Pulley Ratio Help Please

Hey guys, tying to figure out what pulleys I need for my PW. I have a honda gx390, and I want to get this (https://www.powerwashstore.com/P/2049/LegacyHDPumpGTSeries5.6GPM-3500PSINoGT5635R) Pump. I havn’t got a clue what I need pulley and belt wise to run this thing. I tried looking up formulas but they went way over my head.

the Honda runs peak at 3700 RPM and the pump runs at 1450 RPM.

Do I need the engine pulley to be 2.5x the size of the pump pulley? Is it as simple as that?

I’ll include some pics of my current set-up.

I feel like this is such a noob question, but I don’t want to mess up a new pump!

Here are some pics of my belt rive now. I am upgrading the pump. It looks like the pulleys are the same size to me, I couldn’t tell a difference.

Have you called the place you’re going to buy the pump from? If they’re any good they’ll keep the pulleys and mounting plates stocked too. There a site that breaks it down pretty well. I’ll see if I can find it.


online calculator

I havn’t, I will do that.

Thanks for the help!

Yes, the drive end and pump have the same o.d. pulley. They are 5 inch Diameter
Size 3V and use a SH bushing, you would have found that out by googling the part number. You would need a 12" pulley on the pump to spin it at 1500 rpm if you are keeping the drive pulley you have now. That’s absolutely massive so you would step up the drive pulley a few inches. You would then measure center to center on both pulleys and enter into calculator to get your belt length. Those 3v belts you have now are puny though you best step up to BX belts so you’d need both.

Also when you reassemble you need to make sure the key is flush with the face of the bushings. The way they are installed now is too far in on the pump and too far out of the drive. 13 or so ft. lbs on the lil hex bolts or you will crack the sheave.

I would like to see what that pump does spinning at 2.5 times the engine speed. That’s 9k rpm bro

Also, I’d pair that pump with a 18-21hp engine and not with 11.7hp GX390 so what exactly are you trying to achieve with it?



Just found this today. Check out pulley selection guide.


Thanks for the help. I’ll be sure to flush those pulleys up and be careful with the bolts.

I do residential work, mostly soft washing. I am planning on upgrading the engine as soon as possible. My current pump is crap and is giving out, so I had to get a replacement for that first. I know I wont be making the greatest PSI with the gx390 but it’s what I have to work with for now.

You got it and sometimes you just don’t need the psi at all, it just depends on what you’re trying to achieve. I wash heavy equipment all day long with a big pump throttled down to 4.6gpm and 1700 psi at the gun, this allows me to run 14hrs 6 days a weeks if need be.

I don’t know how much concrete you plan on doing but you could run a big pump on that GX390 if you mostly just want to downstream a house wash. When you need to do concrete you would just be needing to slow way down. Maybe even pretreat it with SH with your roof pump and upstream some high PH soap through your PW but the concrete will clean up.