Upgrading my GX390 to 5.5 GPM... question


Thanks jzbreeland.

Yes i can tell a difference. I did a bucket test with the 4, and then with the 5.5… if i remember correctly, it was like a 10-15 second difference. When rinsing with the ball valve, i can feel the increased force of it over the 4. I also use an injector bypass, and it makes a noticeable difference when i bypass the injector.

As far as the 2400 PSI (which with 200 feet of hose, it’s probably closer to 2000… one of these days i will check), i haven’t had any issues. I just cleaned 40k square foot parking lot with the 5.5, a 19" SC, and cold water only. Granted, the parking lot had very little oil, but it did have heavy organic staining. Here’s a picture of it partway through the cleaning:

It cleaned up nicely, but I did have some faint striping over most of the concrete. Not sure if that was due to the low PSI, and/or lack of hot water, or if the striping would have occurred even with more PSI and heat. Here you can see the striping, except for in the second parking spot… that spot i had post treated with straight bleach in a pump up to test if it would remove the striping, and it did. I ended up X-Jetting 10% straight for the rest, and it took care of most of the stripes, except for a few spots here and there that i used a pump up on. I had pretreated by downstreaming straight 10% bleach, with a little EBC mixed in. A stronger pretreat may have helped with the striping also, i suppose.


How much did the pump cost you in the end? Sorry if you mentioned that already. I didn’t see it in the thread.


start at the top you cant miss its the first link posted.


Around $800 with shipping, came with unloader.


That wasn’t the place i ordered from, that was the same model though. Sorry, i should update it or something probably.


@tireshark, now that a year has passed, do you have any positive / negative feedback on how the 5.5gpm worked out on the GX390? I’m in the early stages of figuring out my trailer setup, and this build looks to be a possibility. Edit - I see he hasn’t been on here in awhile. Anyone know how it worked out?


I basically copied what he did and used some of his pics as guidance. It’s a pretty easy swap. If your gx390 is an older machine you may have some trouble getting the pump off. Get some anti-seize to put the new pump on.

I was very skeptical about a 5.5 on a gx390. There wasn’t much info about it…I was worried I wouldn’t get the gpm upgrade I was looking for over my 4/4. One person claimed you can’t get 5.5gpm out such a small engine…they tested one and only got 4.3gpm. I asked a few that had 5.5/2500 belt drives to do bucket tests…only one responded and got a true 5.5gpm.

I went with the hp5535 General pump from Russ Johnson at Southside for a little over $800 shipped. Came with an unloader but you could get one without and put your own on. I tested it after setting the unloader and was blown away by the results. I’ve tested probably 8 times with different tips and mine pulls 6.6 gpm. It’s a house washing beast. With 200ft of hose you’ll be around 2200-2300psi so Driveways you’ll need to pretreat if they’re bad. It’s a great upgrade if you’re into residential.


It should put out the correct gpm. Its the PSI that will be less than spec using the 390 as opposed to the 16/18 hp engines.


I can tell you from experience that even on a hundred degree day after running my udor gear drive for 6+ hours straight that the pump is as cold as ice. No way I’m seeing enough heat transfer to do any damage, even on hot days I have cold condensation on the pump.


I don’t know how true this is, but I asked about renting a bigger surface cleaner from home Depot and the first thing he asked is if my machine was at least 3500psi.


It’s very true that the first thing he asked was if your machine was at least 3500psi.




I called Russ today and ordered the pump with an unloader.


I’ve seen other pumps outperforming their factory stats. A few here that have 4/4 belt drive machine wth a cat pump did bucket tests…one was 5 and the other 5.25gpm (I think the last one was from @Racer) Mine is putting out 1gpm more than stated from the factory…but I’d rather have more gpm and less psi. It runs like a top

I don’t have a psi gauge…but if I’m getting 6.6gpm x 2200 / 1100 = 13.2hp so should be good with a gx390…it might be working hard but hasn’t given me any problems.


What gearbox do you use?


Good question…had to go back to my receipt and look at the number. Here’s the gearbox that came on this pump (HP5535UR-402)


Its just the rule of thumb, but I didnt snap this is for a gearbox pump.


Did it come with the gear box? $800 total?


You can get an udor 8/3000 with gearbox already installed for $800ish


I think it was $820 for everything…pump, gearbox, and unloader shipped to my house. I asked Russ Johnson if I should go with a General or Udor…he recommended the General. I know Udor is awesome and many here have them. I just put a small Udor pump on my backup PW. But so far the General has performed better than I expected.