Torque and horsepower for semi-idiots
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01-20-2013, 02:09 PM
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Torque and horsepower for semi-idiots
By Rex8499
Summary: In an effort to explain how torque and horsepower in cars are related, and what exactly those terms and numbers mean, and which feels faster, I’ve written this in hopes that semi-intelligent people might be able to get at least a rudimentary grasp of the concepts at play. If you aren’t already familiar with how an engine works, you’ll need to start there before trying to read this. Engines are very complicated, and these explanations are simplified, but accurate. There are many other factors at play that I will not delve into here today. Be patient while reading through all of this. You need to understand the basics before getting to the good stuff.
Torque:
At it's most basic level, is the twisting force applied to something. When you put a wrench on a bolt and push down on the end of the wrench, you're applying torque and trying to twist the bolt. Torque = (force applied perpendicular) x (length of the arm the force is being applied on.)
See Picture: Torque
Hand A is pulling with a force of 20 lbs downward, and is 1ft from the bolt. Therefore:
Torque A = (20 lbs) x (1ft) = 20ft-lbs Hand B is twice as far out, so only has to pull with half the force to create the same amount of torque.
Torque B = (10 lbs) x (2ft) = 20ft-lbs.
You'll need to keep this in mind when working on a car. Often a bolt will be too tight, or rusted on, and you wont be able to get it loose. If you get a longer wrench, or put a piece of pipe over the handle to make a longer handle, you can exert more torque on the bolt without working so hard. Or in that case, work as hard as you did the first time, but exert a lot more torque, and hopefully enough to loosen the bolt. One of the tools I carry in my toolbox is a piece of pipe for just this purpose. Let's look inside an engine for a second. When the combustion happens inside, it pushes down on the piston, which pushes down on the connecting rod, which pushes down on the offset of the crankshaft. If you can imagine the center of the crankshaft like the bolt, and the offset arm of the crankshaft as the wrench, and the piston and connecting rod as your arm trying to push down. You're applying a twisting force (a torque) to the axis of the crankshaft.
Crank Torque
In that photo, the torque created on the crankshaft will be: Torque = F * D Depending on how big the cylinder is, and how much gas was ignited, your force (F) being applied downward on the piston, and ultimately the crankshaft arm, varies. Engines burning more gas can push down harder and create higher torque. Multiply this torque by the number of cylinders, giving even more torque overall for a big V8 engine vs just 4 cylinders.
Horsepower:
A unit of measurement for work. Back in the day, 1 horsepower was used to describe the power required to lift 550 lbs by 1ft of vertical distance in one second. 1 HP = 550 ft*lbs/second. A standard horse with a pulley system could lift 550lbs by 1ft per second.
Horsepower
If you have a 2000 lb Ferrari, and a 2000 lb Volkswagon, and you drive both of them to the top of a hill, the Ferrari will get there faster (aka, shorter time), it has more power. Power = (work/time). But you will have done the same amount of work with each car. You’d burn the same amount of gas with each car (here in the ivory halls of acadamia, we’re assuming equal efficiencies as well). The Ferrari can do more work in a shorter time, but it cannot do more work in a set distance.
Horsepower is just a unit of measurement to compare how much work that engine can do over time. How many times per second an engine makes that torque determines horsepower. An engine that wasn't spinning could still be making torque without making any power at all. Just like you can apply torque to a rusted bolt, and the thing never moves. You're applying torque, but not doing any work. Hence no horsepower. Once it starts rotating, you're making some power.
When looking at a dyno graph, the torque and horsepower are related. The typical chassis dyno is not measuring horsepower, it is measuring torque and then CALCULATING the horsepower using this equation. Note: this is a simplified equation, and requires torque in ft-lbs and RPM listed as "X000."
Horsepower = (Torque x RPM) / 5252
As such, the HP and Torque dyno curves will ALWAYS cross each other at 5252 RPM. That's just how the equation works out. Look at this equation for a second and let’s think about it. If you have an engine that makes a huge amount of torque, but it is rotating very slowly with low RPM’s, the Horsepower will be low.
Or, if you have an engine that makes a small amount of torque, but rotates very fast, you could still have a large horsepower number.
If you’ve got the big V8 engine, it makes a lot of torque at low RPM’s, meaning it also has more HP and accomplishes more work in those lower RPMs but it usually has a lower power curve and RPM redline, so the horsepower number in the higher RPMs is limited compared to a motor that could maintain a good torque curve to 10k RPM.
With the small 4-cylinder engine, it makes less torque and therefore HP at low RPM’s with only half as many cylinders, but it usually revs to higher RPM’s and maintains a better torque curve in those RPMs, so the horsepower could still be as high as the big V8 with more torque. If you look at some of the formula one race cars, they often have very small engines that don’t have a huge amount of torque, but they rev to 18k RPM, so their horsepower is crazy high.
So, the horsepower is just a function of the torque at a certain RPM. That's the most important thing to understand from this. THE HORSEPOWER IS JUST
A FUNCTION OF THE TORQUE AT CERTAIN RPM’s.
At lower speeds, the engine makes high torque, but it's making that torque less times per minute with low RPM's, so the horsepower is low because less overall work is being done. As the engine RPM's increases past its most efficient point and peak in the torque curve, the torque decreases for each revolution (I’ll explain why later on). However, it's making that torque more times per minute, so the horsepower starts to increase. Even though it's making less force per revolution, it's revolving (and firing) more times per minute, so the overall work being done is greater, giving more power.
~~~~~~~~~~~~~~~~~~~
I cribbed this from self-post on reddit.com:
Torque and Horsepower Explained...For semi-idiots. : cars
Summary: In an effort to explain how torque and horsepower in cars are related, and what exactly those terms and numbers mean, and which feels faster, I’ve written this in hopes that semi-intelligent people might be able to get at least a rudimentary grasp of the concepts at play. If you aren’t already familiar with how an engine works, you’ll need to start there before trying to read this. Engines are very complicated, and these explanations are simplified, but accurate. There are many other factors at play that I will not delve into here today. Be patient while reading through all of this. You need to understand the basics before getting to the good stuff.
Torque:
At it's most basic level, is the twisting force applied to something. When you put a wrench on a bolt and push down on the end of the wrench, you're applying torque and trying to twist the bolt. Torque = (force applied perpendicular) x (length of the arm the force is being applied on.)
See Picture: Torque
Hand A is pulling with a force of 20 lbs downward, and is 1ft from the bolt. Therefore:
Torque A = (20 lbs) x (1ft) = 20ft-lbs Hand B is twice as far out, so only has to pull with half the force to create the same amount of torque.
Torque B = (10 lbs) x (2ft) = 20ft-lbs.
You'll need to keep this in mind when working on a car. Often a bolt will be too tight, or rusted on, and you wont be able to get it loose. If you get a longer wrench, or put a piece of pipe over the handle to make a longer handle, you can exert more torque on the bolt without working so hard. Or in that case, work as hard as you did the first time, but exert a lot more torque, and hopefully enough to loosen the bolt. One of the tools I carry in my toolbox is a piece of pipe for just this purpose. Let's look inside an engine for a second. When the combustion happens inside, it pushes down on the piston, which pushes down on the connecting rod, which pushes down on the offset of the crankshaft. If you can imagine the center of the crankshaft like the bolt, and the offset arm of the crankshaft as the wrench, and the piston and connecting rod as your arm trying to push down. You're applying a twisting force (a torque) to the axis of the crankshaft.
Crank Torque
In that photo, the torque created on the crankshaft will be: Torque = F * D Depending on how big the cylinder is, and how much gas was ignited, your force (F) being applied downward on the piston, and ultimately the crankshaft arm, varies. Engines burning more gas can push down harder and create higher torque. Multiply this torque by the number of cylinders, giving even more torque overall for a big V8 engine vs just 4 cylinders.
Horsepower:
A unit of measurement for work. Back in the day, 1 horsepower was used to describe the power required to lift 550 lbs by 1ft of vertical distance in one second. 1 HP = 550 ft*lbs/second. A standard horse with a pulley system could lift 550lbs by 1ft per second.
Horsepower
If you have a 2000 lb Ferrari, and a 2000 lb Volkswagon, and you drive both of them to the top of a hill, the Ferrari will get there faster (aka, shorter time), it has more power. Power = (work/time). But you will have done the same amount of work with each car. You’d burn the same amount of gas with each car (here in the ivory halls of acadamia, we’re assuming equal efficiencies as well). The Ferrari can do more work in a shorter time, but it cannot do more work in a set distance.
Horsepower is just a unit of measurement to compare how much work that engine can do over time. How many times per second an engine makes that torque determines horsepower. An engine that wasn't spinning could still be making torque without making any power at all. Just like you can apply torque to a rusted bolt, and the thing never moves. You're applying torque, but not doing any work. Hence no horsepower. Once it starts rotating, you're making some power.
When looking at a dyno graph, the torque and horsepower are related. The typical chassis dyno is not measuring horsepower, it is measuring torque and then CALCULATING the horsepower using this equation. Note: this is a simplified equation, and requires torque in ft-lbs and RPM listed as "X000."
Horsepower = (Torque x RPM) / 5252
As such, the HP and Torque dyno curves will ALWAYS cross each other at 5252 RPM. That's just how the equation works out. Look at this equation for a second and let’s think about it. If you have an engine that makes a huge amount of torque, but it is rotating very slowly with low RPM’s, the Horsepower will be low.
Or, if you have an engine that makes a small amount of torque, but rotates very fast, you could still have a large horsepower number.
If you’ve got the big V8 engine, it makes a lot of torque at low RPM’s, meaning it also has more HP and accomplishes more work in those lower RPMs but it usually has a lower power curve and RPM redline, so the horsepower number in the higher RPMs is limited compared to a motor that could maintain a good torque curve to 10k RPM.
With the small 4-cylinder engine, it makes less torque and therefore HP at low RPM’s with only half as many cylinders, but it usually revs to higher RPM’s and maintains a better torque curve in those RPMs, so the horsepower could still be as high as the big V8 with more torque. If you look at some of the formula one race cars, they often have very small engines that don’t have a huge amount of torque, but they rev to 18k RPM, so their horsepower is crazy high.
So, the horsepower is just a function of the torque at a certain RPM. That's the most important thing to understand from this. THE HORSEPOWER IS JUST
A FUNCTION OF THE TORQUE AT CERTAIN RPM’s.
At lower speeds, the engine makes high torque, but it's making that torque less times per minute with low RPM's, so the horsepower is low because less overall work is being done. As the engine RPM's increases past its most efficient point and peak in the torque curve, the torque decreases for each revolution (I’ll explain why later on). However, it's making that torque more times per minute, so the horsepower starts to increase. Even though it's making less force per revolution, it's revolving (and firing) more times per minute, so the overall work being done is greater, giving more power.
~~~~~~~~~~~~~~~~~~~
I cribbed this from self-post on reddit.com:
Torque and Horsepower Explained...For semi-idiots. : cars
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