Several Jetters Northwest customers have asked about the horsepower requirements of combustion engines as it relates to designing and building jetters to perform at particular gpm and psi specifications. It’s an excellent question relating to the dynamic engine-manufacturing market. Online forums and group message boards have discussed similar horsepower requirement questions as this, so let’s talk about newer realities that affect engine-horsepower ratings.

First, many of the questions about horsepower requirements for running a jetter pump up to a specified gpm and psi load can be put to rest if we understand one thing: The answer is not as much math related as it is marketing related. Search and you’ll find several different jetter-horsepower requirement calculations using: gpm x psi ÷ ___ (insert your preferred factor here). That third factor can vary depending on who you are asking — some use a more conservative factor while others a less conservative factor. Why is there any variation at all? Much of the confusion lies in the fact that some engine manufacturers continue to rate their engines’ horsepower via the more old-school and generous Society of Automotive Engineers J-1995 “gross horsepower” testing protocol, which results in higher horsepower ratings; more conservative manufacturers have switched to the newer SAE J-1349 standards of “net horsepower” test protocol, which result in lower hp ratings that are near actual net brake horsepower, which is more consistent with how electric-powered motors are rated to provide a reliable “net” horsepower capability to handle more continuous loads.

Switching to the newer SAE J-1349 test protocol has resulted in horsepower rating reductions of 15% or more, lowering popular engines that were previously labeled as 24 hp into 20.5 hp engines, and limiting other newer engine models to a rating of 24.8 hp that could have been rated to over 28 hp with the older more liberal SAE J-1995 standards. It’s important to realize that these engines didn’t magically lose power or lose any ability to drive a load; they simply adhered to newer standards put out by SAE to rate the engines’ horsepower capability.

So back to the marketing aspect: Some engine companies are still using the older and more generous SAE J-1995 gross horsepower standard. Why? Maybe so they can advertise a higher hp rating, or perhaps to avoid the time and expense of retesting and recertifying engines, updating websites and printed specs, etc. On the other hand, other engine manufacturers have elected to retest and re-rate engines so they can confidently advertise “certified net horsepower.” Still others only publish an engine’s CC displacement and don’t label a horsepower on the engine at all.

Another newer factor when choosing gasoline-fueled engines for jetters and other power-equipment is the fuel system itself; is it a classic carbureted engine or is it a more modern fuel-injected engine? Engines with fuel-injection along with electronic air/fuel mix controls can typically produce more horsepower than carbureted engines with the same CC displacement. By the way, engines with electronic fuel injection tend to be more fuel-efficient and have cleaner exhaust emissions than their carbureted counterparts. All considered, it’s no wonder why carburetors have long since disappeared from cars, trucks and other vehicles.

The bottom line — and only thing that really matters — is how a jetter or other machine performs in the manufacturer’s testing and in actual field-use for contractors like you. We choose engines based on the net horsepower curves printed by reputable manufacturers and then test our jetters at full-flow and pressure as rated to verify their capabilities, while measuring things like oil and head temperature, load-to-no-load rpm differential, and governor position to ensure our machines are built to work — and built to last.

About the authors: John McBride and Steve Jones from Jetters Northwest have designed and built hundreds of different jetter models and other engine-driven high-pressure pump systems in their 30-plus years with the company.