Horsepower and torque are related measurements of engine output in practical terms, an engines horsepower rating (hp) is most relevant to how well the engine performs at high speed, while it’s torque rating (expressed in pound-feet, or lb.-ft.) is most relevant to low-speed rock-crawling and trailer towing from a standing start. This is because maximum horsepower is achieved at high rpm levels (often in the 5,000- to 6,000rpm range) while maximum torque is achieved at lower rpm levels (often between 200 and 3,500 rpm). Each is a relative factor of the other. If you know one, you can figure out the other.

The term horsepower was derived by British inventor James Watt, who developed the first truly practical steam engine, used for pumping water out of mines. Since such water pumps had been powered by horses, Watt decided that the relative power output of his new engine should reflect a comparison with the number of horses it replaced.

Watt’s horsepower formula was based on several calculations. The pump horse walked in a circular path tethered to a 12-foot lever arm connected to what was, essentially, a crankshaft that turned the pump. Using the value (pi symbol) (=3.14) and the formula

(2 x pi) x radius

to calculate the circumference of a circle, this meant that the horse walked approximately 75.4 feet during each revolution of the crankshaft, i.e. (2xpi) x 12 =75.4.

Watt calculated that the horse traveled with a force of 180 pounds, and that it traveled at a rate of about 144 revolutions per hour, or about 181 feet per minute. By multiplying the two quantities, Watt quantified the horse’s measurable leverage—i.e., its torque—at 32,580 pound-feet per minute. Converted (and rounded off) to 33,000 pound-feet per minute. Watt had his 1-horsepower equivalent.

Engine horsepower is the relationship between known engine torque at specific rpm divided by Watt’s equivalent unit of one horsepower. Simplified, the equation is:

Hp = (rpm x torque) / 5,252

Conversely, to calculate torque the equation is:

Torque = (5,252 x hp) / Rpm

In today’s modern engines, horsepower is calculated electronically on an engine dynamometer, or dyno. Four specific classifications need to be mentioned, as they are often confusing:

Brake Horsepower (bhp): Prior to electronic bench testing, horsepower was quantified as the amount of resistance against a flywheel brake. Although the method is no longer used, the term remains an industry standard.

Gross Horsepower: Absolute maximum horsepower at the flywheel, with no load or drag from auxiliary systems, such as the alternator, water pump, etc. Created in an ideal environment with precisely controlled intake and exhaust flow characteristics, this category is of little practical use.

Net Horsepower: Maximum horsepower at the flywheel, with intake and exhaust systems in place and accounting for load from auxiliary systems. This is what most automotive manufacturers publish as SAE net horsepower.

Real-wheel Horsepower: Measured on a chassis dyno, the maximum horsepower transferred to the ground by the rear wheels. It can be affected by gearing, with lower (numerically higher) gearing tending to indicate a lower torque peak. Considered the most practical measurement of usable horsepower, it is the standard used in all 4WD vehicle tests whenever possible.

Thai, great article on horsepower versus torque.
If I can clarify a few points:

<b>"an engines horsepower rating (hp) is most relevant to how well the engine performs at high speed, while it

"No replacement for displacement."