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Understanding Horsepower Ratings

Q: What does the term "horsepower" mean?

A: "Horsepower" is a measurement of an engine's rate of doing work. Originally, the term horsepower was used by an engineer named Watt, who determined that a strong horse could lift 366 pounds of coal up a mine shaft at the rate of one foot per second. Over time, "horsepower" has acquired a technical meaning: a unit of power equal to 745.7 watts.

Q: How Is horsepower measured?

A: Engine manufacturers typically measure horsepower by operating a "bare" engine (e.g., one that is not yet equipped with accessories or installed in power equipment) at its "rated speed." The "rated speed" for most engines normally is 3600 RPM. The rated speed for engines used in handheld equipment is somewhat higher -- between 4000 and 12,000 RPM. Measuring a "bare" engine at its "rated speed" produces a number referred to as "gross horsepower."

Q: Can I expect my power equipment to achieve the same horsepower as the "gross horsepower ratings of the engine?

A: The actual operating horsepower is typically somewhat lower than the gross horsepower rating. For example, exhaust systems and air filters on the engine will generally reduce its power. Similarly, throttle govenors, which are used to maintain engine speeds under varying loads, also may keep small engines from operating at their "rated speeds" once they are installed in equipment.

Q: Will outdoor conditions impact the machine's actual horsepower?

A: They may. The horsepower of an engine installed in outdoor power equipment can vary depending on numerous factors such as heat, altitude, and humidity. Throttle settings and the resulting horsepower may also fluctuate depending on conditions such as thickness or wetness of the grass, for lawnmowers (and comparable variables for other types of power equipment).