There are a lot of things to consider in motor racing, and two common concepts you will come across are horsepower and torque. But whether you’re a beginner driver or an experienced racer, it helps to understand the difference between torque and horsepower.
Horsepower and torque are both important for racing, as while torque is a measure of the rotational force applied by the engine, horsepower is a measure of how quickly the engine can apply that force. They both determine how well a race car will perform and how fast it will go.
Torque and horsepower are directly related to each other. Below, I will explain how they are related, how they can be measured, and what these measurements can tell you about the car you are driving.
What Is Horsepower?
Before we consider why each of these concepts are important for racing, we need to define a few basic physical concepts first. The first of these is called work. Work is basically the result of a force acting over a distance. If you push something, say a wheelbarrow, across your garden, you are doing work. The work you have done is equal to the force you applied multiplied by the distance.
Work = Force x Distance
Work is often measured in foot-pounds, or ft-lbs, but is sometimes measured in joules (J) or Newton meters (Nm). How quickly this work is done is what we call horsepower in a car. This was first measured by Scottish inventor James Watt (the namesake of the common Watt measurement of power), when he was observing horses pulling weight up a cliff using rope and a pulley.
First Measurement Of Horsepower
He found that the horses (actually ponies, which he then accounted for by doing some basic calculations) could pull 330 lbs of weight 100 ft in one minute. This he designated to be 1 horsepower. This can also be written as 33,000 lbs over a distance of 1 foot in 1 minute, which is the way that horsepower is often equated now. It could also be 1 lb 33,000 ft in one minute, but this is far less practical!
Horsepower was defined to be how quickly a certain amount of work could be done. As work was a measure of force multiplied by distance, this was effectively how far the horse moved the weight. By timing this and finding the distance it could move in one minute, James Watt standardised horsepower, and this has been used as a measurement for the power of motor vehicles ever since.
Cars will have a quoted value for horsepower (HP) or brake horsepower (BHP). BHP is the power of the engine without accounting for any possible power losses. These are not that important for this article, but the subtle difference is that HP accounts for these losses and may be viewed as a more accurate representation of the car’s real power. So, the HP value will be slightly lower than the BHP.
The horsepower of the car is essentially a measurement of how fast the engine can move the car forwards (but it’s a bit more nuanced than that). A Bugatti Veyron may have 1000 HP, and it can reach a top speed of 250 mph (400 kph). However, it weighs around 2 tons. An F1 car might also have 1000 HP but have a lower absolute max speed of around 230 mph (370 kph). However, it weighs less than half of the Veyron.
While the Veyron might take 7 seconds to reach 200 kph, F1 cars are able do it in as little as 4.5s. So, horsepower doesn’t always mean a massive top speed (although 230 mph is still hugely fast), but instead it measures how quickly the engine can accelerate the weight of the car. Although the Bugatti has the same HP, it weighs a lot more and this is reflected in the acceleration time.
Other Things To Consider
Horsepower cannot be taken as a simple value for the acceleration however, as there are far more factors at play. Obviously, there is the weight of the car to consider, but also the effects of downforce and aerodynamics. These are especially important in F1 cars, and it shows that horsepower is not everything when it comes to speed.
What Is Torque?
Torque is a measure of the force required to move an object around its axis. This results in something called angular acceleration, which is essentially what makes something rotate. In the case of a car, it is the crank shaft in the engine that is turning, and the force acting on it is coming from the piston above it moving up and down. The piston pushes down on it with a specific amount of force.
The greater the amount of torque (i.e. the faster the crank shaft can spin), the greater the ability of the crank shaft to perform work. This can be thought of more broadly, with all of the pistons considered, as the capacity of the engine to do work. Horsepower is linked to the torque of the engine via a simple equation that shows horsepower as the rate of applying torque – or completing work.
HP = (Torque x RPM of the engine)/5252
This last number (5252) is known as a constant, and it is used to allow the units of RPM and torque to work and cancel in a certain way when combined. This makes the value of horsepower obtained a much easier number to work with, and as long as you use the same constant for every calculation, you are not changing the math in a different way for any one calculation, giving you consistency in your results.
Although this might seem confusing, essentially what this shows is that horsepower is a measure of the amount of work that can be done at certain engine RPMs with certain torque values.
KEY POINTS• Torque and horsepower are directly related
• Torque is a rotational force
• Horsepower is a measure of how fast something can apply that force
Is Torque Or Horsepower More Important for Racing?
Cars will have quoted peak torque and horsepower ratings, and these are the absolute maximums that the car can achieve, and only at certain RPMs.
The RPM is how fast the engine is spinning the crankshaft, and it accounts for the classic revving sound that is associated with race cars (and indeed non-race cars too). This dependence on RPM is why it is a key component of the horsepower equation. The torque doesn’t depend on the RPM, as it is a measure of the force acting on the crankshaft itself, but horsepower does depend on it.
This is why you will accelerate the car very quickly at lower RPMs, but as you reach the higher RPMs, you need to shift up a gear in order to maintain acceleration. This is because the car has a lot of torque at these lower gear ratios, and then as the RPM increases it begins to run out of torque, and so you begin to struggle to accelerate.
When you shift up a gear, the ratio changes and the revs are brought back down, allowing you to make use of the torque once again. This cycle repeats as you change gears. You will have an optimum torque value at a certain RPM, and this will be different for each car. You will also have an optimum “power band” that is the rev range where the car generates the most power.
This is why you will often see race car drivers push the cars much closer to the redline of the tachometer than you would in your car, as you are much less concerned about the power of the car when driving below the speed limit. What they are doing is getting the absolute maximum amount of power out of the car.
On a racetrack, acceleration is often more important than top speed. This is because the track will have corners and turns that will require the driver to slow down and then speed up again. Rarely on an F1 track will a car drive at its top speed for very long, and even on most NASCAR tracks the driver needs to lift off the gas at some point, before putting their foot down again.
Torque Is More Important Than Horsepower
So, it is clear that acceleration is vital. As horsepower is a measure of how fast the torque can be applied, it is correlated to acceleration. This often makes horsepower a much more commonly quoted statistic for race car drivers. Drivers should understand the torque curve and horsepower curve of their engine (relative to RPM), in order to understand the optimal points to shift gears.
Torque & Horsepower Curves
Torque curves and horsepower curves are essentially graphs of the two variables plotted against the RPM of the engine. An example of a torque/horsepower curve can be seen below, with dummy data used to show the general shape that can be expected from these curves. The actual numbers and point where the two series cross over each other will vary depending on the car.
Note: When using units of lb-ft and hp, the curves will always cross at 5252 RPM, but it varies if you use different units
Example Of A Torque/Horsepower Curve
The optimal RPM range for shifting gears will be somewhere between the peaks of torque and power. The specific optimal rev range will once again vary for each car, with the ideal RPM being that at which, if changing up a gear, you will be able to deliver more torque to the wheels at the higher gear than in the lower gear.
Remember, torque is related to acceleration, and the more torque delivered to the wheels the faster they can accelerate. For example, imagine you are in 3rd gear at 7000 RPM, and going by the graph above experiencing around 100 lb-ft of torque. You are now looking to shift up but are unsure if now is the right time.
If you were to shift up to 4th gear, and your revs dropped to 2000, then you would be dropping down to around 80 lb-ft of torque, much less than 100 lb-ft. So, shifting down would reduce the amount of torque at the wheels, and therefore result in less acceleration.
If, on the other hand, you shifted up and the revs only dropped to 4000, then you would be going to around 120 lb-ft of torque instead. This is greater than the 100 lb-ft you were experiencing in the lower gear at higher revs, and so this would be an ideal shifting point. Note that these numbers are completely made up, but the idea still stands true.
Now, in order to get these optimum shifting ranges, you need a lot of data. Going by feel will get you where you need to be most of the time, but if you want to make it an exact science then you need more information, including real torque/RPM curves, the gear ratios of your car, tire sizes and the final drive ratio.
A Lot Of Data
All of this information is quite hard to deal with if you are not experienced, and that is why qualified racing engineers work with top motorsport teams to help train the drivers to get the most out of their car. A lot of it is done by computers, but the engineers are the ones that program the computers and translate this into useful information for the driver.
Horsepower and torque are two concepts that are often confused with each other, and torque is usually much less well-understood. They are both vital to the performance of the car, but horsepower is the more commonly quoted measure. Drivers should analyze both the torque and horsepower curves to fully understand the acceleration of their car, and the optimal moments to shift gears.
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