Build an 8gpm on a 13 hp

i have a 13 hp honda 4 gpm machine. Im looking to get an 8 gpm pump on there. I will only be using it for house washes i have another machine ill be using for concrete… my question is what exactly do i need to make the switch? What pumps will work? And do i need a gear reducer or something to go between the pump and the engine? If so which one? Any help is greatly appreciated thanks

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This is a guide from AR

13hp max out is 7GPM @ 2700PSI.

General rule is big motor small pump… Big pump small motor = problems.

Cheers JV

That is not a general rule, and isn’t great advice. Please intro yourself and let us know who you are.

Hi, I’m a fabricator who does a lot of work on pressure washers in New Zealand. ARs table is there work not mine, if you know better than them please reference your work. Be it a boat, car, plane, or power washer if you have an engine too small for the load, you’ll have problems. Now if you know something the rest of the engineering world doesn’t about the physics of an over load, I’m all ears. I’ve spoken to enough users and seen enough big pumps on small engines over the past 15 years of doing this to satisfy myself that I’m OK with what I said.

I like how the chart says my 21hp GX 630 will power an 8.5/3500 pump.

Granted it’s only using 17 With the 5.5 I have right now, but still.


Im looking for actual pumps with part numbers so i know which ones will fit my machine and i need to know if i need a gear box or gear reducer. Pretty much what pump will go on my 13 hp honda gx 390

You’re not going to like the SH strength with low pressure/high volume. If anything shoot for a 5gpm/2500 or something.

Then again, I’m only 1.5 years in this, ask the other guys.

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I’m sure I know more than some engineers, less than others. I’ve got a few more years experience than you, but that isn’t your fault. Your advice is still wrong. A bigger pump puts no more strain on an engine than a smaller one. Probably less, as a bigger pump on a smaller engine produces more gpm, not pressure. AR is not really recognized as a decent pump for professional contractors. Stick around. Intro your self. You’ll learn a lot here.


Are you familiar with the 8 gpm 13 hp build?

Formula is psi x gpm / 1100 = up

So for your 13 hp, which is only 11 if it is a Honda,
You would be pushing 8 gpm at 1500 psi. Not bad for washing houses.

Interesting. Have you ever built one or tried one out?

Yes, I’ve made a couple for one of the older guys on this forum in return for some website building. My Landa guy builds them also

Disregard my previous question.

Could you point me in the direction of pumps and gear boxs?. Can i put any 8 gpm pump on there or will only certain specs fit my machine?

Ive been looking at this pump

Here you go:

When you order, add on the hardened steel key to lessen the chances of the key getting eaten up.


I don’t do gear boxes, only belt drive. I wouldn’t recommend an AR either, but you gotta do what you gotta do. You need to measure the shaft on your engine, see that it mates to the gear box you are getting and that it is turning the required rpm. Several threads on here about gear box set ups



I remember doing research on this a while back…there’s a few posts on here about it. But yes you can put a gear drive 8gpm pump on a gx390 but as @Innocentbystander mentioned you’ll only get 1500psi at the most out of it…so it would be great for house washing but garbage for concrete cleaning.

Dovetailing into the conversation…

What happens if you install an 8/3500 pump on a GX630? Motor overworked, less pressure or less GPM?