No announcement yet.

Pressure washer ? on GPM and pressure

  • Filter
  • Time
  • Show
Clear All
new posts

  • Pressure washer ? on GPM and pressure

    If your running say a 4000psi 4 gal per minute pressure washer and you back the pressure down at the unloader do the gal per minute also drop? The reason i ask is Im putting together a mobile detail rig and i have a 190gallon tank onboard which I am running spotless water and i want to conserve it being the distance i need to drive to fill it up but id like the versatility of the bigger pump to hook up to someones garden hose and wash a drive way or what ever else comes up to make a buck. thanks jim

  • #2
    Re: Pressure washer ? on GPM and pressure

    Good question and can't say that I've ever seen this issue addressed. I'd recommend you contact the pump manufacturer and ask them. They'll know for sure. Later!


    • #3
      Re: Pressure washer ? on GPM and pressure

      You can see the ratios, 4.0@2000psi = 2.83 gpm; 3000psi=3.46 gpm. 1000psi=2gpm.

      The .pdf is more readable


      • #4
        Re: Pressure washer ? on GPM and pressure

        Good information!!,,,thanks very much jim


        • #5
          Re: Pressure washer ? on GPM and pressure

          yes and no.

          the pump we use for pressure washing/ jetting is referred to as a positive displacement pump.

          typical is a 3 piston pump. each piston will displace a certain amount of water per revolution. the pump is designed to pump water. the pressure output is a direct result of restricting the outlet at the nozzle.

          so in your case by dropping the pressure at the unloader, you will also drop the amount of water that will squeeze through the nozzle port. but you will still be producing the same amount of bypassed water being circulated into your holding tank.

          the easiest way to lower the volume is to lower the rpm's of the pump. or in this case the engine of the pump. less rpm's, less displacement of water. but remember that in order to drop rpm, you will also need to drop psi or your engine will lug/ stall.

          so dropping rpm's and psi, you can dial in the gpm's much easier.

          with a holding tank, it safer than without a holding tank. the non holding take will cause the water to recirculate around the inlet and will eventually overheat. with holding tank.

          the nozzle chart is designed to help properly size the nozzle based on maximum flow at max. rpm.

          so it's best to drop rpm and downsize your nozzles. but remember you need the h.p to keep from stalling the engine. h.p is rated at the max.rpm of the engine.

          hope this shed some light onto your question.

          phoebe it is


          • #6
            Re: Pressure washer ? on GPM and pressure

            If you happen to have a belt drive pump you can downsize the engine pulley slightly which will lover the pump speed. This will result in both lower volume and pressure at the nozzle. What you really want is to find a good old cast iron Wisconsin engine rated at about 1-1/2 times the max HP your pump would ever need. Then you can run the engine at 1800 RPM rather than 3200-3600 RPM and lower the flow rate. You'll enjoy way less noise and long engine life too.

            Example: Your pump says to use a 10 HP engine. Now if you had a 10 HP engine you would have to run it full out at max rated speed and really work it hard. On the other hand if you had a 16HP heavy duty engine it would not have to work as hard even when reved up and also it would chug along really nice when running slower.

            Hint: Try out a really good pressure washer all setup at full throttle and then slow the engine a little, then a little more. You'll get the idea. A good rental yard should have a good pressure washer you can try out. Just take in a few cold cans of beer and that goes a long way.


            • #7
              Re: Pressure washer ? on GPM and pressure

              As Rick said drop rpm and nozzle size. If you drop rpm only you might stall the engine. You can also back off the unloader to lower pressure too. You need a gauge to see the pressure, with the nozzle size known then the chart will indicate the GPM. 4k/4gpm pw's usually have 13hp Honda GX's [OHV]which have throttle controls so you can alter engine speed much better than the old side valve B&S or Tecumseh due to their better torque curves.


              • #8
                Re: Pressure washer ? on GPM and pressure

                Il back off on both then the unloader and the rpms and see what plays out,,

                I just realized im faced with another challange,I planned on gravity feeding pressure washer and been reading all over thats no good being i have a direct drive pump so i ordered up a flojet on demand pump from Northern
                Im hoping it keeps up the pressure and volume when im working it so i dont burn up my pump,,

                suprises me how many local detailers i see out there gravity feeding there pumps they have to be short lived,,
                thanks again jim