Stop by a newsstand and check out the cover headlines on almost any motorcycle magazine and the first things you'll see are topics such as "Gain 10 easy horsepower," or "Increase horsepower and torque with quick bolt-ons," or "Add pound-feet of torque the easy way." Terms like horsepower and torque are tossed around frequently, but seldom are they defined. Consequently, you are often left with many unanswered questions. Regardless of whether you ride a heavyweight bagger, medium-size cruiser or lightweight Sporty, understanding the differences between horsepower and torque can help you make educated decisions when choosing performance parts for your model of bike and riding style.
Interestingly, horsepower and torque are closely related. The point in the engine's rpm band at which torque is made and the amount of torque made will determine whether the engine is considered a torque- or horsepower-type of motor. Astute engine builders select engine components that improve torque and/or horsepower to optimize a bike's application and the rider's style of riding. In order to understand horsepower, we must first understand something called "work" and then define torque.
In simple terms, horsepower is a measure for the amount of work an engine can perform in a given time. Eighteenth century engineer and inventor James Watt headed the development of the high-pressure steam engine. That led to the need for a method of measuring the amount of work a steam engine could perform over a given amount of time. Because the steam engine could perform work commonly done in that epoch by draft horses, Watt related an engine's work to the work a horse could perform. Using a progression of tests along with empirical data, Watt surmised that moving 33,000 pounds 1 foot in one minute was the equivalent of 1 horsepower, which has become the standard for measuring the force or power output of the internal combustion engine. However, to explain horsepower first requires a discussion of force and torque because horsepower is a calculated number derived by first measuring torque at a given rpm.
Work is the measurement of a force exerted in a straight line. Common items that involve force include an engine's crankshaft, nuts and bolts as they are tightened or loosened, and a bicycle pedal crank. All three examples rotate around an axis. The rotational or twisting force of these items is called "torque." Torque, then, is defined as a measure of the ability of a force to cause twisting or rotation, which is measured in "pound-foot" units of force times the distance from the axis of rotation. As an example, let's assume you have a wrench 1-foot long and apply a force of 100 pound-feet at the end of it. In essence, you are applying a torque of 100 pound-feet. If the same wrench were 3-feet long, 100 pound-feet of force would apply 300 pound-feet of torque. In other words, if your V-twin engine makes 100 pound-feet of torque, it would take 100 pounds of force on a 1-foot lever to stop its rotating motion.
Engine torque is normally measured on a dynamometer and can be defined as the potential to do work. However, unlike horsepower, torque does not take into consideration the element of time, which gauges the rate at which an engine can perform work. An engine's power rating is actually established by first measuring torque at a given rpm and then mathematically calculating horsepower.
1. Force is a pushing or pulling action by one object against another. When force is applied and movement occurs, such as with these bicycle pedals, work is performed by moving the bicycle from one location to another. Torque is a measure of the ability of a force to cause twisting or rotation. An engine's crankshaft, nuts and bolts when they are tightened or loosened, and bicycle pedal crank sets rotate around an axis. The rotational or twisting force of these items is called "torque," which is measured in "pound-foot" units of force times the distance from the axis of rotation.
1. Force is a pushing or pulling action by one object against another. When force is appl
2. Torque is the force that launches this AHDRA gas drag bike from a dead stop. Torque is a measurement of the ability of a force to cause twisting or rotation. It takes torque to launch a vehicle and get it moving; horsepower is what gives the vehicle a high top-end speed. - AHDRA
2. Torque is the force that launches this AHDRA gas drag bike from a dead stop. Torque is
3. An engine that makes greater horsepower has the ability to perform more work in a given amount of time (rpm). After a vehicle is launched, it takes horsepower to power down the track or achieve high speeds on a highway. Top-end horsepower is "king" at the drag strip. But low-end torque rules on the street because it gets a heavy bike moving and makes for more enjoyable street riding. - AHDRA
3. An engine that makes greater horsepower has the ability to perform more work in a give
Now that you know about work and torque, horsepower can be defined. Horsepower is a measurement of how much work (force over distance) an engine can perform while including the element of time it took to do the work. Therefore, horsepower is a function of a given amount of force (torque) acting over a given distance from the axis of rotation within a given amount of time (rpm). A simple example of performing work over a specified distance is applying a force of 1 pound over a distance of 1 foot. This is equivalent to 1 pound-foot of work (force over distance). Still, the definition of horsepower also includes a time factor. So let's now assume we applied a force of one pound over a distance of one foot and did it in one minute. That would be equivalent to a small fraction of a horsepower because James Watt's definition of one horsepower is performing 33,000 pound-feet of work in one minute.
Moreover, the definition for horsepower does not need to remain firm for producing 1 horsepower. As an example, the force, distance and time factors can be varied as long as the result is equivalent to performing 33,000 pound-feet of work in one minute. For instance, applying a force of 66,000 pounds over a distance of one foot in two minutes or applying a force of 300 pounds over a distance of 100 feet in one minute or applying a force of 33,000 pounds over 3 feet in three minutes are all equivalent to James Watt's definition of one horsepower.
The constant 5252 is derived from James Watt's 18th century definition of horsepower. The constant includes the "time factor" into the equation.
Horsepower can be related to the internal combustion engine by associating force, distance, and time in the following manner:
(1) Force is equivalent to the amount of combustion pressure applied to a given square area of the piston dome.
(2) Distance is equivalent to the engine's stroke length.
(3) Time is defined by the rpm or speed at which the engine is rotating.
When designing, building and tuning an engine, it is important to remember the following four principles:
(1) At any given rpm, horsepower is directly proportional to torque.
(2) By increasing torque at a specified rpm, horsepower increases at a corresponding amount.
(3) If torque remains constant but rpm increases, then horsepower increases in direct proportion to rpm.
(4) When torque starts to drop off (beyond the engine's torque peak), as long as rpm increases faster than torque drops, horsepower will still increase. These adages inform us that every engine is a 'torque' engine. And the only difference between a "torque" engine and "horsepower" engine is where the engine makes its torque: torque engines make gobs of torque low in the rpm band while horsepower engines make big torque at the top end.
Another description for horsepower is how much and how often a cylinder fills, and how often the cylinder fires during a given time frame. Keep in mind that since horsepower is a calculated number, the only practical method for determining horsepower is by first measuring engine torque and rpm with a dynamometer.
Inasmuch horsepower is equal to torque multiplied by rpm, an increase in torque at any given rpm increases horsepower at that same rpm. As such, when striving for best performance, wise engine builders concentrate on improving torque rather than horsepower. They also concentrate on improving torque within the most important rpm range the engine operates.
4. These two engines make the same amount of peak torque at the same rpm but the areas under the curve are unequal. Since engine #2's torque curve is broader, it has more area under the curve. Generally, the engine with the broader torque curve accelerates faster and is more suitable for street riding. For optimized performance, maximize torque within the engine's most critical rpm range and don't worry about horsepower.
4. These two engines make the same amount of peak torque at the same rpm but the areas un
5. Here we see that torque peaks at a low 3,000 rpm, then quickly drops off since the engine runs out of air because the induction and exhaust systems are restricted. Horsepower also drops quickly because volumetric efficiency (cylinder fill) is reducing faster than rpm is increasing.
5. Here we see that torque peaks at a low 3,000 rpm, then quickly drops off since the eng
6. Increasing the engine's ability to breathe with more cam timing/lift and higher-flowing induction/exhaust systems will move the torque peak horizontally on the chart to a higher rpm. Horsepower will also increase because the volumetric efficiency is dropping slower than rpm is increasing.
6. Increasing the engine's ability to breathe with more cam timing/lift and higher-flowin
7. Since leverage is torque, a long-stroke engine will make about an identical amount of peak torque as a shorter-stroke engine of equal size. However, the rpm at which the peak torque occurs will be at a lower rpm. Stroker flywheels not only increase displacement but also provide greater leverage for more low-end torque. - Short Block Charlie's
7. Since leverage is torque, a long-stroke engine will make about an identical amount of
No Replacement For Displacement
Essentially, an engine's torque is determined by the percentage of cylinder fill (volumetric efficiency) and cylinder displacement. The greater the cylinder fill, the greater the torque will be. To increase power, it is important to improve the engine's ability to breathe. Peak torque is reached when the engine runs out of air or loses its ability to breathe better. And that is the point of maximum cylinder fill. An engine will continue to make more horsepower even when torque is falling as long as rpm increases faster than torque falls. If maximum torque is the point of maximum cylinder fill, then maximum horsepower is the point where torque begins falling off faster than rpm increases.
While every engine produces torque, an engine that produces peak torque at low rpm is typically referred to as a "torque" engine, while one that produces peak torque at high rpm is considered a "horsepower" engine. The most effective way to increase torque is to increase displacement. For this reason, increasing displacement on a stock Twin Cam engine by installing big-bore cylinders offers a lot of bang for the buck, even though the additional displacement will cause the engine to run out of breath at a lower rpm. Once engine displacement is increased, airflow improvements can be made by installing a free flowing exhaust system, higher-flowing head, larger carburetor or throttle body, bigger cam(s) and less restrictive air cleaner.
Changing the bore/stroke combination for a given size engine displacement can change the engine's power curve. As an example, a short-stroke combination will typically raise the rpm at which peak torque occurs, which will improve top-end power potential at the expense of bottom-end torque. For a low-rpm street engine that does not rev more than 5,000 or 5,500 rpm, a longer stroke is usually more beneficial than a larger bore because increased stroke can make more torque down low, which is where street engines spend most of their time. However, stroking a V-twin can be more costly than installing large-bore cylinders, so it often makes more economical sense to just install big cylinders and be done with it for mild or moderate hop ups.
When high rpm operation is a priority (about 6,500 rpm or higher), a large-bore/short-stroke combo offers the best power potential because it will unshroud the valves and allow installation of larger valves for improved breathing. But this design also requires high-quality heads and valvetrain components to support the engine's high-rpm airflow requirements, which usually results in a costly engine and overkill for most low-rpm street engines. As an aside, most aftermarket V-twin crate engines are big-bore moderate- or short-stroke combinations for durability reasons and not necessarily higher performance. A shorter stroke results in less crank leverage, therefore reduced thrust loading against cylinder walls and less bearing loads. A short stroke also results in less piston speeds for reduced wear and longer piston skirts for improved cylinder sealing. As a result, aftermarket manufacturers have less warranty concerns.
8. Displacement and torque can be easily increased on a V-twin engine with big-bore cylinders. But remember that the engine will run out of air and torque will peak at a low rpm unless the induction and exhaust systems are improved to support the increased airflow demands of the larger displacement. - Short Block Charlie's
8. Displacement and torque can be easily increased on a V-twin engine with big-bore cyli
9. Twin S&S D carbs make big top-end horsepower on a high-revving engine like this one running in AHDRA events. However, twin carbs can reduce low-end torque on a smaller displacement, lower-revving engine. Always match the carb or throttle body (TB) size to your engine's airflow requirements and application. An overly large throttle body on an EFI engine is more forgiving than an overly large carb because the TB only flows air and not fuel. But extreme TB size also reduces air velocity, making tuning more difficult. - AHDRA
9. Twin S&S D carbs make big top-end horsepower on a high-revving engine like this one ru
10. High-flowing cylinder heads are needed to make big horsepower and for increased displacement to support the additional airflow required at high rpm. Port and valve sizes should be matched to the application and airflow requirements so torque is optimized in the most critical rpm band. - Short Block Charlie's
10. High-flowing cylinder heads are needed to make big horsepower and for increased displ
Displacement is "king" for building torque because the easiest and most effective method for increasing torque is to increase engine size. For high torque, a 95ci engine is better than an 88, and a 124ci engine is better than a 95. Size does matter. Although building a low-end torque engine can be easier on engine parts, it's also harder on drivetrain components. Moreover, too much torque can overpower the rear tire and chassis, making a bike hard to launch. Most V-twin street riders are craving more torque down low, but what they forget is that engines have gotten so big, 120- to 140ci or even larger, that they often overpower the available traction. In these cases, the bike's launch can be improved by trading off some low-end torque for high-end horsepower. This is done by moving the torque curve higher up in the rpm band by installing free-flowing induction and exhaust systems, better breathing heads and performance cam(s).
While displacement is "king," remember that similar size engines can have widely varying power levels due to differences in airflow capabilities. All other things being equal, a big engine will make more torque down low than a smaller engine, but it will not necessarily make more horsepower up top. To make more horsepower on the top end, a big engine must be supported with free-breathing cylinder heads, carbs or throttle bodies, cam(s), and exhaust systems to feed the larger displacement at high rpm. For example, installing big-bore cylinders on a bone-stock Twin Cam 88 or 96ci engine will improve low-end torque, but the engine will run out of air at a low rpm (lower than a stock engine) and not develop optimized horsepower for its size. This is because the induction and exhaust systems cannot flow enough air for the big engine at high rpm. Accordingly, volumetric efficiency falls off early in the rpm band, and the torque curve drops like a lead balloon. Since cylinder filling is poor at high rpm, little torque is made up top, which also means that little horsepower is made up top.
Despite your engine combination, always strive to build a balanced engine with airflow matched to the engine's displacement and critical rpm range. Although this requires matching engine displacement with the proper heads, cam(s), carb or throttle body, and exhaust system for the application, which can be costly, it will also result in the broadest, flattest and highest torque curve. This will result in optimized acceleration and the greatest suitability for street riding. Generally, using smaller valves, smaller ports, smaller carburetors and less cam timing will improve low-end torque at the expense of top-end horsepower.
Another key factor for maximizing torque and horsepower is mechanical compression ratio. Most street engines are pump-gas limited, which means that the quality of the gas will dictate the maximum compression and cam timing that can be used without encountering performance problems. In order to optimize torque and throttle response, always maximize the engine's compression ratio to the quality of the fuel, and then match the cam(s) to the compression, engine displacement and rpm range. Depending on several variables, pump-gas engines are normally limited to between 10:1 and 10.5:1 mechanical compression ratio. Move the engine's compression ratio into that range, then build and tune the engine to that delta. If you don't, you are leaving power on the table.
11. Camshaft(s), cylinder heads and stroke length determine where the torque and horsepower peaks occur. Hotter cams, higher-flowing cylinder heads and a shorter stroke length will move the peaks higher up the rpm band. Closely match the cam(s) to the displacement, compression ratio, rpm band and exhaust system for optimized torque and horsepower. Dyno or trackside testing is the only sure way to know how a parts combo performs. - Short Block Charlie's
11. Camshaft(s), cylinder heads and stroke length determine where the torque and horsepow
12. A higher-flowing exhaust system is required when displacement is increased. Typically, larger diameter and shorter length pipes improve top-end horsepower, while smaller diameter, longer pipes favor low-end torque. Stepped headers can broaden the torque curve.
12. A higher-flowing exhaust system is required when displacement is increased. Typically
Whenever a long-duration cam is installed, the effect of increasing the compression ratio is of much greater importance, especially at low rpm. The cam's intake valve closing should be matched to the mechanical compression ratio to achieve optimized corrected compression and optimized performance. At low rpm, a late-closing intake reduces cylinder fill and torque. However, torque lost at low rpm can be regained (at least partially) by increasing the engine's mechanical compression ratio. This will allow the corrected compression ratio to maintain a predetermined level.
Compression ratio is another key optimizing variable. The corrected compression ratio (not to be confused with mechanical compression ratio and also called static compression) is a mathematically calculated number based on cylinder displacement and intake valve closing. It will always be a lower value than the mechanical compression ratio. A corrected compression ratio of 9:1 or a tad higher, depending on engine optimization, is roughly the maximum achievable on a pump-gas engine without encountering detonation. Keep in mind, however, that bike weight, gearing, combustion chamber design, rod length, ignition timing and ambient temperatures are some of the variables determining the maximum corrected compression achievable before encountering detonation.
When designing and building an engine to satisfy predefined performance objectives, the relationship between horsepower and torque is one of the most significant concepts to understand. Keep in mind that everything is a tradeoff; so make decisions wisely. Although all engines make torque, it is where the engine makes torque that will determine whether it is considered a "torque" or "horsepower" engine. Before buying costly parts, determine which type of engine your application and riding style need, then strive to build a "happy" engine by choosing performance components that are compatible and in harmony with one another. Knowledgeable engine builders maximize torque in the most important range of the rpm band and do not get overly concerned with horsepower.
The following equations are used for calculating horsepower and torque:
||torque & rpm
||horsepower & 5252
All Harley Drag Racing Association (ADDRA)
Short Block Charlie's