket test


If you change the rpm of the engine and keep the same nozzle the psi will certainly change but it’s because the amount of water you are trying to force through the orfice is changing creating more or less restriction