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Does Horsepower Determine (PSI) and (GPM) Water Flow?

Updated: Oct 19, 2023

How does Horsepower determine my Pressure (PSI) and water flow (GPM)?

Understanding how Horsepower affects pressure washer performance and capacity:

HP (Horsepower), PSI (Pressure), GPM (Water Flow expressed in Gallons per minute) will determine any pressure washer unit's performance which the unit was designed to operate at. Exceeding those MFG performance ratings will prematurely wear your machine out or cause serious damage. This is true on any brand and all pressure washing equipment: gasoline, electric or diesel-powered machines.

Brake horsepower is a measurement of how much "useful" power an engine can produce. By taking the total power of an engine and allowing for the power lost through friction (in the engine’s mechanical parts, gearbox, water pump, and generator) you can calculate the brake horsepower.

This kind of horsepower is a measure of how much an engine can output with all of the necessary loads applied to it. These loads are things like the alternator running faster to power air conditioning as well as how much force the engine is able to exert and the useful power it can generate.

The equation to calculate horsepower is simple: Horsepower = Torque x RPM / 5,252.

RPM horsepower to torque graph

When it comes to understanding how a dynamometer measures torque and calculates power, it will help to know a few more basic definitions and formulas.

Performance of all pressure washers is determined initially by the HP rating of the motor (power source). Understanding working pressures and water flow of pressure cleaning equipment is important to ensure proper performance of your equipment. The power source (motor or engine) determines what force and pressure the unit can safely operate at. An oversized nozzle orifice (tip hole size) will produce less pressure than the intended design and will not increase the flow due to the pump displacing a predetermined and exact amount of water.

An undersized nozzle will damage the pump, due to the restriction of the smaller orifice opening. Damage to the pump will occur by the restriction that will put too much back pressure on the pump displacing the predetermined flow and exact amount of water it is designed for.

Pressure washing equipment is designed to perform at specific pressures and water flow by the manufacturer. Perhaps this may help understand how it all comes together.

X__ amount of HP will deliver X___ amount of power that is transferred to the pump using a type of drive (direct, belt, gear) and a type of powered motor (gasoline, diesel, electric or hydraulic) power. Additional control devices such as an Unloader, (usually with) a pressure control adjustment in conjunction with properly matched nozzle orifice opening sizes will deliver the desired pressure (PSI) and water flow, represented as GPM (Gallons Per Minute).

Pressure washer pumps are designed to produce a maximum pressure with a fixed water flow using properly matched tip/nozzle orifice (hole opening or size). Tips/nozzles are available with various spray patterns that must be properly matched to perform as desired and prevent pump damage due to improperly matched tips or nozzles. The spray pattern typically ranges in fans of: 0, 15, 25 and 40 degree fan patterns.

More information on Tips can be found on our nozzle chart BLOG.

That all may sound kind of complicated but it’s not that much of a mystery. Call our Expert Support Team with any questions. We are ready to help! Call us at: 800-493-7692

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