Torque vs Horsepower – All you need to know

Torque and horsepower are two of the most common terms used to describe the performance of a car. From car journalists to manufacturers and even your circle of friends, everyone seems to be bragging about how much torque their cars produce. But what do these terms actually mean and why are they interchangeably used? Well, stick around to find out.

Before they started being loaded with tons of tech and features, cars were generally light in weight hence not a lot of power was needed to get the car moving. It is no surprise that the terms, horsepower and torque were rarely used to market cars in the early days.

Take for the example the 1986 BMW E30 M3. The car weighed 1,165Kg (2568lbs) with a maximum output of 200 horsepower. Fast forward to 2023, the G80 BMW M3 Competition weighs in at a staggering 1780Kg (3924.2lbs) with a maximum output of 510 horsepower. In the 37 year time frame, the weight increased by 53% but the horsepower figure also increased by 155% which is very impressive.

These days, even when you go to purchase a cheap daily driver, it’s good to ask for the horsepower and torque figures. This way you will be able to avoid being blinded by the marketing flashy features such as heated and cooled seats, and focus more on the performance aspect of the car. Without further ado, let’s dive into the difference between horsepower and torque and the role they each play in the performance of the vehicle.

What is Torque?

In physics, torque is described as the measure of force that can cause an object to rotate about an axis. In simpler terms, torque is a rotational force. The formula for obtaining Torque is Force (Newtons) multiplied by the distance( Meters) from the pivotal point. The unit of measurement is Newton-meters or Pound-feet depending on your region.

For example, consider tightening a bolt using a wrench. To get the amount of torque needed to tighten the bolt, we measure the distance from the bolt to your hand multiplied by the amount of force exerted by your arm. If the wrench is one meter long and you are exerting 3N of force on the bolt, the torque needed to tighten the bolt is 3Nm (3N X 1M). That’s easy right? But how does a car engine produce torque?

How is engine torque produced?

Well, to answer this, we’ll have to cover how a 4 stroke internal combustion engine works. For a car to convert chemical energy (fuel) to mechanical energy (rotation of the engine), it involves a 4 stroke cycle.

These include;

• Air intake
• Compression
• Combustion
• Exhaustion

Air is necessary for combustion to occur and its no different in a car engine. When a driver presses the gas pedal, the intake valve opens letting in more air. The more pressure the driver exerts on the gas pedal, the more the intake valve opens.

Once the intake valve opens, the pistons inside the cylinder go down creating a partial vacuum which is filled with the air drawn in.

During the compression stage, the pistons move upward, compressing the air-fuel mixture. ( In a petrol engine) The purpose of this step is to raise the temperature of the mixture. This is true according to Gay Lussac’s law, which states that the pressure of a given mass of a gas is directly proportional to the absolute temperature. The more the air-fuel mixture is being compressed, the higher the pressure in the cylinder leading to high temperature of the mixture.

During the combustion stage, air -fuel mixture is then ignited by spark plugs ( for petrol engines), leading to a explosion which pushes the pistons down the cylinders.

In the last stage which is exhaustion, the pistons moves upwards and the exhaust valves open to release the exhaust gases from the engine. This process is repeated thousands of times per minute.

Remember the pistons which move up and down the cylinder. Well, they are attached to the crankshaft via connecting rods. The crankshaft is a rod that is moved by the action of the pistons in the cylinder. The crankshaft is further connected to the fly wheel, clutch, transmission, drive shaft, the axles then the wheels in that order.

With all this in mind, let’s answer the question posed earlier. How is engine torque produced? When the pistons move up and down the cylinder, they exert a force on the crankshaft which is in turn transmitted to the wheels. The length of the crankshaft multiplied by the force it exerts on the crankshaft gives us torque. It is important to consider that torque is not always constant, especially in internal combustion engines. This simply means that if your car is rated at 500 Newton-meters of torque, it won’t always produce this figure. The 500 Newton-meters of torque is the maximum output and can only be attained at higher RPM’s. Now that we have a basic understanding of torque, let’s talk about it’s close cousin, Horsepower.

What is Horsepower?

In physics, power is defined as the rate at which work is done. Since work is Force multiplied by distance, its unit of measurement is Newton-meters. If you recall the formula of obtaining torque it is exactly the same as the one above. So, in place of work, we can substitute it with torque.

Horsepower is defined as the rate at which torque is being produced by the engine. Given the torque figure, we can get horsepower by dividing Torque with time.

The origin of the term Horsepower.

To understand more on how the unit horsepower came to be, we need to take a short history lesson. A famous scientist by the name James Watt, wanted to find an easier way of expressing the power output of vehicles to the common citizens. The need to do this arose from the fact that people at that, time had gotten used to horse carriages and engines were a relatively new concept especially in transportation.

James Watt figured out that it took one minute for a horse to raise 330lbs of coal from a 100ft well. This translates to 33,000lb.ft of load per minute which is now referred to as one horsepower. This way, it was much easier to explain to people, how powerful and efficient engines were instead of horses. As a side note, the average horse produces approximately 15 Horsepower and the average healthy adult produces as much as 1.2 Horsepower 🙂 We have James Watt to thank for this term that is now being thrown around left right and center.

Relationship between Horsepower and Torque mathematically.

Now that we have an idea of what Horsepower and Torque is, how do they relate with one another. Well mathematically, Horsepower is obtained by dividing Torque with time. But its not always this straight forward to accurately determine horsepower.

A more detailed formula of obtaining horsepower is Torque multiplied by RPM divided by a constant 5252. If you are as curious as I am, you may be asking yourself where the constant 5252 came from. Remember the horse from earlier. It was able to pull 330lb of coal from a 100ft well in one minute. To express this figure in revolutions per minute consider the variables below.

• Radius of the wheel – 1ft
• Distance traveled – 330ft
• Weight of the coal – 100lb

For this equation to work, we are going to interchange the distance the coal traveled with the weight of the coal as illustrated above. Don’t worry, the horse still managed to produce 33,000lb.ft in one minute 🙂

Let the 330ft represent the revolutions per minute. Since circumference of the wheel is obtained by 2πr, let’s divide 330ft by 2πr to get the RPM of the wheel.

330Ft ÷ 2πr (r is one ft in this case) = 52.52

Starting to look familiar right? Since the horse lifted 100lb of coal from the well, we will multiply 100lb by 52.52 to get 5252.

This figure represents the revolutions per minute of the 1 foot wheel. Hence the formula:

Horsepower = (Torque X RPM) ÷ 5252

It is worth noting that this formula only works with the pound-feet units. What’s even more interesting about the 5252 constant, is that when you plot a graph of horsepower and torque against RPM’s, the two intersect at 5252. Again, this only works if you use pound-feet units.

How does Horsepower and Torque affect a car’s performance?

Common knowledge dictates that the higher the horsepower and torque, the more powerful the car is. Since torque is the rotating force an engine produces, some vehicles may have an absurd amount of torque as compared to horsepower. A good example of this is heavy commercial trucks as well as tractors. These vehicles need the pushing power to move the heavy machinery. If you observe these vehicles closely, most of them only come in diesel engines.

This is because diesel engines are known to produce a lot of of torque due to the high compression ratios in the engine’s cylinder. This means that the cylinders travel a longer distance as compared to their petrol counter parts increasing the amount of torque produced. But why do these heavy trucks have a low horsepower figure you ask? This is because the rate at which the torque is being produced is significantly lower.

Maximum torque is generally obtained at a relatively lower RPM as compared to maximum Horsepower. Quiz here; if you want to overtake which do you need most, Horsepower or Torque? From the moment you press the gas pedal, you need more torque produced at a faster rate. Essentially, you need both.

Conclusion.

Both Horsepower and Torque play an important role in the functioning of a vehicle. If you are buying a tractor or a lorry, you will benefit from more torque in order to do the heavy towing and carrying heavy items. If you are targeting a sports car, more horsepower is essentially desirable in order to make the sub 3 second 0-60mph. (0-100km/hr). I hope this article has shed some light on horsepower and torque. If you found it informative, be sure to share the information with your pals. Cheers!