Pressure washer pump sizing

Discussion in 'Power Washing' started by cgaengineer, Jun 17, 2010.

  1. cgaengineer

    cgaengineer LawnSite Fanatic
    Posts: 15,782

    I acquired a North Star pressure washer with an I/C Briggs 5 HP and a working pump, the engine needed a gas tank so instead of purchasing a $75 gas tank I used a GX180 Honda to power the pump since I had one laying around. I would now like to purchase a better pump and would like to figure out how to size the pump for the engine. I am thinking no more than 4 gpm would be needed.
  2. Pumptecguy

    Pumptecguy Inactive
    Posts: 71


    I cannot find a GX 180 as a current offering on Honda's website, but I will assume it has 5 HP since the GX 160 claims 4.8 HP and GX 200 claims 5.5 HP. If there is 5 HP to work with then I recommend the following:

    A pump with 4 gpm at 3600 rpm performance
    0-1500 psi unloader - your maximum usable pressure at that flow.
    '2506' spraying systems meg nozzle.

    Here is the wizard behind the curtain:
    GPM x PSI equals load (HP). Take your load number and divide by 1500 to approximate your electric motor HP need. **Gas engines are less efficient, but it is hard to trust mftr to mftr that they are using the same yard stick - the government requires elec. motor mftrs use the same one.
    If you use this formula, you will have a number like 1875 for load that equals 5 HP. I revised down to swag the engine inefficiency.

    Next you are going to want a nozzle to use. The '25' is the fan degree and the '06' is the size. The nozzle sizes are related to each other and do relate to actual flow. An '06 nozzle will provide 0.6 gpm at 40 psi and 6 gpm at 4000 psi. You can decide which one is easier for you to remember. If you pick a larger size you will lose pressure (and not gain any more flow) or pick a smaller nozzle you will lose flow (and gain no increased psi since your engine has a finite amount of power - even if you can adjust the unloader higher). Either way, you will lose something and be less effective.

    In terms of the type of pump, you get what you pay for. If you think the engine will last a long time, then spend the money. Do not buy a 4 at 3000 pump if you can buy a 4 at 2000 for less money. No point in spending money on 50% more performance than you can use.

    I hope this helps and makes or saves you money.

    FCPWLLC LawnSite Senior Member
    Posts: 352

    Good stuff... I use gpm*psi/1100 to figure HP needed with Gas engines.
  4. cgaengineer

    cgaengineer LawnSite Fanatic
    Posts: 15,782

    Sorry, its a GX160, but it does say 5.5 hp. The engine is relatively new so I don't mind spending a couple hundred on a new pump. That's for the info fellows.
    Posted via Mobile Device
  5. JCinNJ

    JCinNJ LawnSite Member
    Posts: 159

    GPM does not change with orifice size, only pressure will go up or down.
  6. Pumptecguy

    Pumptecguy Inactive
    Posts: 71

    You will lose flow at a fixed pressure. I believe if you read the complete post you will understand that I was talking about a fixed pressure (1500 psi). The excess flow beyond the flow at that pressure is bypassed.

    You are correct if you remove the fixed pressure limit, but then you need to assume infinite power which is the opposite of this fella's post. His post is related to 'how much can I get for this amount of power?'.

    This brings up another thing to consider: The pump is always pumping 4 gpm (at 3600 rpm), so the HP calculation always applies in terms of maximum pressure. For example, let's say you have an '04 nozzle instead. The nozzle chart would tell you that at that flow you could achieve 4000 psi, but you cannot with a 5 HP power source. 4 times 4000 equals 16000 - more than twice the HP you have. In this case, the unloader (regulator) will bypass the amount of excess flow to get it to 1500 psi which would be about 2.3 gpm coming out the nozzle. You lose flow.

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