Jason Fenske and the team over at Engineering Explained offer up one of our favorite YouTube channels, as he takes complex engineering topics and simplifies them for the masses. This video touches on the difference between power and torque, which, of course, is critical if you hope to have any understanding of the relationship between the two and how they correlate to engine output.
The definition of the word torque: a force that produces or tends to produce rotation or torsion.
Torque is simple to understand. It’s simply force multiplied by distance. You apply a force to the end of a wrench; that force is a specific distance from the fastener being tightened, which gives you a torque value. It’s the same concept which allows us to measure torque in an automotive engine. Combustion pressure acts upon the piston to push it down (force), which then flows through the connecting rod (distance) to the crankshaft pin and gives the engine its twisting force. Torque can be easily increased by using a longer arm/wrench/connecting rod (distance) to provide greater leverage with the same amount of force.
The definition of the word horsepower: a unit of power equal in the U.S. to 745.7 watts and nearly equivalent to the English gravitational unit of the same name that equals 550 foot-pounds of work per second.
As explained in the video, horsepower is the rate at which work is done. For example, the horsepower required to move a car from one place to another (distance) at a rate of one mile per hour is far less than the power required to move a car from one place to another at a rate of 60 miles per hour. The rate of work at 60 miles per hour is far greater, so more power is required to achieve this. Power, as Fenske explains, is what gives you acceleration.
To relate this to an engine, horsepower can be thought of as force x distance x RPM. How fast the engine is spinning will provide more force on the top of the piston and thus to the crankshaft. The twisting force (torque) is applied to the transmission, driveshaft(s), and ultimately the wheels.
This is most evident with naturally-aspirated engines where the force applied to the piston doesn’t change throughout the engine’s powerband. This beautiful 10,000 rpm Dart LS Next-based bullet—since it don’t have the benefit of additional force applied to the top of the piston via an external super- or turbocharger—has to spin faster to make more power (more force applied to the top of the piston as engine speed increases.) The engine makes more power at 9,500 rpm than it does at 7,500 rpm. As an engine is just a big air pump, it requires efficiency, and the more efficiently it can pump that air though itself, the more power it will make. At low RPM levels, it is not pumping air as efficiently as it is at higher RPM levels.
There’s an interesting correlation in the video as well, comparing two theoretical vehicles with the same mass; one makes 200 horsepower and 100 lb-ft of torque, while the other reverses those figures with 100 horsepower and 200 lb-ft of torque. Although the vehicle with the higher torque value would seem to be the one able to move more quickly (as it offers double the force) Fenske explains that the one with more horsepower is always going to be quicker, because horsepower is the rate at which work is done (moving the car from one place to another.)
He then digs into the relationship between the two, how to analyze the curves, and the best way to maximize hypothetical performance. Is it better to build an engine to operate at peak torque, or peak horsepower? You’ll have to watch the video for the details—which we find fascinating. Hint: it’s all related to gearing.