Car and Driver: Horsepower vs. Torque: What’s the Difference?

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Which is better? Here’s how you quash that bar night debate.

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Yogi Berra, never known to dwell on engine particulars, would have concluded that torque and horsepower are the same thing, only different. Actually, that simplification is partially correct.

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Torque and power are what engines produce when you turn the key and press the accelerator. Air and fuel ignited in the combustion chambers cause the crankshaft, transmission, and drive axles to do the twist. This is the miracle of energy conversion: the potential energy contained in a gallon of recycled dinosaur efficiently changed to the kinetic energy needed for driving.

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Digging deeper, consider these textbook definitions:

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Energy is the capacity for doing work. In this instance, engines perform the drudgery (work) formerly done by horses.

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Work is the result of a force acting over some distance. The US measurement unit for work (and also energy) is foot-pounds. In the International System (SI) work is measured in joules and, in rare instances, newton-meters.

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Torque is a rotating force produced by an engine’s crankshaft. The more torque an engine produces, the greater its ability to perform work. The measurement is the same as work, but slightly different. Since torque is a vector (acting in a certain direction), it’s quantified by the units pound-feet and newton-meters.

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Of course, there’s always an exception. In this case the distinction is static torque, the kind you apply with a wrench to tighten head bolts. To avoid confusion, the units for static torque are traditionally foot-pounds. Just to be contrary, SI sticks with newton-meters for both static and dynamic torque measurements.

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Power is how rapidly work is accomplished. Eighteenth century Scottish inventor James Watt gave us this handy equivalency: one horsepower is the power required to lift 33,000 pounds exactly one foot in one minute. Honoring that contribution, the SI measurement unit for power is the kilowatt.

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Back to Berra’s theorem, torque is the capacity to do work, while power is how quickly some strenuous task can be accomplished. In other words, power is the rate of completing work (or applying torque) in a given amount of time. Mathematically, horsepower equals torque multiplied by rpm. H = T x rpm/5252, where H is horsepower, T is pound-feet, rpm is how fast the engine is spinning, and 5252 is a constant that makes the units jive. So, to make more power an engine needs to generate more torque, operate at higher rpm, or both.

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While thumbnail definitions are great for textbooks, applying them to real engines is another matter. One concern is that every car engine has an idle-to-redline operating range. For example, the Dodge Challenger’s 6.2-liter Hellcat V-8 produces 707 horsepower ONLY at 6000 rpm. It makes substantially less power at idle (only enough to spin engine-driven accessories) and a bit less than 700 horsepower at the 6200 rpm redline. And it delivers its maximum 650 pound-feet of torque ONLY at 4000 rpm.

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Another issue is accurately quantifying the power and torque from a spinning crankshaft. The tool for this task is an engine dynamometer. While that word means ‘power measurement device,’ in practice, the engine’s torque and rpm are measured and its power is calculated using the formula cited above.

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Eddy current dynamometers use a magnetic field to transfer torque from the spinning crankshaft to a lever arm bearing against a static force gauge (known as a load cell) spaced a precise distance from the center of the crank. The other type of dynamometer in common use is a water brake; it uses one spinning and one static set of pump vanes to convey the crankshaft’s torque through a lever arm to the load cell.

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The perfect engine produces ample torque at low rpm and sustains that output to the redline. The amount of torque produced is directly proportional to the air flowing through the engine. Large engines pump more air and, therefore, produce more torque. Boosters—superchargers, turbochargers—deliver additional air to help small engines act large. Of course, appropriate amounts of fuel must be supplied to the combustion chambers but that’s the easy part, especially with electronically controlled injection.

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Making up for the ease of injecting the right amount of fuel, engine designers face several difficult tasks. One is making all the components tough enough to handle the loads they’re subjected to by combustion pressure and, in the case of moving parts, their own inertia. Cooling and lubrication needs are roughly proportional to the power produced. And pumping air into, through, and out of any engine at ultra high rpm is where engineering becomes an art form. Factor fuel efficiency and exhaust cleanliness into the development equation and it’s clear why engine wizards rarely hang out at the water cooler.

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At this point of the discussion, it should be clear that torque and horsepower are like estranged siblings; they’re closely related but don’t have much in common. But what about the greater moral issue confronting mankind in general and car enthusiasts in particular: Which is better?

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We’ll answer that in terms Yogi Berra would appreciate. If torque is the shortstop, horsepower is the pitcher. The game may be diminished without a shortstop but the speed and path of the ball—both solely determined by the pitcher–matter most.

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