How to figure out PSI when you know the HP and GPM?

Clear Shine

New member
I am looking at different engines and wanted to know is there a formula for finding out how much PSI I would be putting out with knowing the motor size and the gear drive GPM pump.....anyone know?
 

Russ Johnson

Equipment Expert
I usually just plug numbers in for psi until I get close to the hp.

[(gpm X psi) /1450] X 1.5 = hp

So, if you've got 8 gpm and 20 hp, try different psi's in the formula.

[(8 X 2500) / 1450] X 1.5 = 20.69...close enough to 20 to work well.
 

Russ Johnson

Equipment Expert
[(psi X gpm) /1450] X 1.5 = hp

If I refresh my knowledge of algebra, I can reformulate this equation to solve for psi knowing the gpm and hp.
 

"Red"

Graphic Designer
What make/model pump do you have or what are the claimed psi/gpm output?

Engine output hp and hydraulic hp (pump output) aren't the same thing. hhp is the horsepower coming out of the pump.

HHP = (PSI X GPM) / 1714

Example: a 3000psi@4gpm pump puts out 7hhp, and a 4000psi@8gpm puts out 18.67hhp.

Question: Would the psi and/or gpm change (much) between a 15hp engine and a 50hp engine (considering all other factors remain constant such as rpm)? Not in my opinion. The engine would turn the pump somewhat easier, but would use considerably more fuel in doing so. Efficiency is achieved by matching the engine and the pump for the best performance.

Working psi is not achieved until a tip is introduced into the system. When there is no restrictive tip (creating the high pressure), the flow has a much lower psi. Ever open a trigger gun without a tip and put your hand in the flow? Does it feel like 3000psi?
 

greg/sd

New member
plug your numbers hit calculate it will tell hp requirments on the right. play with it and you can find psi ,gpm, hp, nozzle size.
 

"Red"

Graphic Designer
I was responding to russ's post everything I wrote came across.

My bad, I thought you were referring to an online calculator or something.

[(psi X gpm) /1450] X 1.5 = hp ---- ((3000 x 4) / 1450) x 1.5 = 12.4 ---- the Honda GX390's net hp is 11.7 (@ 3,600 rpm)

Not sure if this is suggesting that the engine is undersized or what. Where did the formula come from?
 

Scott Stone

New member
I usually just multiply for a general field calculation. Not 100% accurate, but it works well enough. So, for a 5 GPM at 3000 PSI, I mulitple 5x3 for 15 and add 1 for 16. For a 8GPM at 3000 PSI it is 25 HP. I like to keep it simple, because, I am almost as old as Russ, and don't remember my Algebra like I used to.
 

HighTide

New member
So, with these formulas it seems that most of the machines sold today are underpowered. I hope to buy an 8 gpm @ 3,500 psi unit this spring. With the formula above: (psi X gpm) /1450] X 1.5 = hp, one needs a 28.97 hp unit to handle these specs. With the Honda GX690 one of the most popular engines for this setup -- it appears grossly underpowered. Honda's own website spec sheet on the GX690 states net power output is: 22.1 hp @ 3,600 rpm.

With this said and the fact that I will be cleaning a ton of concrete with a ton of hours of full power, where does one go to get a properly powered setup for this type need? Seems like a 30 hp engine is the way to go for the specs I need, or is this not true? Thanks!
 

Russ Johnson

Equipment Expert
David, it's not to say the smaller engine can't handle it...it can, but at the expense of life expectancy.

Hydro Tek uses a 27 hp Kohler on their 30009 (8.5 @ 3000) and the 35008 (7.7 @ 3500). And they work well.

You are right Greg...did not see the hp rating pop up. Good call. I think it's a little underpowered, but a decent guideline.
 

"Red"

Graphic Designer
Any engine, regardless of HP, that will turn a pump at the specified RPM will produce the specified GPM & PSI output.

Pump output is based on RPM not HP. HP is needed only to achieve the desired RPM with a certain amount of efficiency.

This is simple logic folks, not rocket science. The pumps have already been engineered to produce a given PSI and GPM flow at a specific RPM.

Altering the RPM will change the GPM & PSI output. Find the pump specs and attach a motor that will turn it at the specified RPM and you're done.
 

gear junkie

New member
Many engines give their power rating at 3600 rpm but most pumps use 3400 rpms. Always look at the manufacters power curve charts when buying an engine. Honda got the lawnmower industry in trouble because they were overstating their engine specs which is why they stopped stating hp but instead use CC.
 

Russ Johnson

Equipment Expert
Any engine, regardless of HP, that will turn a pump at the specified RPM will produce the specified GPM & PSI output.

Pump output is based on RPM not HP. HP is needed only to achieve the desired RPM with a certain amount of efficiency.

This is simple logic folks, not rocket science. The pumps have already been engineered to produce a given PSI and GPM flow at a specific RPM.

Altering the RPM will change the GPM & PSI output. Find the pump specs and attach a motor that will turn it at the specified RPM and you're done.

Spinning the pump to a specified RPM will produce the advertised GPM. The pressure relies on the nozzle orifice and the engine horsepower.

I agree that any engine, regardless of the horsepower, is capable of attaining the proper RPM, but it takes horsepower to keep it at that spec under load.
 
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