Horsepower Vs Torque

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Horsepower vs Torque:

Horsepower Or Torque?:

What is Torque? It is a force. It can be measured and felt and quantified. If you apply a force to an object and it moves, you have done work.

What Is Horsepower?

It is a number and that’s all. It is a numerical expression of what your engine does.

The force (or amount of energy/work) needed to move a particular object a certain distance is always the same so long as the conditions remain constant. If you apply more force, you will do the same work in less time. Heat is a by-product of work.

How did the term “Horsepower��? come into being? When James Watt (he of the Steam Engine) wanted to rate the power of his steam engines, he used an above average horse to define the basic unit of power. He calculated that a good horse could lift 550 lb (249.5 kg) at the rate of one foot per second all day. So, anything that can lift 550 lb one foot per second is producing 1 Horsepower of work. Therefore, 550 is the defining number for Horsepower. Horsepower is force times distance divided by time with the result divided by 550.

For example:

Work/Time = Force times distance divided by time
= 550 (lb) times 1 (Foot) divided by 1 (second) divided by 550
= 1 (Horsepower).

Or 550 x 1 (=550) / 1 (still equals 550) / 550 = 1


But what if we lift the same weight the same distance but twice as quick?

Work/Time = Force times distance divided by time
= 550 (lb) times 1 (Foot) divided by 0.5 (seconds) divided by 550
= 2 (Horsepower).

Or 550 x 1 (=550) / 0.5 (equals 1100) / 550 = 2


Or if we move the object twice as far in the same time?

Work/Time = Force times distance divided by time
= 550 (lb) times 2 (Foot) divided by 1 (second) divided by 550
= 2 (Horsepower).

Or 550 x 2 (=1100) / 1 (equals 1100) / 550 = 2


What does this mean for our engine?

It is the torque that your engine produces that does all the work. This is a product of the cylinder pressure, the piston area and the length of the stroke. The higher the pressure on the piston, the greater the force transmitted to the crank, the greater the torque produced. A second combustion event will add to the work that the engine can do, so spinning the crank faster means more work can be done in the same time. This is why Horsepower is torque times revolutions for a car’s engine.

The acronym for this is PLAN. This stands for Pressure (in the cylinder), Length (of stroke), Area (of piston top) and Number (of cylinders). If we multiply these factors together and divide the result by 550, we will have a result expressed as horsepower.

In reality, these factors are combined and expressed as torque (as it is torque that a dyno directly measures). And Torque times the distance through which it is applied gives us the basic form of power equation as used to give a number to Horsepower.

Torque (ft/lbs) times Distance (ft) times revs (per second) divided by 550 = Horsepower.

It is the distance and revs part of the equation that make it possible to calculate Horsepower.

Remember that the distance through which the force is applied defines work. A one foot radius (2 foot diameter) circle (because the torque is expressed in foot/pounds) has a circumference of 6.2832 feet and revs per second needs to be multiplied by 60 to get revs per minute (a more usual measure) so we can also multiply the 550 by 60 to keep the equation balanced.

This means --> Horsepower = Torque (ft/lbs) times 6.2832 times RPM divided by 33,000.

Or more simly --> Horsepower = Torque (ft/lbs) times RPM divided by 5252.

This is the version that is most commonly used in the auto industry. When you hear someone rattle this off (assuming you have those types of conversations) you will now know where it comes from.

There is, of course, a metric equivalent. Torque (in Newton/metres) times RPM divided by 9549 equals Kilowatts.

What does all this mean anyway? Look at a power and torque graph produced by a dyno…the torque part of the graph will start to fall even though the power line continues to rise…why? Because, even though less torque is now being produced by each stroke of the engine, it is being produced more times per minute because the engine is revving faster, thereby producing more power. If the torque happened to be decreasing at the same rate the rpm was increasing, then the horsepowe curve would be flat in that range.

Why does torque production drop off? As the engine spins faster, there is less time for a fresh charge of fuel/air mixture to enter the cylinder which results in less pressure pushing the piston down and less torque for each stroke. Eventually, the volumetric efficiency of the engine becomes so poor that not enough fresh charge is being drawn in to the cylinder to make enough pressure on the combustion stroke to make the engine spin faster, so power drops off.
Also, with higher speeds, mechanical inefficiencies increase, further contributing to the drop off in power.

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