So to start with I naturally consulted Google. Most of the top hits for “torque vs. horsepower” are excellent pieces; they break down the math in a very methodical way, so I won’t repeat that excellent work here. Instead I’ll just summarize the basics that are accepted as fact by everyone:
James Watt came up with the concept of horsepower — which is a measure of, interestingly enough, power. 1 HP is the equivalent of 33,000 ft/lbfs per minute. The reason for the complex unit is that we’re accounting for three things with this number: the amount of weight involved, the distance it’s being moved, and how long it takes to do it (that last one is important).
Torque is nothing more than a measurement of twisting, or rotational, force. The easiest way to think of this is to imagine a long shaft — like a car’s axle — and imagine it’s in a room suspended in mid air. Hanging on the bottom of one end is a rope with a weight attached — a very heavy weight.
Now imagine someone trying to, using their hands, twist the shaft so as to lift the weight. Think of them as essentially trying to act like a wench and reel it up. The amount of force they are able to generate to lift the weight in this manner is the torque that they’re able to produce. One unit for measurement of this is the foot-pound. A foot-pound is the rotational ‘force’ generated by hanging a one-pound weight at the end of a 1-foot wrench.
THE COMMON MISTAKE
The mistake most people make when engaging in this debate is considering horsepower and torque independently. Almost everyone argues as if they are separate, unrelated values. They aren’t.
Horsepower = (Torque x RPMs) / 5252
In fact, there’s not a single machine in existence that measures a car’s horsepower. It’s a man-made number. When a car’s performance is tested, its torque is measured using a dynamometer. The measure of an engine’s performance is torque. Horsepower is an additional number that’s attained by multiplying the torque by the RPMs.
THE PHYSICS OF ACCELERATION
So now for the most important thing on the page. What determines true acceleration for a vehicle isn’t really debatable — it’s force divided by mass. The formula for acceleration is seen below.
f = ma
a = f/m
The confusion only comes in determining which force we’re actually talking about.
So we are solving for acceleration and we have a constant mass. We’ve already established that torque is the amount of rotational force being generated at the engine, but we aren’t concerned with the force at the engine. What we’re interested in is the force at the wheels. The force at the wheels is the
f = ma (actually, it includes the radius of the wheel as well, but we’re simplifying).
But remember, the transmission ultimately gives the force to the wheels, not the engine. And that’s the trick to this whole mess.
So that’s where gearing comes in.
Gearing magnifies torque. The torque at the wheels is the torque at the engine combined with the torque magnification given by the transmission through gearing. So the transmission only sees what’s coming off the engine, while the wheels see the resulting force combination of the engine plus the transmission.
That’s what horsepower represents. Horsepower is the combination of the benefits of the engine’s raw abilities combined with RPMs. And RPMs are what allow us to use gearing effectively, which gives us more torque at the wheels.
So a technical answer to the question of, “What makes acceleration: torque or horsepower?”, is torque—but torque at the wheels, not at the engine. And since we’re talking about torque at the wheels and not at the engine, the best answer is horsepower, because horsepower encompasses not only the engine’s torque but the total torque that gets delivered to the wheels and therefore provides the
f = ma.