The distinction between gross and net horsepower lies in both math and marketing.
I was recently asked by a satisfied Jetters Northwest customer to help him understand horsepower as it relates to designing and building our jetters. So I thought, certainly he can’t be the only one, right? After all there are several different forums and message boards out there and all of them have probably hosted discussions that are similar to the one he was in. We could call this post “everything you didn’t care to know about horsepower … but were not bored enough to ask.”
Many of the questions about horsepower requirements for running a jetter or pressure washer pump can be put to rest if we understand one thing: The answer is not as much math related as it is marketing related.
It is easy to look up several different calculations for horsepower using gpm x psi ÷ (insert your preferred method here). Some numbers are more conservative and others are less; however, most of the confusion lies in the fact that some engine builders are using Society of Automotive Engineers “gross horsepower” (SAE J-1995 test protocol) to rate their engines output, resulting in higher ratings, while others have switched to the new standards of “net horsepower” (SAE J-1349 test protocol) which result in numbers closer to bhp (brake horsepower) — much like how you would size an electric motor for a given load.
This switch ended up in rated hp reductions of 15 percent or more, turning engines previously labeled as 24 hp into 20.5 hp engines and giving ratings of 22 hp to engines that could have been rated to 27 hp. These engines didn’t magically lose power or any ability to drive a load; they just adhered to new standards put out by SAE.
So back to marketing: Some engine companies are still using the older and more generous SAE J-1995 “gross horsepower” standard, either so they can advertise a higher rating, or so they don’t have to go through the expense of re-testing and certifying engines. While other engine builders are re-testing engines so they can advertise “Certified Net Horsepower” while others only publish an engine’s displacement and don’t talk horsepower at all.
The bottom line and only thing that matters is how a machine performs in testing and in the field. We test our machines at flow and pressure and measure things like oil and head temperature, load-to-no-load rpm differential, and governor position to ensure our machines are built to work and to last.
John McBride is the Jetters Northwest Production Manager.