The PSI rating of any pump is based on the ability of the weakest component of the pump to withstand the pressure over a given time without failing. In other words it is the ability of the pump to resist the forces of the pressure. Where does the pressure come from? I will get to that in a moment.
Every piston or wobble plate pump is a positive displacment system. Every stroke of the piston will displace the same amount of water. RPM will determne how many of these units of volume will be pumped. Engineers determine the safe effective working parameters for all of the moving components based on longevity of the components and their ability to withstand the stresses imposed by moving an incompressible fluid. Essentially what you have is a hydraulic pump.
All pumps are rated at a set RPM to provide rated GPM/Litres. They also allow for a +/- of 10% minimum and still considered within specification. They are also rated without a nozzle in place.
Now where does the pressure come from? Have you ever put your thumb over the end of your garden hose to get more power? Well, the tip provided by the manufacturer is nothing more than a piece of hardened steel with a very precise hole in the end to provide resistance to flow, therefore increasing the pressure. Using a soap nozzle will demonstrate the pressure lowering available from a larger orifice.
The nozzle chart will show you what your 'actual' GPM and pressure are for a given orifice. Refer to the excellent chart here
http://www.arnorthamerica.com/spraynozzlechart.html
to see what the different sizes will give.
Over-speeding a pump will lower it's life expectancy and void all warranties.
For future conversion reference please use the following formulae...
1 bar = 14.5038 PSI
1 litre = 0.264 US Liquid gallons
1 US Liquid gallon = 3.7854 litres
Hope this helps? Cheers.
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