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A tale of torque and horsepower

To describe the level of output in any given engine, we need to use two different measurement units: Horsepower, and torque.

To describe the level of output in any given engine, we need to use two different measurement units: Horsepower, and torque. This duality has, in my opinion, created a monster.

Torque is grunt — what you use to twist the lid open on a bottle of pickles. The actual dictionary definition is “twisting or rotating force.” In automotive terms, it’s the force that ultimately turns the vehicle’s drive wheels. The strength of that force is expressed in a torque rating.


As an example, let’s look at the most popular car in Canada, the Honda Civic; its 1.8-litre engine produces 128 lb-ft of torque at 4,300 rpm. This denotes the maximum amount of torque the engine can create and how fast that engine is revving when it makes its maximum torque.


But it also produces 140 horsepower (hp) at 6,300 rpm. This is where the fun starts.


Horsepower is a rate — how much torque can be delivered over a period of time. For some reason, the amount of torque used for horsepower calculation has been set at 550 lb-ft per second.


So an engine’s horsepower rating is tied to its torque rating.


But that doesn’t necessarily mean you want an engine designed solely for maximum torque. Generally, to make an engine with a lot of torque (or force) means the engine will turn over (or rev) slowly.


Think of the big, diesel engines in large ships — a lot of force is needed to turn their propellers. Engines like that boast huge torque numbers alongside surprisingly low horsepower figures.


An engine that gives us big — but infrequent — power events is basically useless in a road-going vehicle. Better to have somewhat less torque, but have it delivered more frequently.
Unless you’re obsessed with top speed (horsepower-focused), or need to plough a field with a tractor (torque-focused), it then becomes an exercise in finding a compromise.

 
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