Torque vs HP - discuss :-)

[Tricky]

Want to start an argument with a gearhead? Say that horsepower is more important than torque. Or that torque is more important than horsepower. The resulting fact-free emotionalism, random shouting, and absence of listening will be rivaled by little this side of "discussions" regarding Roe v. Wade. To paraphrase the great philosopher Jeff Foxworthy: "If you can argue about torque and horsepower for more than an hour, you might be a gearhead."

Often, the most passionate cannot define "torque." Nor can they accurately quote the scary math formulas for determining horsepower much less describe its origins. Here, we aspire to remove you from the vast unwashed.



Definitions |
TORQUE is a twisting force that does not require motion. In the case of vehicle engines, it is the force of combustion pressing through pistons, connecting rods, and crankshaft to turn the flywheel.

HORSEPOWER is a measure of work and power created as a marketing ploy by inventor James Watt around 1775 to promote his newfangled steam engine. Textbooks define WORK as force multiplied by distance, and POWER as work divided by time. To cut the physics lesson short, here's the math formula: engine revolutions per minute (rpm) multiplied by torque at that engine speed, divided by the constant of 5252. Or RPM x TORQUE / 5252 = HP

For vehicle engines, torque is MEASURED by a dynamometer. Horsepower is CALCULATED (often by the dynamometer's software). You can tell if your ciphering (or the dyno's software) is correct if torque equals horsepower at 5252 rpm.



Peak Performance |
Carmakers report PEAK torque (in pound-feet) and horsepower along with the engine speed at which those peaks occur. The torque peak indicates where the engine is pulling its strongest. The horsepower peak indicates where torque is dropping like a rock: Increasing rpm can no longer offset falling torque in the math formula. A dyno chart shows the shape of the torque curve: Ideal is a torque curve that quickly rises to near its peak and is flat as eastern Colorado after that.

For drivers, torque is FELT as punch-in-the-back acceleration, while horsepower is OBSERVED by such things as quarter-mile times or headlights disappearing in the rearview mirror.


It All Depends |
To those who say "torque rules," ask if they'd like to drive a vehicle with more than 900 pound-feet of torque, and then point them toward a 250-horse farm tractor. Tell not to be frightened by the tractor's 26-mph top speed and 2,200-rpm redline.

To those who say "horsepower is king," ask if they'd like a vehicle powered by a 900-horse Formula 1 engine boasting a rev limit exceeding 19,000 rpm. Also ask if they're good at changing clutches. That's because that 3-liter F1 mill, which peaks out at about 500 pound-feet of torque around 14,000 rpm, will be required to pull an 18-wheeler. The combination's usual 12.7-liter turbocharged diesel makes some 1,200 pound-feet of torque just above its 700-rpm idle speed, allowing the driver of a fully loaded tractor-trailer to pull forward from a stop in second gear by simply releasing the clutch: No throttle is required to move 80,000 pounds. But its rev limiter kicks in at 2,400 rpm.


Generalizations |
Generally, larger displacement engines make more low-speed torque but have lower rev limits. This is largely because as engine speed increases the energy required to move the crankshaft and other engine parts jumps at the square of the rpm. Smaller engines have fewer and lighter parts so often can turn more rpm, which translates into higher horsepower. However, small, fast-spinning engines don't usually produce low-end torque. (These paradigms can be bent with expensive lightweight parts in the larger engine or supercharging the smaller one.)

Assuming otherwise identical cars and closely matched engines, the one with a bigger, torquey engine will beat the one with a smaller, but higher-revving engine off the line. If the contest is a stoplight-to-stoplight drag that won't exceed 30 mph, the big, torquey engine wins every time. However, if the race is longer the bigger engine will soon reach its redline and the driver will have to shift. The act of shifting—even with an automatic—will cost some time, but far more important is the loss of the torque multiplication of the lower gear. Meanwhile, the car with the higher-revving engine will catch up and, likely, pull ahead. By the time the bigger-engined car needs third gear, the race is over.



Everything's Relative |

Watt's formula shows that torque and horsepower are not opposite sides of an argument. Instead they are inextricably interrelated. The comparison between the tractor engine and the F1 powerplant show a plethora of low-speed torque is important in some situations, while high-rpm horsepower is critical in others.

Here's the Cliff Notes version of this article: Low-end torque rules the street (at least at lower speeds), while high-rpm horsepower is king on the track.

Let the arguments begin.

I agree completely

[Tricky]

Torque vs. Horsepower


If you've been around motorized vehicles for any length of time, you have probably been exposed to the great torque vs. power debate at some point. If not, it goes like this:
"Torque is what makes a bike accelerate, not power."
"Wrong."
Torque and power are inescapably linked by the fact that horsepower equals torque (in ft-pounds) times RPM divided by 5250, so people who talk as if they are independent are full of it. If you have a given torque curve for an engine, you have the horsepower curve also. Knowing how these two numbers work with each other lets you can poke through some of the BS you might read.
First, as usual, a few definitions.
Torque is a twisting force applied to an object, like a wheel or a crankshaft. Note that motion is not required for torque to exist! If you stand on a lug wrench that is on a frozen lug bolt, you are applying a torque to that bolt even though there may be no movement. For our purposes, we will consider that torque is measured in pounds-force feet (lbf-ft) meaning the equivalent of a given force, in pounds, acting on the end of a lever of length in feet. For example, standing with 180 pounds body weight on a lug wrench one foot long yields 180 lbf-ft of torque. A child of 90 pounds standing on a two-foot lug wrench applies the same torque.
Work is the application of force over a distance. Unfortunately, the units used are the same (pounds times feet) but we write this as ft-lb just to distinguish it. The real difference is that in this case, the "feet" part means feet of movement. If you push on a car with 100 pounds of force and maintain that for 30 feet, you have done 3000 ft-lb of work. An easier example is lifting a weight (in pounds) a given distance (in feet). If you use some sort of mechanical advantage, like a winch, you will do the same amount of work because by halving the effort required, you will have to double the distance through which you apply the force to achive the same objective.
Power is the application of work within a finite time. 550 ft-lb of work in one second is one horsepower.
So, let's first go through the numbers to get from torque to horsepower. Pushing with 87.5 pounds (force) on the end of our 1-foot lug wrench applies a torque of 87.5 lbf-ft. No motion yet, so no work and no power. But now let's say the lug bolt loosens slightly and starts to turn, but that same 87.5 pounds of force is needed to keep the wrench turning. For every revolution of the wrench, you are applying 87.5 pounds of force over a distance of (2 * pi * 1 foot) or 6.28 feet, the circumference of the circle that your hand is making, for a total of 550 ft-lb of work. It's only when this system is actually moving that work is being performed. From here, it's a quick step to say that if you work fast enough to turn that wrench once per second, then you are doing 550 ft-lb of work per second, which means you are applying one horsepower.
By the definitions we can see that HP is directly proportional to torque and RPM. "Directly proportional" means there may be a multiplyer involved, so let's find it using our example numbers, remembering that 1 revolution per second is 60 RPM:
torque * RPM * constant = hp
87.5 lbf-ft * 60 rev/min * X = 1 hp
X = 1 / (60 * 87.5) = 1/5250
torque * RPM * 1/5250 = hp
hp = (torque * RPM) / 5250
For internal combustion engines, torque is always given at a certain RPM because they can't generate any torque when they aren't moving. Once they are running fast enough to sustain their own operation, the force that they are exerting against a load can be measured, and the speed at which they are turning can be measured, so the torque (and therefore power) numbers become known.
So, if there is such a fixed relationship between torque and power, why do some people say that a certain engine has lots of power, but no torque? Remember that the connection between torque and power is rotational speed. A sportbike motor might generate 150hp at 14,000 RPM but the torque at that RPM is very small; about 53 ft-lbs. In comparison, a large-displacement twin might peak at 100 hp at 7000 RPM. The torque applied at the twin's 7000 rpm, 75 ft-lbs, is greater than the torque applied at the sport bike's 14,000 rpm but the sport bike makes up for it with a lot more engine speed and ends up with more horsepower.
The street, though, complicates things because the sport bike will probably not be ridden at 14,000 RPM. At 5000 RPM, the twin would likely have more power. This is an artificial handicap; the sport bike wasn't meant to be ridden at that speed since it generates its power by sending the RPM part of the equation sky-high. For street riding, the twin is easier to ride, less prone to stalling as you pull away from a light, and you get that satisfying "oomph" when you twist the throttle. But as the RPM increases, the twin runs out of breath and the race bike, although the torque is low and probably getting lower, continues to make more and more power until it hits its peak at 14000.
[Insert dyno charts for comparison showing less torque but more power for sportbikes at high RPM]
Engines are designed for their intended use. Our twins are designed to yield fairly high torque values at low RPM, because this makes them easy to ride in day-to-day life, and Harley-Davidsons have their torque concentrated even lower in the RPM range than BMWs do. Low-end torque is accomplished by several design traits, one being small valves and intake tubes which create high air velocity into the cylinder for good fuel mix at low speed.
Those effects tend to become a restriction at high RPM, which means that engines intended for high RPM end up with larger valves, larger air intakes, smaller cylinders and other things that let them continue to breathe when other engines start to gasp. Race bike engines have fairly small displacement, which limits the torque that can be produced at the crank. They apply that torque at much higher speeds to get high horsepower (and who can argue that those bikes don't accelerate quickly?).
To a lesser extent, BMW varies these techniques for different bikes. The GS series has narrower intake tubes to give a faster intake charge, giving better fuel/air mixing and better torque at low RPM. Since this becomes a bottleneck at higher RPM, the "power" engine in the RS and RT bikes have larger intake tubes. Swapping the GS tubes into an RS or RT is a common retrofit, as it makes the bike torquier at low RPM where most of us ride. Newer technology in cars, like variable valve timing and variable intake tract length, can give motors the best of both worlds by increasing torque at higher RPM without giving it up at low RPM. Incidentally, Honda has variable valve timing on a motorcycle now.
But to get back to the main point, it is power that moves our bikes down the road. Yes, torque provides the pushing force through the drivetrain, but it needs to happen at some given speed, and those two factors define "power."
Why does torque drop after a certain RPM?
Torque starts to decrease because the engine cannot breathe as well. Due to the speed, the cylinder does not fill with air as well. A designer can get around this problem with "tuned intake" which sets up a resonance to pack the cylinder with air, but it only happens at a certain RPM. The next evolution of design is to make a variable system which packs the cylinders with air at all RPM; this is usually called "variable tuned intake runners" or something like that and involves valves which open and close to create a different size for the airbox and manifold.
Why does power continue to increase after torque decreases? Remember that the power is essentially the product of the RPM and the torque. At first, decrease in torque is small and is not enough to offset the increasing RPM, so the overall product still increases. Eventually the decrease in torque becomes large enough that it outweighs the increase in RPM and we see the power start to drop. Because of this, the power peak will always be after the torque peak.

2*PI*N*T/60000

[Coxy]

More porn!

Originally Posted by Tricky

Everything's Relative |

Watt's formula shows that torque and horsepower are not opposite sides of an argument. Instead they are inextricably interrelated. The comparison between the tractor engine and the F1 powerplant show a plethora of low-speed torque is important in some situations, while high-rpm horsepower is critical in others.

Well said

Engine design is about balance a compramise

[Timo 67]

Quality explanation . I'm not gonna argue with any of that, sounds like you know exactly what your talking about.

I still agree completely

[Coxy]

Yeah, but if the driver is a woman then all this is flawed!

[Tricky]

Originally Posted by Coxy

Yeah, but if the driver is a woman then all this is flawed!