What psi are those gx390 5.5 gpm really putting out?

are those pressure pro belt driven gx390 5.5 units putting out a full 5.5gpm and 3000psi ?

how would a 20" surface cleaner work with that lets say 200ft out

Mine puts out 6 gpm and a smidge under 2500 psi, after 200 feet of hose its closer to 22/2300 and with a 16" surface cleaner I usually have to make passes in both directions to get concrete cleaned… but I’m also only post treating at 1ish%

You will be fine with a 20” SC with that setup……

A 390 cannot make 6gpm@2500 psi. Just a physically impossible. You also cannot alter the laws of friction loss. 16inch surface cleaner is the biggest you can practically use.

Valid, my pressure gauge probably isn’t that accurate to begin with and it barely makes it to 2500 (which could also be the answer to the question I asked in my thread… using wrong sized nozzles to set unloader)

90% of the time you need to always make 2 passes doing residential no matter what your machine is if you really want to clean it or move really slow. If you’re cleaning it right, you should rarely ever need to post treat with more than a HW mix.


my gx390 is getting alittle tired and just doesn’t have the balls behind it for the 20" it does it but sounds like its operating at half sedated.

I have a gx390. Aaa pump… so it’s direct drive. It’s 4200 psi and 4 gpm. I’m trying to do the research but question 1) can you change direct drive into gear or belt? (My impression is you can not). Question 2) can I get this machine to be a 5.5 gpm or at least higher than the 4 because I know the AAA pump won’t last.

  1. Correct, you can not turn a direct drive pump into a gear or belt drive pump… if you wanted to move to gear or belt you would need to buy a new pump and either a gear box or the appropriate pulleys, belt, and skid to convert it.

  2. You can get as many GPM as you want out of that engine, just look at the trash pumps they sell with it… they put out 580 gpm. The problem is that when upping gpm, psi takes a hit… it’s one or the other and after a certain point you end up hitting the tipping point… in the case of a gx390 thats right around 5.5 gpm. Real world you will probably see a little more flow then 5.5 and a little less psi then what the pump is rated for… I was getting a little under 6 gpm and around 2200 psi (according to my cheap gauge) after 200 feet of hose on my belt driven 5.5/2500 marketed machine.

So I can just put a gear drive pump into my 390? It’s already ready for it?

As long as it has a 1" shaft gear box with the pump, yes.

Gear Drive Build

As long as your GX390 is a straight shaft (very, very likely) and not a tapered shaft. All you’ll need is a gear box, a new pump that is gear drive compatible, and some gear oil to fill the gear box, and some non-detergent oil to fill the new pump. Call Russ at Southside Equipment and tell him what you want to put together and he’ll get you set up. I went with the GP pump that he sells. Can’t remember what model it is and I’m not near my rig today but I’ll post a picture on here later of what all you’ll get from Russ. Super simple to put together. Just make sure you have some copper anti-seize on hand to slather on the engine shaft before stabbing the gearbox on there. Look at @TheWizard had happen with his shaft key recently. Don’t want that to happen to yours so be very liberal with the anti-seize.


Perfect. Sounds like Russ is the guy and will be hearing from me a couple times this year!

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I think my issue was the metal quality. The key was made out of 5 finger face slap but luckily the shaft was made outta pure Slayer.

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All the stock keys are basically a mix of cheap carbon steel and a few different alloys mixed in there. I’ve still never had one disintegrate like yours did but I smother those things in copper anti-seize anytime the pump is off. I did have one split in half once but I think that was an improper installation from the factory as it was a low hour machine and the pump grenaded way prematurely.

He’s helpful. You can probably shop around and find all the pieces from different vendors and maybe save a couple bucks but it’s convenient to have him ship you everything you need and you can have that thing mounted and running in a couple hours no problem.

SO changing my pump would bring down pressure got it… but at the change down to 2500 psi is that worth it for flat work? I know Gpm wise going from 4 to 5.5 which would really be like around 6gpm would be nice for house wash. At that 2500 am I going to see drastic efficiency loss on surface cleaner? 16’’ SC and 25020 tips.

Also Russ is a gangster. Hes the best.

After 200 feet of hose with friction loss and all that you will be closer to 2200/2300. You might need to go over some areas twice, walk slow, or pre-treat/post-treat with a hotter mix then typical but it is doable.

Are you sure it’s engine trouble? Multiple other things could make it weak.
1st I’d check the unloader
Check the prefilter to the machine
Make sure your not sucking air anywhere
If it is engine issues it may just need a tune up