Car Drift

Torque vs. Horsepower – What is the Difference?

We hear the words torque and horsepower, usually in connection to cars, when motorheads debate furiously over which is better and essential when it comes to building and driving automobiles. The torque vs horsepower debate is old and sometimes considered redundant, but to understand it fully, we must look at the science behind the two terms.

Horsepower and torque are units used in physics to talk about acceleration and are used most often concerning moving objects, especially vehicles. To fully comprehend the difference between torque and horsepower, let us look at how physics defines the units.

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What is torque and horsepower – according to physics?

Let’s start at the very beginning – with energy. Energy is the capacity an object has for doing work. There are several kinds of energy, which we will not discuss here, but in this context, the engines of a car perform the work and have the capacity to do the work. Work here means the result of any force acting over some distance. Work is the result of using energy and how much we see produced. It is measured in joules in the International System of units or Newton meters.

Torque determines an engine’s ability to perform work and has the same units. It is a rotating force that the engine’s crankshaft produces. As a shaft is driven by a series of cranks and pins to which the engine’s connecting rods are attached, the crankshaft’s function inside a car is to convert reciprocating motion into a rotational motion of the car, or in other words, to produce torque.

The more torque an engine can produce, the higher is its capacity to do work. But there are exceptions to this rule – there is a phenomenon called static torque. It is by no means a rare phenomenon – when you use a wrench to tighten a bolt, the force you apply is static torque.

Let’s move on to power- James Watt defined power as the rapidness or rate at which work is completed. This way, adding that the power required to move 33,000 pounds over a distance of one foot in one minute is one horsepower. The measurement unit for horsepower, according to the SI, is a kilowatt.

Power is the rate of completing work or applying torque over a given amount of time or the time taken to complete a strenuous task. Usually, in physics, horsepower equals torque multiplied by rpm. RPM is revolutions per minute, or how fast the engine is spinning. The equation looks something like this:

H = T x rpm/5252

H stands for horsepower, T (Torque) is pound-feet, and 5252 is the constant that makes sure the proportions are in place. This equation shows that an engine needs to generate more torque to generate more power or operate at a higher rpm. In short, if you multiply the torque of the axis’s rotational speed per minute, you will get a figure equal to the engine’s final output.

How does this translate to real-life engines?

While these definitions work great in theory, it does not translate exactly in a practical context where the theories have to be applied to real engines. Most car engines have an idle-to-redline operating range – a vehicle’s redline is the maximum engine speed possible for a vehicle, internal combustion engine, or traction motor without causing damage to the internal components.

For instance, the Dodge Challenger’s Hellcat V-8, with a capacity of 6.2 litres, can produce 77 horsepower only at 6000 RPM. The power it produces while it is idle is substantially less and produces its maximum of 650 pound-feet torque only when it reaches 400 RPM.

It is also difficult to correctly quantify variables like power and torque from a spinning crankshaft, even with the use of power measurement devices like engine dynamometer. But experts still rely on the equation we saw earlier to measure such variables in practice. There are two types of dynamometers using which we can measure torque and power, one of which is the eddy current dynamometer, the other being a simple water brake.

How much torque does the perfect engine have?

An ideal engine should produce enough torque at low rpm to sustain that output to the redline. It should produce torque at an amount that is directly proportional to the air flowing through the engine. This is why large engines produce more torque – they pump out more air.

Boosters work on this concept – when there is additional air delivered to small engines by boosters like superchargers and turbochargers, they help them function on a larger level, provided appropriate fuel is supplied to the combustion chambers. This is easy to do because we have developed a method to control the injection of the precise amount of fuel electronically.

What is not easy is to make sure all the components can withstand the high loads and pressures they will be subjected to, especially in the case of moving parts, which have to deal with their own inertia. Cooling and lubrication are also paramount when it comes to producing power, as is factor fuel efficiency and exhaust cleanliness. Clearly, an automobile engineer has to think about a myriad of problems and their solutions before declaring the horsepower of their masterpieces.

Can you tell the difference between torque and horsepower in real life?

Now that we know what torque and horsepower mean in physics, let us look at what it means in automobile terms – torque is the turning ability of the engine or its ability to turn its flywheel. Horsepower, on the other hand, denotes the total power output of the engine.

Another facet of the debate that is usually ignored is the fact that you need a lot of experience to even differentiate or measure the torque and horsepower in a car without a dynamometer. You cannot really feel the change unless there is at least a 10% difference unless the seat of your car is particularly calibrated for this purpose. You probably will not be able to distinguish the characteristics of an ordinary car with any accuracy.

In layman’s terms, the force you feel pushing you against the seat when a car accelerates is torque. The speed the automobile has achieved at that point of acceleration (or at the end of it) is horsepower. Most automobile engineers use the unit Brake Horsepower, which is the amount of power produced by an engine after subtracting the amount that gets lost due to friction. They measure torque conversion at pounds per foot of rotation around one point.

Clearly, wondering which is more important is irrelevant once you understand the difference between torque and horsepower because one cannot exist without the other. However, the balance between the two and their interactions with each other. You can change the way an engine works by manipulating one or the other, which is what the torque vs horsepower debate usually boils down to.

Many claims to prefer certain cars over others because it provides better horsepower or torque and this difference come from the fact that no two engines can be the same (at least when we are considering different models), and no two units pulled from the same line will be the same on an engine dynamometer. Dynamometer readings are affected by air, temperature and humidity, and even forced induction, so we would need to complete the readings in a fully controlled environment as well. This is because there is no real machine that can measure a car’s horsepower very accurately.

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Can you customize torque vs. horsepower?

In a way, yes. All these measurements and the trouble it takes to get the figures to pay off because we can shape the characteristics of power using the numbers – a long-stroke engine, for example, generally gives more torque than a short-stroke engine. Engineers also regulate the cam timing to change the power delivery – by advancing the cam timing, they effectively increase the low-down torque, and by slowing it down, they increase the RPM horsepower.

To increase the power of an engine cost-effectively, automobile engineers usually use forced induction, by supercharging or turbocharging the engine. While supercharging gives higher engine torque, turbocharging provides extra horsepower. While both methods increase power, there are so many other factors involved to pull it off in a practical context, like the fuel, engine design etc.

Fuel, especially, is a complex component, because, at one point in the history of automobiles, diesel was considered to be the ideal track fuel, while petrol was used for motor cars. These days, diesel cars are extremely popular, and petrol cars are used frequently on the race track. In fact, manufacturing giants like Audi and Peugeot entered the 24-hours Le Mans motorsport race with nothing but diesel cars because the fuel economy and subsequent torque capacity of such cars are ideal for endurance racing.

What do you need – torque or horsepower?

To settle the question of torque vs horsepower, one needs to ask what the application of either is in the situation. If you need brute force, to pull a metal stump out of the ground, for instance, horsepower will not be much help, but if you need to win a race, it most certainly will. The balance between the two is what manufacturers have been trying to perfect for decades now.

One can also look at it this way – torque can get you off the line, but horsepower can get you ahead. If two evenly matched race cars A and B are going at it on the tracks, and A has more torque at low RPMs while B has higher RPM horsepower, who do you think will win?

While race car A will be quicker to accelerate off the line and will lead in the beginning, by the time it shifts to second gear, it will lose its acceleratory bump, and by third gear, it will lose its lead altogether, allowing race car B to win the race.

If you juxtapose a heavy diesel dump truck with an exotic sports race car, you will find that the dump truck produces a lot of low-end torque, while the race car will give you high-range RPMs. Here, the pivotal factors will be overall vehicle weight and gearing set up.

Both torque and horsepower are pivotal to performance racing, and auto students try to learn more about the play between the two by making internal modifications and using diagnostic equipment to test the engines.

When it comes to day to day applications, the choice between vehicles that produce high torque or high horsepower depends entirely on your need for either. If you live in a steep and hilly area and require vehicles to pull heavy weights up difficult inclines, you will need a vehicle that produces a lot of torque at a low RPM. Preferably, choose one that has low gearing, which will ensure that most of the engine’s torque goes to the wheels when the RPMs are low.

If you choose a vehicle with higher horsepower, it will have to be revved heavily if you want it to make steep climbs. The lower fuel efficiency this produces will not be an advantage either. However, the second car with higher horsepower will allow you to race past the previous one on a highway, since it produces power at a higher rpm, and is geared to fully utilize that power. Diesel engines can be used for carrying heavy loads as well as for high speeds, and the only difference between the two applications is how the vehicle is geared.

Where is the future of torque in electric vehicles?

All this talk about fuel becomes inconsequential with the invention of electric vehicles because electric power can deliver unimaginable amounts of torque from a standstill. This means that the acceleration will be brisker than ever before, even with the weight of hundreds of batteries within the car. The Tesla Model S P90D broke the record for an all-electric production car quarter-mile sprint by completing it in just 10.9 seconds. A Bugatti Veyron can complete the same in 10.175 seconds – the gap is evident, but electric cars are closing in on fuel-based cars very quickly.

In conclusion, there is a theoretical difference between torque and horsepower, and they are not to be used interchangeably. However, one cannot exist without the other and the debate depends entirely on what your use is for either.