Both of these engines make the same peak torque at the same rpm but have unequal areas under the curve. The torque curve for engine #2 is broader, resulting in more area under the curve. Normally an engine with a broader torque curve accelerates faster and is more suitable for street riding. For best performance, maximize torque within the engine's most important rpm range and let horsepower fall where it may.
This power chart shows that torque peaks at a low 3000 rpm and then drops off quickly because the engine runs out of breath due to restricted induction and exhaust systems. Horsepower also drops off quickly because volumetric efficiency (cylinder fill) is dropping more quickly than rpm is increasing.
By increasing the engine's ability to breathe (more cam, higher-flowing induction/exhaust systems), the torque peak will move horizontally on the chart to a higher rpm, and horsepower will increase because volumetric efficiency drops more slowly than rpm is increasing.
It doesn't matter whether you stop by your local motorcycle dealer or favorite Friday night hangout; sooner or later, chitchat gravitates to performance. And sometime during the conversation, the word "horsepower" is tossed around with abandon. Typically, someone says they dyno'd their engine and it made huge horsepower. Another guy quips that he is building a monster motor and expects to get 150hp. Another performance guru says he bolted on a new carb and picked up big horsepower. Nevertheless, the common thread in the conversations is often "horsepower."
Clearly, horsepower has panache. Although the term is tossed around frequently, no one really defines it. And to make matters even more confusing, the word "torque" is often tossed into the conversation. As such, questions often left unanswered are what is horsepower, how do you define it and where does torque fall into the mix?
Truth be told, horsepower isn't something discernible by touch. Instead, it is a measure for the amount of work an engine can perform in a given time. Eighteenth century engineer and inventor James Watt spearheaded the development of the high-pressure steam engine. That led to the need for a method of measuring the amount of work the steam engine could perform over a given amount of time. Since the steam engine would 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 1hp, which has become the standard for measuring the force or power output of the internal combustion engine. But 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 measured as a force exerted in a straight line. However, common items that involve force, such as an engine's crankshaft, nuts and bolts when they are tightened or loosened and a bicycle pedal crank, rotate around an axis. The rotational or twisting force of these items is called "torque." Torque is a measure of the ability of a force to cause twisting or rotation, which is measured in "pound-feet" units of force times the distance from the axis of rotation. For example, if you have a wrench 1 foot long and apply a force of 100 lb-ft at the end of it, you are applying a torque of 100 lb-ft. If the same wrench were 3 feet long, 100 lb-ft of force would apply 300 lb-ft of torque. In other words, if your V-twin engine makes 100 lb-ft of torque, it would take 100 pounds of force on a 1-foot lever to stop its rotating motion.
Today, engine torque is typically measured on a dynamometer and can be defined as the potential to do work. Unlike horsepower, however, 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.
Horsepower is a measurement of how much work (force over distance) an engine can do 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 lb-ft of work (force over distance). However, the definition of horsepower also includes a time factor. So let's now assume we applied a force of 1 pound over a distance of 1 foot and did it in one minute. That would be equivalent to a small fraction of a horsepower, because James Watt's definition of 1hp is performing 33,000 lb-ft of work in one minute.
Furthermore, the definition for horsepower does not need to remain rigid for producing 1hp. The force, distance and time factors can be varied as long as the result is equivalent to performing 33,000 lb-ft of work in one minute. For instance, applying a force of 66,000 pounds over a distance of 1 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 1hp. Following are the equations for calculating horsepower and torque.
**horsepower = torque x rpm /5252
torque = horsepower x 5252 /rpm**
The constant 5252 is derived from James Watt's 18th century definition of horsepower. The constant enters the "time factor" into the equation.
What It All Means
We can relate horsepower to the internal combustion engine by associating the three factors involved--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 turning.
The following are four important principles to remember when designing, building and tuning an engine: (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) Even 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. What these axioms tell us is 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 lots of torque low in the rpm band while horsepower engines make gobs of torque on the top-end.
Horsepower can also be described as how much and how often a cylinder fills and how often it fires in a specified time. However, keep in mind that horsepower is a calculated number, so the only practical method for determining horsepower is by first measuring engine torque and rpm with a dynamometer.
Since horsepower is equal to torque multiplied by rpm, any increase in torque increases horsepower at a given rpm. That is why it is better to concentrate on improving torque than horsepower for achieving the best engine performance. When designing or building an engine, always concentrate on improving torque within the most important rpm range the engine operates.
Engine torque is primarily determined by the percentage of cylinder fill (volumetric efficiency) and 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.
Although all engines produce torque, an engine that produces peak torque at low rpm is generally 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 engine will 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 and less restrictive air cleaner.
Juggling the bore/stroke combination for a given size engine displacement can change the engine's power curve. For example, a short-stroke combination will typically raise the rpm at which peak torque occurs. This will improve top-end power potential at the expense of bottom-end torque. For a low-rpm street engine that doesn't see more than 5000 or maybe 5500 rpm, a longer stroke is usually more helpful than a bigger bore, because increased stroke can make more torque down low 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.
If high-rpm operation is a priority (roughly 6500 rpm or higher), a large-bore/short-stroke combo offers the best power potential, because it will unshroud the valves and allow larger valves for improved breathing. But this 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. By the way, most aftermarket V-twin crate engines are big-bore moderate- or short-stroke combinations for durability reasons. A shorter stroke results in less crank leverage, therefore reducing 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.
Regardless of what engine combination you build, always strive to build a balanced engine where airflow is matched to the engine's displacement and critical rpm range. This requires matching engine displacement with the proper heads, cams, carb or throttle body and exhaust system for the application, which can be costly. But it will also result in the broadest, flattest and highest torque curve, providing optimized acceleration and the greatest suitability for street riding. In general, smaller valves, smaller ports, smaller carburetors and less cam timing will improve low-end torque at the expense of top-end horsepower.
Compression ratio is another key factor for maximizing torque and horsepower for optimized performance. Most street engines are pump gas limited, which means that the quality of the gas will dictate the maximum compression and cam that can be used without encountering performance problems. To optimize torque and throttle response, always maximize the engine's compression ratio to the quality of the fuel and then match the cam 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. Get 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.
When installing a long-duration cam, the effect of a compression ratio increase is of much greater importance, especially at low rpm. The cam's intake valve closing should be matched to the compression ratio to achieve 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 allows the corrected compression ratio to maintain a predetermined level.
Corrected compression is a mathematically calculated number based on cylinder displacement and intake valve closing and 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 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 hitting detonation.
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 more cam.
Although displacement is "king," keep in mind 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 over a smaller engine, but it will not necessarily make more horsepower up top. To make more horsepower on top, a big engine must be supported with free-breathing cylinder heads, carbs or throttle bodies, cams and exhaust systems to feed the larger displacement at high rpm. For instance, installing big 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 good horsepower for its size. This is because the induction and exhaust systems cannot flow enough air for the big engine at high rpm. Consequently, 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.
The relationship between horsepower and torque is one of the most important concepts to understand when designing and building an engine to satisfy predefined performance objectives. Remember that everything is a trade-off, so make your 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 you need and then strive to build a "happy" engine by choosing performance components that are compatible and in harmony with one another. Smart engine builders maximize torque in the most important operating range and let horsepower fall where it may.
What's the difference? * Which is better? * Does it really matter?
FORCE & TORQUE
Force is a pushing or pulling action by one object against another. If force is applied and movement occurs, such as in the case of this bicycle, 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-feet" units of force times the distance from the axis of rotation.
POUND-FEET & TORQUE
Ground-pounding torque is what launches this AHDRA Top Fuel drag bike from a standing stop--and blurs this photo. Torque is a measure of the ability of a force to cause twisting or rotation. It is measured in "pound-feet" units of force times the distance from the axis of rotation. It takes torque to launch a vehicle and get it moving; horsepower is what gives the vehicle higher mph or top-end speed. --Special thanks to the AHDRA
HORSEPOWER & RPM
The more horsepower an engine makes, the more work it can do in a given amount of time (rpm). Once a vehicle is launched, it takes horsepower to motor down the track or cruise at high speeds on the highway. Top-end horsepower is "king" at the dragstrip or long oval tracks. But torque down low rules on the street, because it gets a heavy vehicle going and makes for a more pleasant-riding streetbike.
Leverage is torque. A long-stroke engine will make roughly the same amount of peak torque as a shorter-stroke engine of equal size, but the rpm at which the peak torque occurs will be at a lower rpm. Stroker flywheels increase displacement and provide greater leverage for more grunt down low.
Big-bore cylinders are the easiest way to increase displacement and torque on a Twin Cam engine. However, the engine will run out of air and torque will peak at a low rpm unless the induction and exhaust systems are modified to support the increased airflow demands of the larger engine.
CARB & THROTTLE BODY
Twin S&S; D carbs will make tremendous high-end horsepower on a high-revving 139ci AHDRA Street Pro engine like this one. But they will also kill torque down low on a smaller-displacement, lower-revving engine. Be sure to match the carb or throttle body size to your engine's requirements. An excessively large throttle body on an EFI engine is more forgiving than an overly large carb, because the throttle body only flows air and not fuel. However, an excessive throttle body size also reduces air velocity and will make tuning more difficult.
High-flowing cylinder heads are required to make big horsepower on any engine. They're also required when displacement is increased to support the additional airflow needed at high rpm. Port and valve sizes should be matched to the engine's airflow requirements and application so torque is maximized in the most important rpm range.
CAMSHAFT, CYLINDER & STROKE
Where torque and horsepower will peak is determined by the camshaft, cylinder heads and stroke length. Bigger cams, higher-flowing cylinder heads and a shorter stroke length will move the peaks higher up the rpm band. For best torque and horsepower, closely match the cam to displacement, compression ratio, rpm band and exhaust system. Dyno or trackside testing are the only ways to know for sure whether the combo works.
PIPES & STEPPED HEADERS
Increased displacement requires a higher-flowing exhaust system. Generally, 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.