Best GX390 pump and reduction gearbox?

Hi all,

I have two spare GX390’s and I’m looking to use one for solely flat work.

So I need a reduction gearbox and a pump with 3000psi and a flow of 5GPM or more.

Can anyone recommend a pump they have or have used?

Thank you!

I’ve had a look here, but there are so many “series”?!

Look into Udor, I would never consider anything else


I’ll do that now! Thank you :slight_smile:

Don’t suppose you know a specific pump?

I’ll email them and ask too.

Check out there website, I think… look under the gear driven pumps and it tells you what size motor you need to achive the specs on them pump, there service is awesome Randy at the service department is always eager to help

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If you’re looking to use it for all flatwork I’d go with the Udor 5gpm pump with gearbox. 3k psi is good plus you’ll lose a few 100psi through the hose. The General 5.5 is great and is best for housewashing but lacks the psi for all flatwork.

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@Chesebro I’m in Australia, so I’ll email some suppliers here :slight_smile:

@Barry1 do you have a model for that 5GPM
@ 3000? Or a link I can send to the Australian supplier?

Wow. Udor are expensive pumps hey?

Here you go. Just need to put an unloader on.

This one is a little over $700


Such a price difference!

I ordered from those guys in December too. The shipping and export fees they charged came to the same price as the items I bought.

God knows what they would charge for a pump :tired_face:

Update: ahhh that one is only 18LPM/5GPM and the one quoted is 20LPM (5.3GPM?)

Barry, that probably won’t work on the GX390. It neds 16hp to put out those numbers. Even the Udor 5gpm/3000 needs 14.5…the GX only puts out about 12-13hp.

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Hmm while researching, I was looking at kilowatts and the GX390 fell short for most pumps I like the specs of.

The guy who sent the quote knows his stuff though - he’s a reputable seller here in Australia, he said this…

Which pump did he recommend?

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7th post down :+1:t2:

I emailed Udor directly, will let you know what they say. If anything I’d imagine 2500psi to be more realistic.

Reason being, at some point I’ll need to upgrade my pump too so looking at this pretty closely.

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Great stuff! Thanks for the assist :slight_smile:

I’ll post back with my findings too :ok_hand:t2:

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8.7 kilowatts is the maximum for the GX390 - so you are right and the guy is wrong.

The 18/24S barely meets the requirements too?

Or is that maximum power? Which doesn’t makes sense…

I was looking at upgrading in the near future and the pump I will probably choose is from here

scroll down to model CKC 12/28s 21LPM(5.5gal) 4k psi @15hp I also have 13ph, was told I would lose some psi, should be still 2.5k-3k psi.

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Hey @steve076

That’s where I got the quote from :blush:

That one needs 11.3KW, the 21/28S. That’s a big drop to the max KW power of the GX390.

Also, he recommended the BK series, not the CK.

I don’t know much about KW ratings and how they affect the flow and psi etc. I’ll call them tomorrow and have a chat.

If can’t get 3000psi and 5GPM (or more) I might get the GX630/690 and get rid of all these little GX390’s I have.

Udor cost more but they are bulletproof. Udor did a test where they ran a pump dry for about 6 hours with no issues. I had an udor with over 4k hours, never rebuilt or repacked. The side cracked. They replaced the pump for free, put my old pump in a new casing for free and thanked me for years of supporting them. They will Outlast any engine mated to them twice over.


Why do you think many gx390s are paired with a 4k psi / 4gpm pump? Those numbers “technically” don’t work with a gx390 (4000x4gpm)/1100=14.5hp. But yet everyone has them. It’s the same with the very popular General HP5535 which many have here (even me) paired with the gx390 rated at 5.6gpm and 3500psi. But the psi drops to about 2500 psi.

That 5gpm Udor pump will work with a gx390…of course he’s not going to get 3500 psi but could get in the high 2000s