When I bought my first real car I became (for the first time) moderately curious about vehicle performance. I set about to see how it compared to my previous car performance-wise and was immediately struck by a pivotal question:

What is the most important attribute for acceleration—horsepower or torque?

My first approach was to ask those immediately around me and call some friends that were into cars and/or racing. The results were not satisfactory. I got a few decent responses, but nobody could explain the relationship to me in a way that I could understand.

Most people have strong opinions on this topic, but no actual grasp of the science.

All this did was confuse and intrigue me. One thing I did figure out is that none of the people arguing were using hard science as the foundation for their argument; they were invoking science, but were doing so very sloppily. Well, that wasn’t enough for me so I set out to find the real answers.

The fundamentals

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.

Horsepower: 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: 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 winch and reel it up.

Notice this doesn’t say anything about how fast you’re twisting.

The amount of force they are able to generate by twisting is the torque that they’re able to produce.

Stop thinking of horsepower and torque as completely separate

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.

Why HP and Torque cross at 5252 RPMs

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—which they are not.

Horsepower = (Torque x RPMs) / 5252

This equation is the second most important thing on this page, and it’s the reason that anyone telling you that horsepower and torque should be considered equally and separately is significantly mistaken. Horsepower is the product of torque and another value—(RPMs divided by 5252). It’s not unrelated, separate, or different.

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 at all debatable—it’s force divided by mass. The formula for acceleration is seen below.

f = m a

Which means…

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 radius of the wheel matters some as well.

The force at the wheels is the f in f = ma .

But remember, the transmission ultimately gives the force to the wheels, not the engine. And that’s the trick to this whole mess!

Gearing is the translator between the engine and the wheels

So that’s where gearing comes in—it’s the magnifier for acceleration, given what the engine can produce in power.

Gearing magnifies torque, which is why it’s so important in racing.

This is why the fastest racecars work at extremely high RPMs.

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! It’s 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.

And torque at the wheels is the f in f=ma .

Conclusion

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 becauase acceleration is torque at the wheels, the real answer is horsepower , because horsepower encompasses not only the engine’s torque but the total torque that gets delivered to the wheels.

Notes