This isn't a "How To" and I don't know which forum to put it in. So, if it's in the wrong place I'm sure it will get changed.
During the last 40 years of reading Car and Motorcycle magazines I have often seen letters written that ask what is the difference between Engine Torque and Engine Horsepower. I’ve never seen the right answer written so, before I get too old and die, I thought I’d answer the question in a simple, mostly non-mathematics way.
First, an Engine’s Maximum Potential Torque is related to that Engine’s Displacement times it’s Efficiency. There are many other factors that contribute to that amount of torque, such as how far open is the throttle (obviously an engine makes more torque when the throttle is opened 70% than it does when the throttle is only opened 40%), type of camshafts used, tune of the exhaust system, etc. But if we take a state of the art engine (most of which are created by the Japanese these days) then we can assume that everything is pretty much maxed out. This means that for different engine displacements we get different amounts of maximum torque at the engine that is approximately .07 times the displacement in cc’s. So a 600cc engine could produce about 42 ftlbs of torque and a 1200cc engine could produce about 84 ftlbs of torque and a 1600cc engine could produce about 112 ftlbs of torque. These torque numbers are at the engine’s crankshaft and vary a little depending on the engine’s state of tune (like increasing the compression ratio would increase the torque output and decreasing the compression ratio would decrease the torque output).
Second, an Engine’s Maximum Potential Horsepower is NOT related to the Engine’s Displacement. Horsepower is related to the Engine’s Piston Area times it’s Efficiency. The engine’s stroke has no bearing whatsoever on the engines horsepower output, only the rpms where that output will be available at. Horsepower is related to Torque times RPM. Torque, as discussed in the previous chapter is directly related to displacement. Therefor, Horsepower is Displacement times RPM. But the engine’s RPM is inversely proportional to the length of the stroke: a 100mm stroke engine can rev to only half the rpms of a 50mm stroke engine and the engine’s stroke times the engines redline will equal approximately 609,600. This means that a 101.6mm (4") stroke revved to 6,000 rpms would be about right for a good longevity street engine and a 50.8mm (2") stroke engine could safely rev to 12,000 rpms. These numbers come from keeping the average piston speed to some amount, increasing the piston speed would reduce the engine’s reliability and is not desirable for a long life street engine but it can be done for a short life racing engine. So, Horsepower now equals Displacement divided by Stroke and Displacement is Piston Area times Stroke. Therefor, Horsepower is equal to Piston Area times Stroke divided by Stroke which means that Horsepower is equal to Piston Area.
To put this into algebra:
Horsepower = Torque x RPM (by definition)
Horsepower = Displacement x RPM (substituting Displacement for Torque)
Horsepower = Displacement x 1/Stroke (substituting 1/Stroke for RPM)
Horsepower = (Piston Area x Stroke) x 1/Stroke (Displacement = Piston Area x Stroke)
Horsepower = Piston Area (since Stroke x 1/Stroke = 1)
The Maximum Horsepower of an engine is approximately .0078 times the engines piston area (though this varies widely because different states of tune and different states of longevity that are built into the engines). Piston Area is one half of the bore multiplied by itself multiplied by Pi multiplied by the number of cylinders (unfortunately, some High School math has just crept in, sorry). So if you have a 77mm bore, 4 cylinder engine you could make about 145 horsepower at a safe street rpm (which would be determined by the stroke, which would also determine the engine’s displacement but not it’s horsepower). You can always increase the rpms to an unsafe level to make more horsepower or detune the engine to make less horsepower and last longer, being more reliable.
OK, so now you have a very basic, rudimentary understanding of where Engine Torque and Engine Horsepower come from but what are they used for? The simple answer is given in one sentence: Horsepower accelerates your bike, Torque breaks the drivetrain components. This is the answer to the question: What is the difference between Engine Horsepower and Engine Torque. If you want to know how quick and fast your motorcycle is you need to know it’s Horsepower. If you want to know whether the clutch will hold up or the tire will spin then knowing the Engine’s Torque and it’s overall gear ratio’s will do the trick.
An example, if you add a supercharger to your bike and do not increase it’s rpms then the Torque and Horsepower will increase. Knowing that the Horsepower increased means you know that your bike will accelerate faster but knowing that the torque increased means you know that your clutch will slip and your tire will spin and both must be upgraded to take advantage of the increased engine torque. Also, with the increase in torque the primary drive may be too weak as may be the transmission and final drive . Any increase in the Engine’s Torque must be met with an increase in the torque capabilities of the entire drivetrain or you will simply break components.
Another example, you increase the cam’s duration and intake and exhaust porting and size. This will increase the rpms your engine revs to and increases the torque by a little bit. The horsepower will increase because both the torque and rpms have increased and the same issues with increased torque as described above apply. However there is the increased rpms that now have to be delt with. All rotating parts have a diameter, main bearings can be, say, 1" or 2" or even 3" in diameter and the 1" bearing can be revved to 3 times the rpms that the 3" diameter bearing can be revved to. So if you start with a 3" bearing and a 4,000 rpm limit then your new engine with it’s 1" bearings can rev to 12,000 rpms. But if you are just modifying an existing engine you can’t change the bearing diameters for the crankshaft, camshaft, transmission, etc. and any increase in their rpms will mean more strain on the parts and the oil that protects them. At some point in rpms the oil will sheer and the part will fail. This is the problem with increasing the engine’s rpm capabilities, you don’t control the diameter of the rotating parts and how the oil will protect them.
For an example of using Engine Torque vs. Horsepower in calculating a bike’s acceleration we can have two riders on two identical bikes pass an 18 wheeler together. The engines produce 85 ftlbs of maximum torque (at about 3,000 rpms) and "I’m Mr. Torque" is producing about 80 ftlbs of torque at 50 mph and "I’m Mr. Horsepower" is also producing 80 ftlbs of torque at 50 mph. "I’m Mr. Torque" is smiling as he watches his engine’s torque climb toward it’s 85 ftlb peek, from 80 to 81 to 82 etc. However, "I’m Mr. Horsepower" watches his torque drop as he accelerates, from 80 to 79 to 78 etc. Soon "I’m Mr. Torque" has reached 66 mph and his engine is making 85 ftlbs of torque when all of a sudden, splat, he gets embedded on the front of the oncoming 18 wheeler. "I’m Mr. Horsepower" has safely passed the 18 wheeler they were passing and didn’t get hit by the on coming truck. WHY??? Because "I’m Mr. Torque left his bike in 5th gear when doing the pass and "I’m Mr. Horsepower" had downshifted into 3rd gear. Both bikes make the same amount of torque to start and the one in 3rd gear makes less torque as it accelerates but the torque that matters isn’t Engine Torque, it’s Rear Wheel Torque, which is Engine Torque time the overall gear ratio. The overall gear ratio of these bikes in 5th gear is about 3.5 to 1 and in 3rd gear it’s about 5 to 1. At 66 mph the bike in 5th gear makes about 298 ftlbs of torque at the rear wheel (85 ftlbs Engine Torque times 3.5 gear ratio) but at 66 mph the bike in 3rd gear makes about 375 ftlbs of rear wheel torque (75 ftlbs Engine Torque times 5.0 gear ratio). The bike in 3rd gear will always out accelerate the bike in 5th gear (well, within it’s rpm limit anyway) and it doesn’t matter how much Engine Torque either of them makes. Knowing the Engine Torque without knowing the gear ratio is meaningless and won’t tell you anything about the bikes acceleration. Now, looking at the horsepower that these two bikes made we find that "I’m Mr. Torque" started out with a little less than 35 Hp and climbed to about 48 Hp by 66 mph. "I’m Mr. Horsepower" started out with about 52 Hp and climbed to about 62 Hp by 66 mph. Horsepower always wins when it comes to acceleration. Engine Torque means nothing.
Basically I just want to say "Horsepower accelerates your bike, Torque breaks the drivetrain components". Engine Torque alone has nothing to do with vehicle acceleration. Engine Torque times overall gear ratio divided by the rolling radius of the drive tire does have something to do with acceleration but most people leave out the gear ratios and the tire radius and just claim that engine torque moves the bike. Those people are wrong.
A side note: Engine Power Bandwidth. The lower the rpms that an engine runs at, the broader the power bandwidth, which is good for the street. So, given two 100 HP engines, one with a 5,000 rpm redline and one with a 14,000 rpm redline, I would always want the 5,000 rpm engine for street use because it will make so much more bottom end power it will be easier to drive on the street, even though it won’t be any faster when racing against the 14,000 rpm engine.
During the last 40 years of reading Car and Motorcycle magazines I have often seen letters written that ask what is the difference between Engine Torque and Engine Horsepower. I’ve never seen the right answer written so, before I get too old and die, I thought I’d answer the question in a simple, mostly non-mathematics way.
First, an Engine’s Maximum Potential Torque is related to that Engine’s Displacement times it’s Efficiency. There are many other factors that contribute to that amount of torque, such as how far open is the throttle (obviously an engine makes more torque when the throttle is opened 70% than it does when the throttle is only opened 40%), type of camshafts used, tune of the exhaust system, etc. But if we take a state of the art engine (most of which are created by the Japanese these days) then we can assume that everything is pretty much maxed out. This means that for different engine displacements we get different amounts of maximum torque at the engine that is approximately .07 times the displacement in cc’s. So a 600cc engine could produce about 42 ftlbs of torque and a 1200cc engine could produce about 84 ftlbs of torque and a 1600cc engine could produce about 112 ftlbs of torque. These torque numbers are at the engine’s crankshaft and vary a little depending on the engine’s state of tune (like increasing the compression ratio would increase the torque output and decreasing the compression ratio would decrease the torque output).
Second, an Engine’s Maximum Potential Horsepower is NOT related to the Engine’s Displacement. Horsepower is related to the Engine’s Piston Area times it’s Efficiency. The engine’s stroke has no bearing whatsoever on the engines horsepower output, only the rpms where that output will be available at. Horsepower is related to Torque times RPM. Torque, as discussed in the previous chapter is directly related to displacement. Therefor, Horsepower is Displacement times RPM. But the engine’s RPM is inversely proportional to the length of the stroke: a 100mm stroke engine can rev to only half the rpms of a 50mm stroke engine and the engine’s stroke times the engines redline will equal approximately 609,600. This means that a 101.6mm (4") stroke revved to 6,000 rpms would be about right for a good longevity street engine and a 50.8mm (2") stroke engine could safely rev to 12,000 rpms. These numbers come from keeping the average piston speed to some amount, increasing the piston speed would reduce the engine’s reliability and is not desirable for a long life street engine but it can be done for a short life racing engine. So, Horsepower now equals Displacement divided by Stroke and Displacement is Piston Area times Stroke. Therefor, Horsepower is equal to Piston Area times Stroke divided by Stroke which means that Horsepower is equal to Piston Area.
To put this into algebra:
Horsepower = Torque x RPM (by definition)
Horsepower = Displacement x RPM (substituting Displacement for Torque)
Horsepower = Displacement x 1/Stroke (substituting 1/Stroke for RPM)
Horsepower = (Piston Area x Stroke) x 1/Stroke (Displacement = Piston Area x Stroke)
Horsepower = Piston Area (since Stroke x 1/Stroke = 1)
The Maximum Horsepower of an engine is approximately .0078 times the engines piston area (though this varies widely because different states of tune and different states of longevity that are built into the engines). Piston Area is one half of the bore multiplied by itself multiplied by Pi multiplied by the number of cylinders (unfortunately, some High School math has just crept in, sorry). So if you have a 77mm bore, 4 cylinder engine you could make about 145 horsepower at a safe street rpm (which would be determined by the stroke, which would also determine the engine’s displacement but not it’s horsepower). You can always increase the rpms to an unsafe level to make more horsepower or detune the engine to make less horsepower and last longer, being more reliable.
OK, so now you have a very basic, rudimentary understanding of where Engine Torque and Engine Horsepower come from but what are they used for? The simple answer is given in one sentence: Horsepower accelerates your bike, Torque breaks the drivetrain components. This is the answer to the question: What is the difference between Engine Horsepower and Engine Torque. If you want to know how quick and fast your motorcycle is you need to know it’s Horsepower. If you want to know whether the clutch will hold up or the tire will spin then knowing the Engine’s Torque and it’s overall gear ratio’s will do the trick.
An example, if you add a supercharger to your bike and do not increase it’s rpms then the Torque and Horsepower will increase. Knowing that the Horsepower increased means you know that your bike will accelerate faster but knowing that the torque increased means you know that your clutch will slip and your tire will spin and both must be upgraded to take advantage of the increased engine torque. Also, with the increase in torque the primary drive may be too weak as may be the transmission and final drive . Any increase in the Engine’s Torque must be met with an increase in the torque capabilities of the entire drivetrain or you will simply break components.
Another example, you increase the cam’s duration and intake and exhaust porting and size. This will increase the rpms your engine revs to and increases the torque by a little bit. The horsepower will increase because both the torque and rpms have increased and the same issues with increased torque as described above apply. However there is the increased rpms that now have to be delt with. All rotating parts have a diameter, main bearings can be, say, 1" or 2" or even 3" in diameter and the 1" bearing can be revved to 3 times the rpms that the 3" diameter bearing can be revved to. So if you start with a 3" bearing and a 4,000 rpm limit then your new engine with it’s 1" bearings can rev to 12,000 rpms. But if you are just modifying an existing engine you can’t change the bearing diameters for the crankshaft, camshaft, transmission, etc. and any increase in their rpms will mean more strain on the parts and the oil that protects them. At some point in rpms the oil will sheer and the part will fail. This is the problem with increasing the engine’s rpm capabilities, you don’t control the diameter of the rotating parts and how the oil will protect them.
For an example of using Engine Torque vs. Horsepower in calculating a bike’s acceleration we can have two riders on two identical bikes pass an 18 wheeler together. The engines produce 85 ftlbs of maximum torque (at about 3,000 rpms) and "I’m Mr. Torque" is producing about 80 ftlbs of torque at 50 mph and "I’m Mr. Horsepower" is also producing 80 ftlbs of torque at 50 mph. "I’m Mr. Torque" is smiling as he watches his engine’s torque climb toward it’s 85 ftlb peek, from 80 to 81 to 82 etc. However, "I’m Mr. Horsepower" watches his torque drop as he accelerates, from 80 to 79 to 78 etc. Soon "I’m Mr. Torque" has reached 66 mph and his engine is making 85 ftlbs of torque when all of a sudden, splat, he gets embedded on the front of the oncoming 18 wheeler. "I’m Mr. Horsepower" has safely passed the 18 wheeler they were passing and didn’t get hit by the on coming truck. WHY??? Because "I’m Mr. Torque left his bike in 5th gear when doing the pass and "I’m Mr. Horsepower" had downshifted into 3rd gear. Both bikes make the same amount of torque to start and the one in 3rd gear makes less torque as it accelerates but the torque that matters isn’t Engine Torque, it’s Rear Wheel Torque, which is Engine Torque time the overall gear ratio. The overall gear ratio of these bikes in 5th gear is about 3.5 to 1 and in 3rd gear it’s about 5 to 1. At 66 mph the bike in 5th gear makes about 298 ftlbs of torque at the rear wheel (85 ftlbs Engine Torque times 3.5 gear ratio) but at 66 mph the bike in 3rd gear makes about 375 ftlbs of rear wheel torque (75 ftlbs Engine Torque times 5.0 gear ratio). The bike in 3rd gear will always out accelerate the bike in 5th gear (well, within it’s rpm limit anyway) and it doesn’t matter how much Engine Torque either of them makes. Knowing the Engine Torque without knowing the gear ratio is meaningless and won’t tell you anything about the bikes acceleration. Now, looking at the horsepower that these two bikes made we find that "I’m Mr. Torque" started out with a little less than 35 Hp and climbed to about 48 Hp by 66 mph. "I’m Mr. Horsepower" started out with about 52 Hp and climbed to about 62 Hp by 66 mph. Horsepower always wins when it comes to acceleration. Engine Torque means nothing.
Basically I just want to say "Horsepower accelerates your bike, Torque breaks the drivetrain components". Engine Torque alone has nothing to do with vehicle acceleration. Engine Torque times overall gear ratio divided by the rolling radius of the drive tire does have something to do with acceleration but most people leave out the gear ratios and the tire radius and just claim that engine torque moves the bike. Those people are wrong.
A side note: Engine Power Bandwidth. The lower the rpms that an engine runs at, the broader the power bandwidth, which is good for the street. So, given two 100 HP engines, one with a 5,000 rpm redline and one with a 14,000 rpm redline, I would always want the 5,000 rpm engine for street use because it will make so much more bottom end power it will be easier to drive on the street, even though it won’t be any faster when racing against the 14,000 rpm engine.
And only God would know what number you'd use for that, well, I guess the Europeans would know also.
