A horsepower of another color.

Technically, one horsepower is the amount of power needed to move 33,000 pounds one foot in one minute, or 745.7 watts.

In practice, when dealing with engines, it's not that simple. There are a number of ways of determining the horsepower of an engine. Some examples:

* Indicated horsepower: Power developed within the cylinders of the engine. (This will be quite high.)

* Gross horsepower: Power measured at the flywheel of a bare engine or a stripped engine not installed in a tractor; the engine is equipped only with the parts needed to make it run.

* Net horsepower: Power obtained at the flywheel while the engine is equipped with all the accessories needed to make it operate by itself.

* PTO horsepower: Power obtained at the tractor power take-off (PTO). The PTO or belt tests made during the Nebraska tractor tests do not reflect any corrected results, only observed results. (See below).

* Drawbar horsepower: Power available at the drawbar of the tractor to pull equipment. The Nebraska tractor drawbar tests are only observed results: no corrections are made for temperature, humidity, baromecric pressure or wheel slippage.

* Peak brake horsepower: The highest horsepower developed without any speed limitation. These runs are often made during a very short time.

* Maximum intermittent brake horsepower: Considered the maximum usable horsepower; the customary definition used when referring to tractors.

* Continuous-brake horsepower: (Used mainly for power unit terminology); rating recommended for operation under continuous conditions.

* SAE horsepower: A Society of Automotive Engineers formula intended to determine the approximate brake horsepower of an engine.

The formula is horsepower = (D2 x N) / 2.5

D = the bore of the cylinder in inches

N = the number of cylinders

This formula assumes that the effective pressure is 90 pounds per square inch, that the piston speed is 1000 feet per minute and the mechanical efficiency is 75 percent.

To add to the confusion, correction factors are sometimes used to make the results of engine tests more directly related to the real-world working environment. This includes corrections for altitude, temperature and humidity.

(Adapted from a 1965 ASAE technical paper by H. J. Slotbower, product engineer, International Harvester Co.)

Steam horsepower is figured on yet a different scale.

Roughly and generally speaking, one steam hp equals 3 gas hp. Here's how steam hp is determined.

The key is PLAN, where P = the average pressure; L is the length of the piston stroke in feet; A is the area of the piston; and N is the number of revolutions (RPMs).

PLAN / 33,000 = horsepower.

Example: We have a steam engine that has a 3" bore, 4" stroke, and operates at 1,000 RPM at 100 pounds of pressure. With a little rounding off that gives us:

(Pressure) 100 x (Length) 0.333 x (Area) 7 x (RPMs) 1000 = 233,100 / 33,000 = 7.06 hp.

The horsepower of a steam engine must be matched to the horsepower of the boiler. Boiler horsepower is its capacity for generating steam. According to one old formula, boiler horsepower is determined by dividing the pounds of water evaporated into steam in one hour by 34.5.

However, boiler horsepower is usually determined as a fraction of the heating surface. Depending on the type of boiler and its construction, this can vary from 6-8 square feet of heating surface per hp (for a plain cylindrical boiler) to 10-12 for a water tube boiler to as much as 20 for a vertical boiler. (Source: Boilers; Types & Design, International Correspondence Schools, 1907; reprinted 1998 by Lindsay Publications, PO Box 12, Bradley IL 60915)