10 Things You Always Wanted to Know About Engines

This article is copied from the September 1999 issue of Hot Rod Magazine

By Ray T. Bohacz

       How can torque and horsepower always be equal at 5,252 rpm?  The variations in cam profile, airflow, and compression ratio, let alone different engine designs, must surely prove this wrong.  It is difficult to comprehend that no matter what you do to your engine, the power and torque will always be equal at this crankshaft speed, albeit greater than when the engine was stock. 

        The easiest way to fix this in memory is to realize that a dynamometer measures torque, and that horsepower is a mathematical derivative of that value.  Torque can be defined as the amount of work an engine can perform; horsepower is how quickly the work can be accomplished.  Engine rpm is the key to horsepower.  The power equation is:

hp = torque x rpm / 5,252

        Let’s compare an engine to a person stacking cement bags on a truck. Worker A is able to lift a maximum of 100 pounds; that represents his torque rating.  If worker A takes one minute to load each 100-pound bag, it would follow that in one hour, he could stack 6,000 pounds of cement.  Worker B can also lift 100 pounds, but he can load two bags in one minute; in one hour, he could load 12,000 pounds of cement.  Workers A and B have the same strength, but B can do the work faster.  If A and B were engines instead of flesh and blood, we would say that engine B has more horsepower than A, since more work was accomplished in the same time period.  In the early 1600’s James Watts established the basis for this equation as a means of comparing a steam engine to the draft-horse standard.  Since no two horses have the same strength, torque is a true indicator of the ability to do work.  Since the constant in the denominator is 5,252 and is divided by the rpm, at 5,252 revolutions per minute, both the numerator and denominator are equal, which equals one.  Torque (at this engine speed) is multiplied by one, which explains how horsepower and torque are equal.  To convert power to torque, the equation becomes:

            torque = 5,252 x hp / rpm

        Different standards are used to rate power, e.g., net, gross, corrected, flywheel, and drive-wheel ratings.  Regardless of these factors, the equation still remains the same:  it reflects the amount of work accomplished over time.  (NOTE: To complicate matters, cylinder pressures are the true indication of an engine’s ability to do work, but at the aftermarket level, we talk power and torque, not cylinder pressure.)