The Definitive Torque vs. Horsepower post
Year after year, I keep seeing the same old debates about torque vs. horsepower. Everyone seems to have a general idea that we are referring to power, but most don't really understand the relationship of torque to horsepower. Therefore, I've decided to try and explain it as best as I understand it. (caveat, I am not an engineer so feel free to clarify and correct any mistakes I make below.)
First of all, a bit of math. (Torque x Revolutions Per Minute / 5252 = Horsepower). Even if you aren't good with math, the important thing to note in this equation is that Torque times RPM divided by a constant equals horsepower. If we ignore the constant, we see that torque times RPM determines horsepower. In other words, horsepower is dependent on RPM and the faster your motor turns at a constant torque, the more horsepower you will get. (In practice, torque falls off, the faster the engine spins)
OK, so what is torque anyways? Let's look at a simple example of a bicycle. The motor on a bicycle is typically your legs and assuming you don't have clips on your pedals, the maximum amount of power you can apply to a pedal is determined by the weight of your body. In other words, if you stand on one pedal and apply all your weight to it, you will apply the maximum amount of torque to the pedal and gears. But here's the funny part. If your bike is in top gear and you start from a stand still, you could apply your entire body weight as torque to the pedals and still not move. Assuming the bike is held upright, you could conceivably stand right up on one pedal all day long generate maximum torque and generate zero horsepower. Why, because your RPMs are zero and as we have already seen, torque times zero RPMs equals zero horsepower.
So how about if we started off in 1st gear on our bike. As most of you know, 1st gear is the easy gear and lets you move uphill or get started with very little effort. The downside is that your bike won't move forward very fast in 1st gear, but you can apply your torque in such a way as to get the RPMs going and hence, generate horsepower that will move you.
Thinking about our bike again, try to imagine that you are comfortably cruising along in 4th gear at 10 miles per hour and you change gears to 5th gear; what happens? If you maintain the 10 miles per hour, you will find that your pedals will turn slower, but you will need more power in each stroke to maintain the same speed. To get the maximum speed out of your bike, you need to apply your maximum torque to the pedals and then turn the pedals as fast as you can.
Well it's exactly the same thing with internal combustion engines. The torque from your engine comes from the combustion of fuel in your cylinders. In general, the bigger the cylinder and the more efficient the combustion chamber and the more energy your fuel source has, determines your maximum torque. But like we saw in our bike example, lots of torque times low RPMs doesn't generate much power. I like to think of torque as the amount of power generated by a single revolution of the engine. (That is not technically true, but that's the way I like to think of it)
In the great gas vs. diesel debate, it often comes down to this. Diesel engines typically produce more torque and run at lower RPMs. Two comparable gas and diesel engines could produce the same amount of horsepower, but typically, the diesel will run slower and would theoretically be more efficient because it is wasting less power in the form of heat.
But there is one other very important thing that usually no one considers when looking at horsepower. The horsepower rating that you always see quoted by everyone is the horsepower at the maximum torque and RPM. Meaning, you only get that maximum horsepower in top gear at top speed and depending on how your vehicle is geared, you may never be able to achieve that top gear at maximum RPM because your vehicle is too heavy or has too much wind resistance or has some other friction. This is why gearing is so important.
So what are the key takeaways from all this?
1) Horsepower is a function of RPM times torque. The faster your engine turns, the more horsepower you will have.
2) You will probably never see the maximum horsepower your engine produces because you will rarely, if ever, go to your maximum RPMs in top gear.
3) In general, the larger the cylinder, the more torque that cylinder will produce.
4) Diesel fuel has more energy in a given quantity than does gasoline and by default will produce more torque, all else being equal.
5) Gearing is important.
People often say that it's horsepower that moves us up mountains and that is 100% true. The difference is that a high torque engine will produce that same horsepower at a lower RPM which often results in quieter operation and more fuel economy.
First of all, a bit of math. (Torque x Revolutions Per Minute / 5252 = Horsepower). Even if you aren't good with math, the important thing to note in this equation is that Torque times RPM divided by a constant equals horsepower. If we ignore the constant, we see that torque times RPM determines horsepower. In other words, horsepower is dependent on RPM and the faster your motor turns at a constant torque, the more horsepower you will get. (In practice, torque falls off, the faster the engine spins)
OK, so what is torque anyways? Let's look at a simple example of a bicycle. The motor on a bicycle is typically your legs and assuming you don't have clips on your pedals, the maximum amount of power you can apply to a pedal is determined by the weight of your body. In other words, if you stand on one pedal and apply all your weight to it, you will apply the maximum amount of torque to the pedal and gears. But here's the funny part. If your bike is in top gear and you start from a stand still, you could apply your entire body weight as torque to the pedals and still not move. Assuming the bike is held upright, you could conceivably stand right up on one pedal all day long generate maximum torque and generate zero horsepower. Why, because your RPMs are zero and as we have already seen, torque times zero RPMs equals zero horsepower.
So how about if we started off in 1st gear on our bike. As most of you know, 1st gear is the easy gear and lets you move uphill or get started with very little effort. The downside is that your bike won't move forward very fast in 1st gear, but you can apply your torque in such a way as to get the RPMs going and hence, generate horsepower that will move you.
Thinking about our bike again, try to imagine that you are comfortably cruising along in 4th gear at 10 miles per hour and you change gears to 5th gear; what happens? If you maintain the 10 miles per hour, you will find that your pedals will turn slower, but you will need more power in each stroke to maintain the same speed. To get the maximum speed out of your bike, you need to apply your maximum torque to the pedals and then turn the pedals as fast as you can.
Well it's exactly the same thing with internal combustion engines. The torque from your engine comes from the combustion of fuel in your cylinders. In general, the bigger the cylinder and the more efficient the combustion chamber and the more energy your fuel source has, determines your maximum torque. But like we saw in our bike example, lots of torque times low RPMs doesn't generate much power. I like to think of torque as the amount of power generated by a single revolution of the engine. (That is not technically true, but that's the way I like to think of it)
In the great gas vs. diesel debate, it often comes down to this. Diesel engines typically produce more torque and run at lower RPMs. Two comparable gas and diesel engines could produce the same amount of horsepower, but typically, the diesel will run slower and would theoretically be more efficient because it is wasting less power in the form of heat.
But there is one other very important thing that usually no one considers when looking at horsepower. The horsepower rating that you always see quoted by everyone is the horsepower at the maximum torque and RPM. Meaning, you only get that maximum horsepower in top gear at top speed and depending on how your vehicle is geared, you may never be able to achieve that top gear at maximum RPM because your vehicle is too heavy or has too much wind resistance or has some other friction. This is why gearing is so important.
So what are the key takeaways from all this?
1) Horsepower is a function of RPM times torque. The faster your engine turns, the more horsepower you will have.
2) You will probably never see the maximum horsepower your engine produces because you will rarely, if ever, go to your maximum RPMs in top gear.
3) In general, the larger the cylinder, the more torque that cylinder will produce.
4) Diesel fuel has more energy in a given quantity than does gasoline and by default will produce more torque, all else being equal.
5) Gearing is important.
People often say that it's horsepower that moves us up mountains and that is 100% true. The difference is that a high torque engine will produce that same horsepower at a lower RPM which often results in quieter operation and more fuel economy.
