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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 the year 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.
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 them 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.
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My Thinking 
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