What’s the Difference Between Horsepower and Torque?

Rather than offer a technical explanation few people will understand, let’s talk about what torque and horsepower feel like.

You’re idling at a stoplight. Or maybe an IHRA starting line. Someone rolls up to you and revs their engine. Gives you the nod. It’s on.

You wind up the engine, the light turns green and you dump the clutch. Instantly the visceral force of the engine kicks you in the chest and slams you back in your seat. The tires smoke and scream. Your chest shakes as if home to a hive of angry hornets.

That’s torque. It’s the sheer grunt and force that transforms your vehicle from placid show horse to ferocious stallion.

This 15-second video illustrates.

play-sharp-fill

As the rpm climb, horsepower takes over. Streetlights zip by, the expansion joints in the asphalt go from, “clack…clack…clack” to “clackclackclack” and the engine effortlessly pulls through the gears toward redline and a seemingly endless horizon. You could drive forever.

That’s horsepower. It’s the speed that carries you to victory. Or to the next stoplight once you return to reality.

Your dyno map to success

You can see the relationship between torque and horsepower in just about any dyno map. Here’s one I grabbed from the boys at Engine Masters – Presented by AMSOIL, a great YouTube show for gearheads.

Notice how torque is higher at low rpm, and horsepower is higher at high rpm. The two meet at 5,252 rpm. That’s because of the following equation:

Horsepower = Torque x Engine rpm/5252

Because torque and rpm are divided by 5,252, torque and horsepower are equal when the engine speed is equivalent to 5,252 rpm.

What, exactly, is torque?

Simply put, it’s a twisting or turning force applied to an object such as a wheel or crankshaft. In automotive applications, torque measures the engine’s ability to perform work. The force created by displacement of engine cylinders spins the engine crankshaft, and the transmission applies this torque to the wheels, moving the vehicle. The more force applied to the crankshaft, the more torque developed and the more work the vehicle can do.

And horsepower?

While torque measures turning force and the engine’s ability to perform work, horsepower measures how fast the engine can perform the work. Engine horsepower ratings indicate how much power an engine can produce similar to how light bulb wattage indicates how much power the bulb will use.

The amount of horsepower an engine can deliver is directly proportional to the level of torque generated by the crankshaft, which is directly proportional to the total displacement capacity of the engine.

(Did you know the term “horsepower” owes its origins in part to beer? Get the story here.)

As they say, there’s no replacement for displacement.

Because there is a limitation on the maximum displacement an engine can generate based on the size of the vehicle into which you stuff the engine, there is also a limitation on the amount of torque the engine can produce, which in turn sets a limit on the engine’s maximum horsepower.

Which is better?

That’s a question no one can answer except you. While both are necessary to drive your car or truck, the answer depends on what you’re trying to do. For most casual motorists, neither is better than the other. They just want their family sedan or SUV to navigate the grocery store parking lot and highway with ease.

But if you’re hauling a load of supplies or trying to win the trailer-pull competition at Diesel Power Challenge, you want the added grunt of a high-torque engine. If you’re trying to set a personal best lap time at the local track, you want an engine designed to maximize horsepower.

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