‘Horsepower Sells’
Over the years, manufacturers in the small-engine business have used horsepower ratings as a marketing tool. It is a very competitive business and in reality “Horsepower Sells.” It doesn’t always sell for the right reasons, but it does sell. If you can buy a 22 hp motor from one brand for the same money as a 19 hp from another most of you would opt for the larger motor.
To state horsepower the industry has commonly used the SAE J1940 rating standard. This standard will continue to be the most common standard used but the way the engines are rated but it has changed for 2014. In particular the horsepower listed on the engine label will be closer to the actual horsepower that your new tractor engine actually produces.
In the past the standard permits the advertised rated value (the label on the engine) to be 85% of the test values.” For example, on an engine labeled 19 hp, the actual horsepower of the engine could be anywhere from 16 to 19 hp. Under the new revision to SAE J1940 the actual horsepower listed on the motor will be at least 95% of the actual, tested horsepower. In other words, an engine labeled as 19 hp will actually produce 18-19 hp at a certain rpm.
This change to the standard marks a change for the outdoor power equipment industry. I believe it will make it easier for you to understand the horsepower labeling compared to “usable” power.
Craftsman has always been committed to giving you all the information you need about the products they offer and they have put together a fact sheet to help you understand the changes. I want to thank Craftsman for allowing me to use this information here.
From Craftsman: (my notes are in italics)
What exactly is changing with the revised standard?
The previous version of SAE J1940 required that the engine produce 85% of the labeled power with 95% confidence. The revised standard requires that the engine produce 95% of the labeled power with 95% confidence. 
The revised standard also requires that the rpm be included in the rating. In accordance with the revises SAE J1940 standard, you will see revised ratings on many tractor engines sold by Sears. Engines will have the same output, and same cutting performance as they did before.
Does that mean the power output will change?
No. Briggs & Stratton and Kohler single cylinder engines will have the same power output and same performance as they did before. Nothing has changed in the design, performance or serviceability of the engines as a result of this new standard, although the advertised horsepower will. “For example, if we test an engine at 10 hp,” says Knott, “the old standard permitted us to advertise 11.5 hp. Under SAE J2723 we will label it as a 10.2 hp. unit. Kohler twin cylinder engines are newly designed for 2014.
Will the horsepower ratings be different?
Yes. The revised HP rating standard results in a more precise rating of engine output. In most cases, the HP will be lower in 2014 than it was in 2013. (For example, the 21 hp Briggs Platinum will now be labeled 19 hp at 3600 rpm)
Does this revised standard affect all engines?
The revised standard will be applied to all Briggs & Stratton and Kohler engines; however, the labeled power rating will change only on select single and v-twin cylinder engines.
Kawasaki rates engine horsepower per SAE J2723 guidelines for their commercial engines. All their engines labeled “Critical Power” meet these standards. These guidelines require that most production engines produce at least 98% of their rated values, which is 3% tighter than the SAE J1940 standard. It is unclear what standard they use for the lawn tractor engines.
When is the revised SAE standard effective?
The revised SAE J1940 standard was published on October 18, 2012 and is effective one year after that date. (Engines made after Oct, 18, 2013 are required to use the new labeling)
When will I expect to see the revised ratings?
All 2014 season product (both engines and end products) will be rated in accordance with the revised standard. (If the HP is listed on the hood it will also reflect the new labeling)
Will this revised rating apply to all engine brands?
Any engine manufacturer that utilizes the SAE J1940 rating standards will now need to follow the requirement that the engine produce 95% of the labeled power with 95% confidence, and the requirement that rpm be included in the rating. (The engine in the 30 inch rider and the economy tractor uses cc’s and does not use a hp labeling)
Does this new rating system affect engines sold outside the United States?
The rating system that is used on all rider engines for the European market will remain unchanged. (Craftsman does sell tractors over there) For other markets rating in HP or Torque, the revised SAE J1940 rating standards will be followed. (Canada and Australia?)
The other brands have not sent me information yet on how they will be using this new rating system for their lawn tractors and zero-turns. As they let me know I will add their info to this post.
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