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Horsepower vs Torque: Simplified explanation for beginners

26th June 2023, 15:45 by Utkarsh Chaudhary

When the RPM changes, torque changes and so does the horsepower.

BHPian Cresterk recently shared this with other enthusiasts.

Note

This post is intended to explain a concept to beginners without much of an engineering background so it might be a bit too oversimplified for the mechanical engineers among you.

All enthusiasts whether they be of car, bike, airplane, boat or lawnmower variety have heard the phrase "Horsepower sells cars but torque wins races".

What if I tell you that this very phrase itself was made up by some clever salesman or advertising agency to sell cars and mislead people?

Anything with an engine is subject to comparisons and debate among people who are interested in it. This can range from casual bragging in coffee shops and bars about whose car is better; to intense, cuss-filled debates at 3 am in the youtube comment section on a video of some obscure quadricycle from Taiwan.

The horsepower vs torque debate is almost always at the forefront of these discussions.

"Oh, your car has only 90hp?"

"Yeah but it has 220nm torque so it's faster than you think."

"My crossover has 103hp"

"Oh but it has only 134nm of torque? That's just too low. Can it even handle 5 passengers? Going up a hill?"

"My BMW has 250hp and 350nm of torque"

"Well my Mercedes is a torque monster with 440nm so the horsepower being 190hp doesn't matter"

etc

You have already seen the equation for horsepower many times : Horsepower = (Torque * RPM) / 5252

Where torque is in Lb.Ft

Horsepower = (Torque * RPM) / 7127

Where torque is in our more familiar newton meters (nm)

I know, I know, math is boring and half of us have our eyes glaze over while looking at equations. Things would be way easier if it was just a bar graph so we can go back to comparing Fortuner vs Endeavour power figures. Bigger number is good. Smaller number is bad.

So let's change the way we look at this. We are all familiar with plots of land, yes?

A plot of land is measured by its area. The bigger the area, the more space you actually have to build structures etc. Everyone would like more area. The more area you have, the more the value of the land. Well depending on the locality.

For the sake of simplifying this example, let's consider all plots of land to be rectangular shaped

So Area = length * width.

Now length and width can be different numbers. Sometimes, the length is more. Sometimes the width is more. Sometimes it's completely equal and your plot is a square.

Now imagine someone comes up to you and claims the area is meaningless. He says length is what is actually important. He says plots of land with more length are better and he gives you an example of 2 plots of land. One with 60m length and 25m width and another with 50m length and 27m width. He says that plot 1 is more valuable than plot 2 because the length is more.

Now area of plot 1 = 60 x 25 = 1500 square meters

area of plot 2 = 50 x 27 = 1350 square meters

Now clearly the extra value is because the overall area of plot 1 is more. But he is adamant that it's because of the extra length and that area is just some figure made up by real estate agents.

Now someone else says that his land is worth much more because it's 70m long. He owns a plot of land that is 70m long and 50m wide. His total area is 3500m squared. The reason it's worth more is because the overall area is way more. Not because of the length is more.

Another guy has a 50 m-long stretch of land with a 2m width. Area of his land is 100 square meters. Meanwhile, you have a plot of land that is 150 square meters. It's 15m long and 10m wide. He says what are you even going to do with land that is just 15m long? That's way too low. He doesn't even know what width or area is.

You show them all a plot of land that is 30m long and 200m wide. You show them how the value of this land is way more. They stand around scratching their heads trying to come up with an explanation for why some land with just 30m length is worth more instead of accepting that overall area is what actually matters and thus width needs to be taken into consideration too.

Saying you own a land that stretches 500m from here to there might feel good. But in the end, if it is just 1m wide, it is useless other than for some very niche applications. Like parking a bunch of scooters end to end. Maybe if you are starting an electric recharging station, it might be useful. But for all other practical purposes, it is useless. Same for land that is 500m wide and 1m long.

Now back to cars and our torque conundrum

Horsepower = (Torque * RPM) / 7127

The equation itself is pretty simple. It says that horsepower is directly proportional to torque and rpm. In other words, if either torque or rpms increases, then horsepower also increases.

If you are designing an engine and need it to make more power, whether it is for better acceleration, higher top speed, going offroading, up steep mountains, carrying more weight, or towing:

then you can either : increase the torque, or increase max RPMs.

Theoretically, either of these will do but practically, there is one major drawback that I will get to later.

Either of these will increase horsepower which is the actual measure of work being done and you can then just gear the car appropriately for acceleration/speed/towing capacity etc.

If you get yourself a car with 100nm of torque at redline and it can rev all the way to 9000 rpm, it will make 126 hp.

If you get yourself a car with 250nm of torque at redline and it can only rev to 4000rpm, then it will make 140 hp.

You will see how the power figures are similar despite one engine making 2.5x more torque.

Now that this part is clear, let's move on to the more interesting part. Engines do not produce the same amount of torque across the entire rpm range. It changes along with the RPM.

"Wait what? So when the rpm changes, torque changes as well as horsepower?"

That's right. This is why cars are listed with peak torque + what rpm it can make said torque.

In the diagram above, you can see that torque (darker line) isn't just a straight line. It increases and decreases. This 2.6l engine from Mazda makes around 120nm of torque at 1000rpm (idle) and then increases to around 210nm at around 5500 rpm and then decreases as the rpm keeps increasing to end at 190nm at 8500rpm (redline).

"So does this car produce max acceleration/towing power when driving at 5500rpm?"

No, that's another often-seen misconception. Even as the torque decreases, the rpm increase is so much that horsepower keeps increasing, up to 8200rpm where the horsepower line also starts to go down. This is because torque starts falling sharply, more than the RPM increase can counter. So around 8200rpm is where the car produces maximum power and thus that will be where it can pull the most weight or accelerate hardest. Revving past this point will actually lead to power decreasing.

Here is a torque graph of popular SUVs. You can see how the torque changes according to the RPM changing. All of them start out pretty low at idle, go up sharply and forms a peak due to the turbo and then fall off sharply as well. The Outlander 2.4 is an exception because it is naturally aspirated and a petrol. It has a very linear torque curve and redlines much higher at 6400rpm.

You might have also noticed that the car with the lowest torque is not the car with the lowest power.

This change in torque according to rpm means that it also leads to a sharp increase and decrease in power for the turbo diesel SUVs. This is why experienced drivers will tell you to upshift faster in a diesel and why dieselheads will tell you not to go to redline unless you just want a whole lot of smoke and noise without much go. You might have experienced this yourself when you tried to drive a diesel SUV fast.

Real world example:

In the case of Endeavour 3.2, you make the highest torque (peak torque) from 1800-2500rpm. But the highest power is made at 3000rpm, meaning around this is where the car can pull the most weight and accelerate hardest.

In the case of the Outlander petrol, this limitation is higher up, letting you go almost all the way to redline without any loss in power. And that is where the CVT will hold the rpms if you go full throttle, giving you that hated boooooom sound associated with CVTs. But also giving you decent power and acceleration from a 2.4L NA engine.

Meanwhile, the CRV and Kodiaq diesel make less power than the Outlander despite having much more torque.

Apologies for the poor quality image but this is the power and torque curve of a Mercedes E250 diesel and E250 petrol.

You might notice that the diesel makes much more torque than the petrol.

However, the petrol produces more power due to it being able to rev higher. 220 hp at 6000rpm (petrol) vs 200 hp at 3700 rpm (diesel).

However, when you take off gently from a stop light, you start off at around 1000rpm. Which car makes more power at 1000rpm?

Looking at the graph, we can say that the diesel makes around 70hp due to it also making around 250lbft torque at 1000rpm.

While the petrol makes only around 25hp due to its lower 125lbft torque at 1000rpm.

So when you are starting off normally, there is a lot more power available for the diesel. This is why torquier cars feel faster under normal driving conditions.

However, if you floor the gas pedal, the diesel will have only 200hp vs the petrol's 220hp, meaning it will be slower in an extended drag race. The petrol will also rev up faster due to its lighter internals.

Your 400nm diesel SUV doesn't feel effortless taking off in 3rd gear at idle just because the high torque numbers mean that horsepower is irrelevant, its because the high torque numbers mean it produces higher horsepower at lower rpm.

Now on to the limitation I mentioned earlier

This is a Tata SE 1613. It has a 5.7L engine producing 136 HP of power and 490 Nm of maximum torque. Now you might wonder what's the point of using such a large engine if it can barely make 136hp. Well, that's because it can produce it right at 2400rpm.

"Well, that's still not very impressive. Why did you choose this old truck when there are plenty of newer Tata trucks with 300hp and 1100nm of torque from 1100rpm like the Tata Signa?"

Well, I chose this as an example because even with these low power figures, it can still carry 16 tons. This is due to gearing. With low enough gearing, you can carry large loads even with low power figures. It's not going to be fast, and it can barely reach 80kmph, but it will keep chugging along.

This is for the offroaders who claim they can't go offroad without massive power figures or the people worrying about whether their small crossover will not be able to climb hilly roads because the power figures are too low. Oh, it will climb them alright, you just won't be breaking the speed limit while doing so. People have been driving on steep hills with less power for ages, you won't be stranded at the bottom. Just don't forget to downshift and rev it up.

Now then why can't we just use a high-revving, smaller, lower-torque engine that makes the same horsepower in trucks?

Well theoretically, you could if the gearing was low enough. But imagine trying to start off with a full load at 6000 rpm. Even ignoring the wear on the clutch, there would be the overall inefficiency of having your engine run at 6000 rpms or higher throughout the journey to make the required power.

Practically though, it would just bend the internals from trying to move the extreme weight since you need lighter engine components such as pistons and crankshafts to reach high rpms.

Likewise, the heavy engine of a truck cannot be tuned to reach high rpms because the pistons and other components are built so heavy to withstand the massive loads. The pistons have such momentum to them that they cannot reverse directions fast enough for the engine to rev up anywhere near the redline in a car.

Now a little bit about electric motors

This is the torque curve for a bunch of Teslas. Notice anything interesting?

Every single one of them has the torque start out max right from idle. There is no increasing torque with RPM like in a petrol or diesel engine here, maximum torque is available right from the start and it continues in a flat line for quite a while. This is why these electric cars accelerate so fast from a stop and then gradually fizzle out at higher speeds as the torque and later power drops.

This instant torque also means that electric offroaders would really change the game. No more shifting to low range and having to rev up, no more having to maintain a faster speed than you would like to keep the engine in its power band. You could control your low speed with utmost precision so you can crawl over obstacles without facing any lack of power. No more breaking your sideboards rock crawling on a steep climb.

Doesn't sound too exciting? Well no worries, you can always use a cheap ICE offroader for fun without worrying about damage so we can have the sounds and thrill associated with it. This is not an electric vs ICE car thread anyway.

This is the dyno results of an old Toyota Prius hybrid. Dyno charts for hybrid and electric cars aren't always accurate due to the different nature in which they produce power. A useful observation here is that torque is max right at the start because of the electric motor but power keeps increasing with RPM due to the petrol engine reving up.

There is no conventional transmission in a Toyota hybrid. E-CVT is about as similar to CVT as email is to mail. There are no belts or pulleys or clutches, just a simple planetary gear set. The engine cannot provide more torque to the wheels by "downshifting" so what it does is spin faster to generate more electricity for the electric motor, which will then provide the torque to the wheels necessary to get moving.

At higher speeds, engine speed will gradually match the wheel speed. This is a complex design that requires an entire thread to explain properly so I will just leave it at this oversimplified explanation. If people are interested, I will make another thread to explain Toyota hybrids and their changes across the generations.

Hopefully, all of this helps you understand and judge different cars better than just comparing maximum torque numbers. If you want to compare engine performance, the best option is to go test drive it yourself.

If you want to do it from the comfort of your home, then a horsepower/torque/rpm graph is a good option that tells you actual meaningful data. Maximum value of torque by itself is useless. Acceleration timings can also help since it also takes the gearbox into account.