# Building a pressure washer, question about the HP required equation

Hey everybody, I’m piecing together a pressure washer and am at a point where I don’t 100% know what pressure range my machine will be at. Looking at unloaders and regulators at the moment and I think I should match them to the psi output of the pump motor combination and not the max pump statistics, so I feel this is probably an important number to have before I start buying additional components.

Scrounging locally for used deals, I have a honda gx620 20hp motor and a general pumps tsp1821 pump (5100 psi / 5.5 gpm) that I am rebuilding with new seals. If I go off the assumption that my motors drive constant is 1100 and using the equation psi = (hp)(gpm)/(drive constant) I will produce 4000 psi. Being a math-oriented person this doesn’t seem to be a very accurate approximation as I’ve seen several different websites say gas engine drive constants can be as high as 1250, in which case my system would produce 4500 psi. I’ve called honda a few times but at this point, I’ve been stuck on hold and have yet to speak to a representative there who can give me this drive constant number. Is there an equation using the information in my gx620 manual I can derive the drive constant from?

Don’t sweat it, you’re almost never going to use more than 3k psi unless you’re trying to bore a hole in something. We wash houses around 60-100psi, rinse at the same. Wood gets around 1k psi, brick between 2k and 3k psi if you’re knocking off lichen, and driveways get 3k psi and below.

Also, I’ve never heard that 1100 referred to as a drive constant, I just thought it was a number somebody came up with that worked. What I DO know is that your pump needs 1450 rpm, so don’t forget to price in a gear box or a belt setup. And a buffer tank.

My main use is sandblasting big pieces of equipment, mostly just my personal industrial hobby stuff. So I defiantly need things to be as maxed out as I can get them, If anything just because I can.

Oh dang! Well then everything I said in the second post still stands. I’ve never used a sandblasting attachment on my PW, but you still need a gearbox or a pulley setup for that pump to run on that motor