The Great Torque vs. Horsepower Debate

[phenom86]

Another good read, very informative for everyone.. i've read this years ago when i used to own a supercharged WS6, great article... grab a drink cuz its long and im sure some of you will be confused but in the end you will understand..

Horsepower or torque: Which is better? It's a never-ending debate among those who build engines for a living. Take, for example, the following statements, all from recognized engine gurus:

"Horsepower sells engines and cars. But torque moves the car." - Kevin McClelland, Flowmaster Mufflers
"I build an engine for strictly horsepower! It's easier to get the car down the track with more power. Horsepower gives you more miles-per-hour, and a better e.t." - Joe Sherman, Joe Sherman Racing Engines

"Horsepower is what pushes, pulls, shoves, or drags our rides through the friction-filled world. If we want to go faster, it takes more horsepower. If we want to go slower, it takes less horsepower." - Harold Bettes, SuperFlow

"It takes torque to get a car moving." - **** Miller, **** Miller Racing

"Want to run faster? It's all about horsepower! But if you have more torque, you'll have more horsepower!" - Judson Massingill, School of Automotive Machinists

Who's right? As we'll see, they all are! Everything starts with the concepts of "force" and "work."


Thrust
Thrust is the force that drives your car forward. It doesn’t matter how much torque your engine makes, what your gearing is, how large your wheels are, how shiny your muffler is, or how many stickers you have on your doors – thrust determines how fast your car can accelerate. In racing, acceleration is what really matters, and there are only a few immediate ways to make your car accelerate faster:

ΣF = ma
Where, ΣF= the sum of the forces acting on the car
m = the mass of the car
a = the acceleration of the car

If you want to increase acceleration, you need to reduce weight, increase thrust, or decrease other forces that oppose thrust (namely drag).

Torque
Torque is a force applied about a moment arm (commonly described as a “twisting” force). Instead of a force that propels an object forward, think of it as a force that causes the object to rotate. If I apply a force about some point, the torque applied is the product of the distance to the point and the portion of the force which is perpendicular to that distance. The equation used to calculate torque is:

τ = Fd
Where, τ = torque
F = the portion of the total force acting perpendicularly to the distance vector
d = the distance between the fulcrum and the point at which the force F is applied

Note that torque is instantaneous. It doesn’t matter how long or the distance over which the force is applied.

Work
Work is the energy used to move an object. Forces and torques might always exist, but unless the object has moved, no work has been performed. If I try to move an object, but I can’t make it budge, then I’m performing no work. I’m using a lot of energy to develop force, but it’s being used to create heat, not to perform work. Once the object moves, then I’m using some of that energy to perform work. The equation used to determine the work performed on an object is:

W = Fx
Where, W = the work performed
F = the force applied to the object
x = the displacement of the object

Work can also be performed on a rotating object, such as a crankshaft. The equation in this case is slightly different:

W = τθ
Where, W = the work performed
τ = the torque applied
θ = the angular displacement (in radians)

Instead of force, torque is used, and the displacement is expressed in radians instead of feet or meters. If an object makes one revolution, its angular displacement is 2π radians (or 360°), if it makes ¼ revolution, its angular displacement is π/2 radians (or 90°), etc. Based on this equation, it should be apparent that torque is not a measure of work. If torque is applied to a shaft, but it does not rotate, no work is performed. It’s a little confusing since both have the same units, but they do not measure the same quantities.

Power
Power is a measure of how much work is performed (or energy used) per unit of time. It doesn’t measure how much torque is applied over time, just the work performed. For engines, the unit “horsepower” is used. If an engine produces 1 hp, then it can perform 550 ft-lb of work (not torque) every second. It doesn’t matter if the engine produces 1 ft-lb of torque or 100 ft-lb of torque. The equation for power is:

P = Fv
Where, P = power
F = the force applied
v = the velocity at which the force is applied

For a rotating object, it’s:

P = τω
Where, P = power
τ = the torque applied
ω = the rotational speed at which the torque is applied

For engines, we commonly see the equation: Horsepower = (Torque)*(RPM) / 5252

However, what does this really mean? Where does the constant 5252 come from?

Calculating Horsepower
As mentioned, power can be calculated by multiplying the torque applied to a rotating object by the speed at which it rotates. However, the base unit for rotational speed is radians per second, and we normally measure engine speed in revolutions per minute. Therefore, if we want to use rpm, we need to convert. Since there are 2π radians in one revolution, and 60 seconds in one minute, we have to divide by 60 and multiply by 2π:

P = (Torque) * (RPM) * (1 min / 60 s) * (2π rad / 1 rev) = 2π*(Torque)*(RPM) / 60

Next, we want power expressed in horsepower, not ft-lb/s (which is the base unit). Therefore, we need to divide by 550:

Horsepower = 2π*(Torque) *(RPM) / (60 * 550) = 2π*(Torque) *(RPM) / 33000

Since 33000/2π is about 5252, we just use: Horsepower = (Torque)*(RPM) / 5252

Now you can see that it’s not just a random equation with a meaningless result. It’s a measure of the rate at which energy is produced and the rate at which work is performed.

So What?
By now, you’re probably getting tired of my physics lesson, so I’ll move on. How do we use this information to design our engines? Do we want to produce tons of torque like a diesel engine or gobs of horsepower like a motorcycle engine?

Work Explained
Force is a pushing or pulling action of one body against another. Depending on the resistance to the application of force, it may or may not result in movement. Say you try to push on a stalled car with 125 pounds of force, but it remains stuck in the mud. You've exerted force, but no movement has occurred because the car (being heavier than you) has too much resistance.



If force is applied and movement does occur, you've performed work, or the movement of an object from one position to another. For example, if you use a hoist to lift a 600-pound engine 6 feet in the air, the work done would be 6 feet x 600 pounds or 3,600 foot-pounds (ft-lb).



Work = D (distance moved) x F (force applied)

Torque Explained
By definition, work is calculated as a vector force, exerted in a straight line. But engines (as well as nuts and bolts when they are tightened or loosened) rotate around an axis. The expression of this rotational or twisting force is called "torque," which is measured in units of force times distance from the axis of rotation. If you have a 1-foot-long wrench and you exert a force of 10 pounds on the end of it then you apply a torque of 10 pound-feet (10 lb-ft). If the wrench were 2 feet long, the same force would apply a torque of 20 lb-ft. When an engine is said to make "200 lb-ft of torque," it means that 200 pounds of force on a 1-foot lever is needed to stop its motion.



To avoid confusion in the U.S. measurement system, the unit of measure for torque is the pound-foot (lb-ft), while for work it is the foot-pound (ft-lb). Remember, work and torque aren't exactly the same. Movement must occur for work to be done, but that doesn't necessarily hold true for torque: Exerting 10 lb-ft of torque on a bolt that's already been tightened to 50 lb-ft won't produce any movement.



If torque does produce movement-as is the case with your engine unless it's locked up-any "distance" traveled as the crank rotates is equal to the circumference of a circle, not a straight line ... so 1 lb-ft of torque produced during one revolution actually is about equal to 6.28 ft-lb of work or mechanical energy. Huh? Just recall your geometry: Find a circle's circumference by multiplying its radius (r) by 2(pi). With a 1-foot lever:


2(pi)r x 1 lb-ft of torque = 2 x 3.1416 x 1-foot lever x 1 lb-ft = 6.2832 ft-lb of work

Power Explained
Torque and work measurements tell us how much has been accomplished, but provides no clue how fast a given amount of work (or torque) is done. That's the job of power, an expression of the rate or speed at which work is performed. The more power that is generated, the more work is done in a given time-period.
Suppose it takes a constant 100 lb-ft of torque to spin a nut onto a bolt one complete revolution. Your girlfriend takes 10 seconds to do this. You, being a real stud (pun intended), take only 5 seconds to perform the same task. You would be twice as powerful, because you performed the same work in half the time.



In the U.S. system, power is expressed as "horsepower" (hp). One hp is the amount of power it takes to perform 33,000 ft-lb of work in 1 minute, as based on 18th century engineer James Watt's observations of the work performed by a strong horse as it operated a gear driven mine pump by pulling a lever connected to the pump (sounds like a dyno, doesn't it?).


Because 1 hp represents the production of 33,000 ft-lb of mechanical energy per minute, horsepower equals ft-lb/minute divided by 33,000, as expressed by Figure 1.Here, D is the distance in feet the weight is to be moved; F, the force in pounds required to move the weight; and t, the time in minutes required to move the weight (F) through distance (D).



Figure 1



An engine dynamometer measures torque (lb-ft), not mechanical energy (ft-lb); and an engine's "time" is expressed in revolutions per minute (rpm). Since 6.28 ft-lb of work is about equal to 1 lb-ft of torque, you can substitute the equations in Figure 2.


Figure 2






What The Math Tells Us
Looking at this horsepower equation, several relationships are apparent. First, knowing that dividing the same number by itself cancels it out (because the product is "1"), it's obvious that at 5,252 rpm, the horsepower and torque values are always equal. That's why an engine's torque and horsepower curves always cross at 5,252.



Figure 3




Second, assuming engine displacement is fixed, raising the engine's operating rpm range is the most effective way to make big naturally aspirated horsepower numbers. Suppose an engine makes 400 lb-ft at 3,000 rpm. The equation tells us that at 3,000 rpm it would produce 228 hp. But if the engine made 400 lb-ft at 6,000 rpm, it would produce 457 hp. It's true that doubling the torque output to 800 lb-ft at 3,000 rpm would also yield 457 hp, but that large an increase on a given displacement engine is unlikely in the real world without forced induction. It is simply unrealistic to expect a normally aspirated engine to produce both big torque and big power numbers under 6,000 rpm-unless the engine is really huge.


The High-RPM Engine
Horsepower is what gets you down the track quickly, and world-class professional race engines are built to achieve maximum horsepower. Unless they are giant mountain motors, inevitably this means they're spinners. The extreme example are the tiny Formula I motors that make big power numbers but turn upward of 18,000 rpm to do it, and use seven-speed close-ratio gearboxes to stay within their narrow operating bands. With a "spinner," the bottom-end must be up to the task (that means premium parts, dead-nuts machining, and meticulous assembly), the compression ratio should be as high as practical for the available fuel, the cam needs to have sufficient lift and duration, the valvetrain must be stable at rpm, and the cylinder heads must be capable of flowing sufficient air at high rpm-or the engine won't live long enough to reach its full theoretical power potential. All that costs money...lots of money.



And the inevitable result is that raising the torque peak to make more top-end horsepower means less torque down low. That's OK in a relatively lightweight car, where more bottom-end may just overpower the available traction anyway. But to get up into the usable powerband, steep (high numerical) rearend gears and (if running an automatic trans) a really high stall-speed torque converter are required.


The Low-RPM Engine
High-rpm may be fine for a single-purpose racing engine, but the results are not always fun in day-to-day driving. Street-driven cars just don't spend most of their time above 5,000. Even if you don't mind buzzing the engine high on a daily basis, it takes a high degree of competence to achieve consistent, maximum performance from a quick-revving, closely geared engine.
Some engines aren't happy at high rpm-and never will be. Their bearing diameters are too large, their strokes too long, and their head-flow capacities too poor to really work upstairs. As Olds expert **** Miller points out, "The big-block Olds is incapable of living at high rpm. The 455 has the longest stroke of all stock big-blocks on the market. Its 3-inch-od main bearings are also the biggest. At 6,000 rpm the Olds main bearing [rotational] speed is nearly the same as a small-block Ford at 9,000 rpm!"


Most street cars are relatively heavy, sometimes carrying as much as 12-14 pounds/hp. To overcome a heavyweight's inertia-the property of a body by which it tends to resist a change in its state of rest or motion-requires lots of initial leverage. And if nothing else, torque is leverage.


All this seems to argue in favor of building for low-rpm performance. Many builders call this "building for torque" or "assembling a torque monster." But in the final analysis, it's really a matter of semantics. Whatever the term, the goal is maximizing engine output in the rpm range where the engine spends most of its time-whether it's because of inherent design limitations, or because of intended use, or a combination of these factors. Even with low-rpm setups, the engine that makes the most torque at equivalent rpm points will make more power-and should be faster.


Area Under The Curve
Notice we said "the rpm range where the engine spends most of its time." Merely considering peak numbers is misleading. In almost every case, it is better to look at the average area under the power curve rather than simply at peak numbers, because a broader, flatter curve generally delivers superior performance to a peaky curve. As SuperFlow's Harold Bettes puts it, "Some engines [that] have a power curve that looks like a tabletop [instead of] a mountain range in profile are pure pleasures to drive." Comp Cams' Scooter Brothers adds, "If it's a Comp Eliminator, Pro Stock, or Winston Cup car, maybe peak power is the answer," because these engines operate in a relatively narrow rpm band. "But for the dual-purpose car, torque must be flat for an extended period of time."


According to David Reher, "We look at the average horsepower within the rpm range we run in; we don't look at the peak number. Anytime you can pick up the average, that's an increase. But you don't want to lose power somewhere else."

"The wider the powerband, the better the acceleration," says turbo wizard Ken Duttweiler. "The best examples are variable-cam engines like the Honda VTEC-they'll pull down to 500 rpm and accelerate to 7,000!" And Norm Brandes at Westech Automotive (of Wisconsin) adds that high-strung, peaky motors "are easier to get out of tune. A carb on a good 'torque' motor sees a much stronger manifold signal, so it's more forgiving. The same holds true with electronic engine management; the computer will be much happier with a broad curve."



Generally the rpm range that is most important is the area between peak torque rpm and peak power rpm. The car should be geared so that you shift 400-500 rpm beyond peak power, and the engine "falls back" to just beyond the peak torque point. Assuming a similar operating range, the engine with the greater area under the power curve between the power and torque rpm peak points makes for the better combination.


On an engine with the most "area under the curve," the torque falls off less rapidly after hitting its peak, so in that sense you are always building for best overall torque, in order to produce the best overall power. This is where Scooter Brothers of Comp Cams gets his axiom, "build for torque, and horsepower will take care of itself," but remember the end goal is always to generate maximum power within your engine's operating rpm range. "There's not one thing that isn't a tradeoff," David Reher points out. "That's the most critical thing in engine-building: deciding where and when to make those tradeoffs." Harold Bettes adds, "It is the package with the greatest area under the power curve that has the advantage. Remember, you cannot have horsepower without torque, but you can have torque without horsepower!" It's called a dump truck.


Source: www.hotrod.com

[wu_dot_com]

thanks for the good read. though it may be too much techinical talk for many :P

[serialk11r]

Thanks for the writeup, but at the core is really the question, given the same shortblock, would you rather have a "torque" setup or a "horsepower" setup? When the engine is naturally aspirated, the "torque engine" is automatically giving up power at the top end to get a bigger percentage of peak power earlier. The only way to expand the range at which power is within some given amount of peak is to reduce the power by moving the torque down, decreasing power at the top end. For an "exciting car", the capability should be at the top end, otherwise there's no point in using a motor that didn't come from a tractor or dump truck, because that's what gives you maximum performance out of 1st gear.

[phenom86]

Originally Posted by wu_dot_com

thanks for the good read. though it may be too much techinical talk for many :P

yeah its very technical but once you read and understand your question will be answered.. My ultimate question is why do we have a dip in our torque band, whats the cause of it and why is it there.

[grodenglaive]

great article:happy0180:

[ahausheer]

Good read. Still confused. I think we look to often at peak numbers which can hold little meaning. Electric cars can have modest torque and power numbers but can rip around a race track or drag strip, seems to suggest the shape of the curve is equal (or perhaps more important) than peaks numbers. Seems like a flat torque curve in the expected RPM range is best?

[phenom86]

Originally Posted by serialk11r

Thanks for the writeup, but at the core is really the question, given the same shortblock, would you rather have a "torque" setup or a "horsepower" setup? When the engine is naturally aspirated, the "torque engine" is automatically giving up power at the top end to get a bigger percentage of peak power earlier. The only way to expand the range at which power is within some given amount of peak is to reduce the power by moving the torque down, decreasing power at the top end. For an "exciting car", the capability should be at the top end, otherwise there's no point in using a motor that didn't come from a tractor or dump truck, because that's what gives you maximum performance out of 1st gear.

your right, ive seen on this forum is HP concern when we are introduced to new products. I dont want to have a too high HP car but a car thats truly balanced.. I'd like to get rid of the torque dip while im not running forced induction, if we can get rid of the dip with proper tune i'd be a happy man.

[DarkSunrise]

A high hp, low torque engine is fine if you have proper gearing and are willing to downshift.

[ahausheer]

Originally Posted by phenom86

yeah its very technical but once you read and understand your question will be answered.. My ultimate question is why do we have a dip in our torque band, whats the cause of it and why is it there.

Or why is there a large bump in torque in the low rpm range. A dip implies its going lower than some expected amount. I think its actually an increase in torque compared to the curve of most NA street motors.

[serialk11r]

Originally Posted by ahausheer

Or why is there a large bump in torque in the low rpm range. A dip implies its going lower than some expected amount. I think its actually an increase in torque compared to the curve of most NA street motors.

That's right, look at the 3S-GE. Nearly the same setup, complete shit low end torque.

To get rid of the torque "dip", simple! Shorten the headers and manifold, you'll lose midrange torque, and it'll look smoother lol.

[acro]

Good read explains a lot. I don't mind shifting and love high rpm's so I know what I'd want

[Frostyman]

Excellent read. Nothing beats a face full of math on a lunch break!

[Rayme]

Originally Posted by phenom86

yeah its very technical but once you read and understand your question will be answered.. My ultimate question is why do we have a dip in our torque band, whats the cause of it and why is it there.

Combustion engine limitation, the valve train, piston design, bore, stroke, etc. makes it almost impossible to have a completely flat torque curve along the operating RPM. There's always going to be a perfect RPM for the power to be made (which is where the torque peaks). I have the impression that the torque dip we have in our engine would be worst if it wasn't for the DI, it really helps making torque at low rpm.

Electric motors on the contrary...

[Captain Insano]

This is all known to me, but I do particularly like this article (thanks for posting) due to some things it points out - how high HP can be misleading. Some people mistakenly think because F1 and indycars have stratospheric HP at 18000 RPMs that HP is king... But that's where those cars operate and that's OK because their entire power curve and car are designed around that concept (light weight and therefore low end torque not as important for accelerating from low speeds). Article concludes with high torque dump truck. For that application, torque is perfect!

Write up reiterates more than once that torque and horsepower are both important, but depending on what you are doing will dictate which is more important for a given application. And ultimately what is most important is area under the curve where your vehicle will spend most of its time....