Please double check my math!

So I just put together a new Udor pump with Honda Gx390. It puts out 6gpm. In 1:15 it draws 1/2 gallon through the injector. So out of 15 gallons it has 1 gallon of solution. So if I was to pull straight 12.5 it would hit the wall at about 1.87%. Which is strong for most of the houses around here. If I do a 50-50 solution that’ll put me right about .9% on the wall. Which has worked for most jobs so far. Does all of this sound correct?

Ok I realized where I was making a mistake. I believe I am actually putting .83% on the wall if I pull straight 12.5.

Its been a long day and i realized i have only had coffee to drink so far lol


@Deanswc is this machine able to shoot as high as your other machines? Is it a belt drive, gear drive?
I have an old gx390 and was thinking of putting a 8gpm on it.

Pro tip know your ratios but we’re not putting a man on the moon. As time goes buy you really just know what it takes to clean a surface


What model pump is this? I’ll be replacing the pump on my 390 soon. How much psi are you getting?

Udor sales rep said to use GC3017s or for gear drive GC3017GR for 8 gpm

This would be for house Washing only I’m guessing. No way this would be sufficient for flatwork? I would be curious to know what height this combination would reach.

Theoretically, it should reach the same height as any 8 gpm machine using identical plumbing and shooter tip.

Most sources say you need 2000-2500+ psi at the SC for effective flatwork cleaning. So this setup would not be sufficient for that purpose. You might get 1800 psi at the pump.

On a side note, I bet we’re 5-10 years away from seeing 2-speed gearboxes for pressure washers. You could run it at a lower speed for lower gpm with a higher PSI for flatwork (or for higher concentration DS’ing), and then switch to the higher gear for faster rinsing.


@Clean_Blue @Harold
Udor Pumps MKC1824GR Plunger Pump, Gear Reduction: MAX 5.0 GPM @ 3500 PSI, 3400 RPM (1'' Shaft) - Dultmeier Sales

This is the pump that I added to my engine. It shows that I have 3000 psi at the gun surprisingly. Not sure how but that’s what the numbers show. It was late by the time I finished setting the unloader and did a bucket test and pressure test. I can check what the height and distance was when I go back to the shop. But from what I remember it looked pretty good.


@Deanswc that’s great! 3000psi how much extra was the gearbox?

It came together with the pump for $716.


They charge separately for it here in Australia!

The gearbox for that exact same pump is $633 on top of $1349 for the pump.


It’s a 5 gpm pump and not a 5.5 gpm pump which is why you’re getting 3000 and not 2500.

The 390 is an 11HP engine so you’re actually losing a little psi along the way.

There’s no real difference from 2500 to 4000 psi anyway. And that’s a great deal on that pump.

Yeah I figured I would get about 2800 but what surprises me is it also puts out 6gpm as well as 3000psi. I am very happy with the purchase. Since I only do residential at this point I would rather have 2 of these than 1 8gpm.


That’s awesome!

I thought the gx370 was 11hp, looks just like the 390, but isnt.

GX390 net horsepower is 11.7. Stated horsepower is usually not what you’re actually getting. GX390 is stated 13 HP

Bucket test? I can see 2800 or 6gpm, but not both

@Innocentbystander I can do a video of it when I go back to the shop this week. Maybe I am doing something wrong but that’s what it showed. When I did the bucket test I took everything off the end of the hose and then timed how long it took to fill up a 5 gallon jug with the pump running. It was filling at 1 gallon every 10 seconds. Maybe I need to use a certain tip to test this?

Then to check the PSI I put a pressure gauge between the end of the hose and the gun. When I let off the gun and restrict the flow the guage goes up to 3000. Is this the correct process to check?

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Without any back pressure on the pump, the engine has nothing to struggle against. I have a hunch that it tends to rev a little higher without any load. So the most accurate way to do a bucket test is with a full pressure tip and a bucket with a lid with a small hole in it to hold the splashback.

Real world house washing, though, you’re using low pressure tips. So less load on the motor, and a little more output from higher rpm. But the difference is likely negligible.

For pressure test, take your reading with a full pressure tip, with the trigger pulled. The 3000 psi you saw isn’t sustained. It’s the spike pressure where the unloader kicks in. Use a nozzle chart to select the tips for 5@3000, and then 6@3000. See what you get from each.

Usually the unloader is set a couple hundred psi higher than the rated pressure for the machine.