However, I have a 3500 psi machine. Shouldn’t I be choosing the tip based on 3500 psi instead of 4000 psi?
example of the calculation of the table above:
nozzle size = GPM * square root of (4000/ desired PSI)
6.5= 4 GPM * square root of (4000/1500)
Shouldn’t I be using a 6.0 nozzle instead?
6.1= 4 GPM * square root of (3500/1500)
A 6.0 nozzle would be 1556 psi if you wanted to be exact.
What about the hose length PSI drop? My 3500 psi machine has 200 ft of hose and I am told that the hose drops 300 PSI for every 100ft you use. This would give my machine 2900 PSI to work with at the end of the hose, right? Shouldn’t I be choosing the tips based on the 2900 psi at the end of the hose instead?
example of the calculation of the table above:
nozzle size = GPM * square root of (4000/ desired PSI)
6.5= 4 GPM * square root of (4000/1500)
Shouldn’t I be using a 5.5 nozzle instead?
5.6= 4 GPM * square root of (2900/1500)
A 5.5 nozzle would be 1534 psi if you wanted to be exact.
Sorry for all the math but I wanted to make sure I understand this before buying a bunch of tips using a chart that I think will be giving me the wrong pressures…
Nozzle size numbers are based off 4000 psi, but you don’t need to worry about the rating on your machine when figuring out the size. It just makes the nozzle size numbers have meaning, so #4 nozzle for 4 gpm, #8 for 8 gpm etc. GPM and desired pressure are all that matters. Your 3500 rating on your machine is just what the pump is rated at and nothing else, has nothing to do with sizing nozzles.
Of all the charts out there, this one is my favorite as far as being easy to use and also gives what the actual orifice diameter is.
If you want 1500 psi, I’d try 6.5 first, but might need to go 6 depending on how long of hose you have. You’ll have losses and not all pumps and engines are exactly the same. The chart is a good starting point, but you’ll need to test.
I have seen that chart also. It is also cacl’d based on 4000 psi. So you are saying a 2000 psi machine and a 4000 psi machine would use the same nozzle size to output 1500 psi as long as the gpm of both machines is the same?
I would have thought that a 4000 psi machine would require a nozzle with a hole that was larger to drop the pressure 2500 to 1500 psi than a machine rated at 2000 psi that only had to drop the pressure 500.
4000 is just the max that the pump can produce. If both pumps output the same GPM and both pumps are capable of reaching the PSI you are wanting then the nozzle orifice is the same size.
It seems counter-intuitive, but I never was able to wrap my head around fluid dynamics. The only project I ever abandoned was rebuilding a Turbohydromatic 400 transmission… Thank you for being patient and explaining it to me.
Ok last question about this then I am done. You are saying that machine PSI doesn’t matter only GPM and nozzle size to get the desired PSI. So you are saying my 4 GPM 3500 PSI machine will output 4000 PSI using a #4 nozzle?
It will try to. The 3500 rating is either all the pump was designed to handle, or all it can do paired with the engine it comes with. Typically if you see a 4 gpm 3500, it’s a smaller engine, so maybe the pump is good for 4000, but that engine just can’t push it.
Usually you see a 4/4000 has a 390cc and a 4/3500 only has a 270cc engine.
Remember, the output of the machine (pressure and flow) are directly related to the horsepower of the engine. You can have a pump rated for 4gpm and 4000 psi, but if you put a 5hp engine on it, you only get one or the other, not both.
Shouldn’t a gx390 and ez4040g pump be giving me 4000 psi? A gx390 is 13 hp and can be used for more powerful pumps. I only need a little over 8 hp for 3500 per your formula.
hp =(psi*gpm)/1714
8.168028005 =(3500x4.0)/1714
I decided I needed a live test to get away from math for a bit.
This is a used pressure washer that has a sticker that says “water cannon max pressure 3500 PSI”. The unloader valve is turned clockwise all the way. I rebuilt the carb and replaced some stuck valves springs in the pump. Engine runs good and starts on one pull.
I just ran a test using a 0040 nozzle and got 4 gallons in 60 seconds after increasing the rpms on the motor to make sure I was in the proper spot. However, my pressure gauge is saying 3300 psi right before the gun. I do have 50 ft of 3/8 hose on it also. This should give me 3455 psi, right? I do have a new swivel, ball valve and pressure gauge on it as well.
Are pressure gauges reliable? Should I adjust the unloader valve? I have read on here that you shouldn’t. Why would they use 3500 instead of 4000? Or does it even matter since psi doesn’t matter?
