DEFINING HORSEPOWER AND TORQUE
Posted: Sun May 23, 2004 8:08 am
Engines don’t make horsepower; they convert fuel into torque. Torque is the twisting force imparted to the crank flange and then transmitted to the transmission and the rest of the drivetrain. To some degree torque is the grunt that gets things moving, and horsepower is the force that keeps things moving. An engine is most efficient at its torque peak, wherever that happens to occur. Below the torque peak, engines generally have more than enough time to fill the cylinders; above the torque peak, they don’t have enough time to completely fill the cylinders. This is generally beneficial in that it lets engines produce most of the desirable grunt work (torque) at lower engine speeds, which means reduced wear-and-tear and better fuel economy. The ability to extend an engine’s speed-range allows it to stretch that torque curve out farther, provided that the high-speed efficiency is there to make horsepower.
Power is torque multiplied by engine speed to produce a measurement of the engine's ability to do work over a given period of time. The story of its origin is well known, but worth repeating, briefly. In the 18th century, steam engine inventor James Watt sought a way to equate the work his steam engine could perform to the number of horses required to perform the same task. Watt performed simple tests with a horse as it operated a gear-driven mine pump by pulling a lever connected to the pump. He determined that the horse was capable of traveling 181 feet per minute with 180 pounds of pulling force. This multiplied out to 32,580 lbs-ft per minute, which Watt rounded off to 33,000 lbs-ft per minute. Divided by 60 seconds, this yields 550 lbs-ft per second, which became the standard for 1 horsepower. Thus, horsepower is a measure of force in pounds against a distance in feet for a time period of one minute. By substituting an arbitrary lever length for the crankshaft stroke, you can calculate the distance traveled around the crank axis in one minute multiplied by engine speed (rpm) and known torque to arrive at the formula for horsepower:
Because torque and rpm are divided by 5252, torque and horsepower are always equal at 5252 rpm. If you solve the equation at 5252 rpm, the rpm value cancels out, leaving horsepower equal to torque. If you plot torque and horsepower curves on a graph, the lines will always cross at 5250 rpm (rounded off). If they don't, the curve is undoubtedly bogus.
Torque is the static measurement of how much work an engine does, while power is a measure of how fast the work is being done. Since horsepower is calculated from torque, what we are all seeking is the greatest-possible torque value over the broadest-possible rpm range. Horsepower will follow suit, and it will fall in the engine speed range dictated by the many factors that affect the torque curve.
Increased displacement is the easiest way to achieve increased torque. Very large cylinders and a long stroke offer the greatest cylinder volume and overall piston area for the fuel charge to push against the crankshaft or lever, if you will. Stationary industrial engines that produce tremendous amounts of torque are typically quite large. The mass and bulk of one of these engines makes extremely large displacement engines impractical for use in cars.
We are limited to displacement values that are easily packaged within the confines of your typical automobile engine compartment. The practical limit is between 400-500 cubic inches for most large automobile engines. Big-block engines in this range deliver tremendous torque, and they are easier on parts for the same amount of power output. Engine builders have stretched displacement out as far as 800 cubic inches with highly modified cylinder blocks and crankshaft strokes, but these engines are not practical or economical for general high-performance applications.
This leaves us searching for ways to increase torque in smaller engines by increasing efficiency through the manipulation of mechanical components, gas dynamics and thermodynamics (to increase and harness cylinder pressure). There are many ways to do this, but most involve some sort of tradeoff somewhere in the power curve. To a great degree, we are forced to build engines for greater efficiency within a chosen engine speed range. Some combinations will function very well at low speeds, others will be strong in the mid-range, and still others will only run hard at a high rpm. The key is selecting the combination of components that will stretch and fatten the torque curve (improve efficiency) as much as possible in the driving range we prefer. Our saving grace is the relatively forgiving nature of internal combustion engines wherein torque dissipates gradually as engine speed increases. As long as the induction system can carry the airflow demand created by the cylinders at high engine speeds, the torque curve will remain broad. This allows engine speed and horsepower to carry the engine farther in the rpm range before the net effect of induction restrictions at high engine speeds chokes off efficiency. The following are some basic methods for increasing torque and, thus, horsepower across the typical range of modern-performance engine speeds.
Power is torque multiplied by engine speed to produce a measurement of the engine's ability to do work over a given period of time. The story of its origin is well known, but worth repeating, briefly. In the 18th century, steam engine inventor James Watt sought a way to equate the work his steam engine could perform to the number of horses required to perform the same task. Watt performed simple tests with a horse as it operated a gear-driven mine pump by pulling a lever connected to the pump. He determined that the horse was capable of traveling 181 feet per minute with 180 pounds of pulling force. This multiplied out to 32,580 lbs-ft per minute, which Watt rounded off to 33,000 lbs-ft per minute. Divided by 60 seconds, this yields 550 lbs-ft per second, which became the standard for 1 horsepower. Thus, horsepower is a measure of force in pounds against a distance in feet for a time period of one minute. By substituting an arbitrary lever length for the crankshaft stroke, you can calculate the distance traveled around the crank axis in one minute multiplied by engine speed (rpm) and known torque to arrive at the formula for horsepower:
Because torque and rpm are divided by 5252, torque and horsepower are always equal at 5252 rpm. If you solve the equation at 5252 rpm, the rpm value cancels out, leaving horsepower equal to torque. If you plot torque and horsepower curves on a graph, the lines will always cross at 5250 rpm (rounded off). If they don't, the curve is undoubtedly bogus.
Torque is the static measurement of how much work an engine does, while power is a measure of how fast the work is being done. Since horsepower is calculated from torque, what we are all seeking is the greatest-possible torque value over the broadest-possible rpm range. Horsepower will follow suit, and it will fall in the engine speed range dictated by the many factors that affect the torque curve.
Increased displacement is the easiest way to achieve increased torque. Very large cylinders and a long stroke offer the greatest cylinder volume and overall piston area for the fuel charge to push against the crankshaft or lever, if you will. Stationary industrial engines that produce tremendous amounts of torque are typically quite large. The mass and bulk of one of these engines makes extremely large displacement engines impractical for use in cars.
We are limited to displacement values that are easily packaged within the confines of your typical automobile engine compartment. The practical limit is between 400-500 cubic inches for most large automobile engines. Big-block engines in this range deliver tremendous torque, and they are easier on parts for the same amount of power output. Engine builders have stretched displacement out as far as 800 cubic inches with highly modified cylinder blocks and crankshaft strokes, but these engines are not practical or economical for general high-performance applications.
This leaves us searching for ways to increase torque in smaller engines by increasing efficiency through the manipulation of mechanical components, gas dynamics and thermodynamics (to increase and harness cylinder pressure). There are many ways to do this, but most involve some sort of tradeoff somewhere in the power curve. To a great degree, we are forced to build engines for greater efficiency within a chosen engine speed range. Some combinations will function very well at low speeds, others will be strong in the mid-range, and still others will only run hard at a high rpm. The key is selecting the combination of components that will stretch and fatten the torque curve (improve efficiency) as much as possible in the driving range we prefer. Our saving grace is the relatively forgiving nature of internal combustion engines wherein torque dissipates gradually as engine speed increases. As long as the induction system can carry the airflow demand created by the cylinders at high engine speeds, the torque curve will remain broad. This allows engine speed and horsepower to carry the engine farther in the rpm range before the net effect of induction restrictions at high engine speeds chokes off efficiency. The following are some basic methods for increasing torque and, thus, horsepower across the typical range of modern-performance engine speeds.