What would you do if you were me? No wrong answers... lets here it

Fair warning, lots of words and rambling (I tried to cut it down), but I figured it best to write my thoughts out, plus nobody I know understands all this jargon and I’m pretty sure my wife is tired of just nodding her head in agreement. So I turn to you guys for help:

I have an inoperable direct drive 4 gpm pump attached to a GX390 sitting in the garage. I’m considering getting a new pump for it so I have a backup machine (running a 5.5/2500 belt drive). I currently have a 65 gallon buffer with no intention or space of expanding it, so I’d like to stay around the 5.5 mark, unless the overwhelming consensus thinks moving toward 8 would be doable… homes around here average 4 output, and I’ve come close to running out my buffer on two occasions (maid started laundry and dishes while I was cleaning, and a new build with terrible pressure/elevation change/200+ feet of garden hose) so I don’t think I wanna risk moving all the way to an 8 because of that, but maybe 6 or 7 would work? I’d also like to get closer to 3000 psi, currently, after 200ft of hose, I’m cleaning concrete at about 2200 and it’s not always cutting it (literally), but I also know based on the formulas that an engine upgrade may be needed to achieve that hight of psi without sacrificing gpm. I’m leaning towards gear drive this time around, again going back to space/layout for my truck… plus I’ve already used direct and belt so why not try out gears and make a first hand account on what I prefer.

From what I’ve read here, it seems Udor pumps are the way to go. I’ve been looking at the B, C, and BK series specifically. The B series seems to give me the most similar pump to what I currently am using with the 5.5/2500 but I could go down to the 5 gpm pump to increase my psi and get closer to 2900 psi. The C series is interesting for the sole purpose of moving up in gpm, however without also increasing my engine size (and probably buffer tank) even just moving to the 7 gpm my psi would drop down to 2,000 or less which would not be beneficial. Finally the BK series would be similar to what I’m currently using, but would give the flexibility to have more psi if I ever upgraded my engine, though I don’t think I’ll even need much above 3500 psi, it’s nice to know it’s there, but might be a mute point. However, I’m unsure if one series is inherently better than the others, or if it simply depends on specific needs and what you’re willing to spend. So any advice or alternatives to this predicament would be appreciated. So what would you do if you were me? No wrong answers… lets here it.

Edited to add: Also looking into flow actuated unloaders and blocks… ZK1 and Zeromatic still the recs?

I used a direct drive 4gpm for my first season. I fretted about buying an 8gpm. I got one from Diego in NC with an udor pump. It was a hell of a jump. I probably should have gotten a hot water 5.5gpm or 8 instead but I’ve been very pleased with it. It made me much more efficient with house washing and I can wash 3 stories from the ground without having to drag an a frame ladder everywhere.

I believe you can hook an 8gpm pump to a gx390 but it would only be doable for house washing. You wouldn’t have the psi for anything else. Do your own research of course but I feel like I had read that when I was deciding what I wanted.

For what it’s worth my gx390 has sat in the garage collecting dust since I upgraded to the 8 and it’s been so reliable that I’ve never had to use it aside from a time where I needed to do a house on the beach and needed it’s wheel mobility.

From my understanding I believe 8 gpm is doable with the gx390, but it’s around 2000 psi so not super practical for surface cleaning.

Don’t bother trying to go over 5gpm with that 390, at the end of 200’ hose it’ll barely do any concrete.

An 8/3000 is a giant leap in productivity, if you can find one used or finance a new one it’s seriously worth it.


I mean, at a certain point you can’t make something cheap like that do everything well otherwise we’d all have gx390s with a certain setup for Max efficiency :joy:

There’s a reason why people are using an 8gpm at 3000 psi as a standard and I mean that not sounding snarky. I looked into hooking two 4gpm units together, building my own harbor freight setup, etc and from what I researched I just went Honda and Diego’s stuff is set up for the long haul in terms of longevity. Well worth the 6k or so I spent in my opinion…but I was stressing over my decision until I used it and realized how much quicker I was able to get stuff done.

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8gpm if you think you can feed it, as you’ve explained. there’s no real reason you can’t just use 2 unloaders with quick connects, separately setup one for housewshing, and a trapped unloader one for the psi surface cleaning. if wanting the zk1 user it for the softwashing. it’s a little tiny bit time consuming but it will work. you have to get the surface cleaner and plug it in anyway, just an extra step to disconnect and reconnect whichever unloader.

I totally get that, I guess I’m left wondering if there is a middle ground between what I’m currently using and 8/3000.

This is an interesting idea… I’m trying to picture it in my mind, this would still involve two separate machines?

Yea I’m currently getting roughly 5.75 gpm at 2200 psi at the end of 200ft… starting to think going with a bigger engine might be the move.

just trying to maximize use of one 390. 2 unloaders will do it.

The steps up are 4gpm, 5.5gpm, 8gpm machines.

I think you could put on a 5.5 gpm pump on your Honda and get enough psi to do concrete still. That would be your middle ground.

I currently use a pressure pro 5.5 gpm 2500 psi belt driven machine, at the end of 200 feet I’m getting about 2200 psi which doesn’t always cut it for the concrete around here… but this build would be as a back up or potentially an upgrade (in gpm or psi) if possible before making the jump to a bigger machine/buffer tank.

That gx390 is probably the same or right around the same up as what you’re currently working with. I think you’ll need more HP to get what you want.

Correct they are both GX390… just wasn’t sure if getting a pump that was different gpm or psi then what I currently have would be worthwhile (dropping down to an Udor 5gpm 2900psi pump to gain psi or moving up to their 7gpm 3600psi pump (only would be getting 2000psi with the GX390 but would eventually upgrade to a 16HP) to gain more flow. Either way the goal is to eventually get to 8/3000 but like I said I don’t have the room/money currently for bigger buffer/engine/pumps.

How much would the pump be? 800+1k? To me it doesn’t make sense to do that, I would save towards that upgrade instead of trying to put lipstick on a pig especially if your end game goal is an upgrade down the line

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I’ve seen prices from 700-1200 for pump and gearbox depending on which set up… basically going this route but gear drive, and suffer with 2000psi until eventually upgrading the engine to get the psi back up?

I can tell you from experience a GX390 with a 4 gpm direct drive pump cleans concrete better & faster than a 15.8hp Predator with a 6.8 gpm pump. That changed when I ditched the Predator & put a GX630 in it’s place. Upgrade after upgrade costs a lot of money in the long run. ZK1 all day long.


i did have a 4gpm-4k on a 420cc motor. and i thought it seemed much better and faster than the 5 5 geardrive i swapped in. never got any tiger stripes either. and 1 pass too. but rinsing sucked,glad you mentioned it.

I’ve thought about going back down to a 4/4000 but keeping it belt or gear to pull water until I can move up to 8/3000… when it was working I felt the 4 did better then the 5.5 on everything but rinsing… but even then it felt negligible at best.

Here’s what I would do if I were looking to upgrade a GX390 5.5gpm… ZK1 unloader first. You’ll kick yourself for not doing it sooner. Next save for a new Honda V twin while keeping an eye on the classifieds for a good deal on a used one with low hours. Once you have it on there, you can open the unloader up & get better performance from the pump you have. A battery & a fuel cell will be needed but the result is electric start & no more running out of fuel every other hour. No more jerry cans, Top it off at the pump & run it all day. After that you can add any pump you want.