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?