Engine Tuning: Understanding the Origins of Today’s Car Tuning, Performance Tuning and ECU Remaps Part 2

This collection of articles consists of four parts. They will help to explain the origins of Engine Tuning, explanations of the engine tuning process and how, the now popular car tuning and performance development industry works.

This article is Part 2 of 4 and looks at how to calculate an engine’s horsepower. Part 1 gave a brief introduction to how Horsepower first came about.

So to calculate an engine's horsepower at some given speed, you use the following formula:

power = Torque × RPM ÷ 5252

To prove this formula we must look at the proof, which is as follows:

Rev the engine to the desired speed. Measure the torque at that speed. Multiply the torque by 2 * PI * the engine speed in RPM. This gives you:

X foot-pounds × 2 × PI × REVOLUTIONS ÷ MINUTE

Which, if you look closely, is a FORCE (in pounds) times a DISTANCE ("one foot × 2 × PI × revolutions" is the distance the tip of our one-foot moment arm travels, in feet) divided by TIME (in minutes). As we know, FORCE times DISTANCE divided by TIME = POWER.

We now have power expressed in foot-pounds per minute. To convert to horsepower (33000 foot-pounds per minute, remember), we simply divide by 33000.

So Horsepower = Torque × 2 × PI × RPM ÷ 33000

so Power = Torque × RPM ÷ 33000÷ (2×PI)

so Power = Torque × RPM ÷ 5252

This incidentally, implies that an engine's torque and horsepower curves cross at 5252 RPM.

So, as you can see, power and torque are very closely related, but it is important to realize that they are different. Remember that you can be applying a lot of torque for no result. Looking at the fact that power is the rate of doing work, it is obvious that if you are doing no work, you generate no power!

So, no matter how hard you push that spanner, if the bolt doesn't turn, you generate no power.