What is the difference between horsepower vs. torque? Does horsepower or torque matter?

Torque is what you feel. It's what gets you moving. Horsepower is what keeps you moving. More detail? Okay!

What is torque?
So, first thing first: torque is a measure of force. It's measured in pound-feet (lb-ft). So, one pound acting on a lever one foot away from an axle will be one lb-ft of torque. Okay. Cool. Imagine a really rusted on bolt. If I took a wrench a foot long, fit it on the bolt, and stood on the end of it, I'd be applying 170 lb-feet of torque to that bolt. I'd also have amazing balance.

What is horsepower?
Note: this is going to all be about axles and rotation and stuff, so while technically, horsepower doesn't measure force around an axis, you will almost certainly never hear it used anywhere except talking about motor vehicles, so that's the context we'll use.

Horsepower is basically a measure of torque over time. James Watt (the guy who the unit watt like for lightbulbs is named for) came up with horsepower while trying to describe how much work a steam engine could do. 1 horsepower is 33,000 lb-ft per minute. So, you can see lb-ft in there. 33,000 seems kind of arbitrary (and it is!). If we just ignore that number for a sec, you can see that horsepower is a measure of lb-ft / minute. Which means that horsepower is a measure of how much torque is applied over time.

Officially, to find the horsepower at any given moment, you take:

Torque (RPM) 2pi

Torque we already get. RPM is revolutions per minute (so, how many times the thing spins). 2pi is the unit circumference of a circle. 2(pi)r describes the circumference of any circle, and since the radius, r, of a unit circle is 1, the r isn't needed. Anything multiplied by 1 is itself, after all.

So, what is the the difference between horsepower and torque?
Torque is a measure of strength/force, and horsepower is a measure of strength over time.

Okay, so for a little bit longer, let's ignore some other things! (Ignoring things makes life really easy. Not recommended, but ask your favorite procrastinator, they'll tell you.) For now, we're going to ignore gears and ratios and all that stuff. Which, for our sake, means that one revolution from the engine equals one revolution of the axle.

On Subarus, most wheels are going to have a radius from the center of the axis to the road of about 13 1/4 inches, which is roughly 1.1 feet. Let's pretend that 1 lb-ft of torque is enough to make the wheel go all the way around exactly once over 1 minute. That means that 1 lb-ft of torque/minute moves you about 3/4's of an inch shy of seven feet. At this rate, you'd need to have 12.7-ish lb-ft of torque to get 12.7-ish revolutions per minute so you could go 1 mile per hour. And if you crank up the torque, you can crank up your mph. You'd need a little over 764 lb-ft of torque to go 60 mph.

The 2015 Outback 3.6R hits 174 lb-ft of torque, and it can tow 2,700 pounds. Soooo, how does that work?

Here's where it gets a little complicated.

Your car has a transmission. The transmission is really where torque and horsepower start to really do anything for you. There are other parts of your car that will have similar functions to the transmission (for different reasons), so the transmission isn't the be-all end-all of turning engine power into your car's movement, but particularly because of towing, it makes the most sense to talk about the transmission.

An easy(er) way to understand transmission:
On a regular 21-speed bike, you have a front cassette where the pedals are. On that cassette are three sprockets: small, medium and big. On your back tire, you have a rear cassette with seven sprockets that go from big to small.

If the sprockets are the same size, for every one time your pedals go around, your wheels go around once. If the front sprocket is bigger than the back sprocket, then you go proportionally farther. So, if your front sprocket is twice as big as your back sprocket, for every time your pedals go around, your wheels go around twice. And if your front sprocket is smaller than the back sprocket, you go as proportionally as far. For instance, if your front sprocket is half the size of your back sprocket, for every revolution of the pedals, the wheels will go halfway around.

If you're biking on flat ground, you can have your bike in whatever gear you feel comfortable pedaling in. If you're biking downhill, you want a high gear to pedal, because a high gear will mean more RPMs (and you're already getting a bunch from gravity). A low gear downhill won't actually get you moving any faster. If you're going uphill, you want a low gear. A low gear helps at low speed, with loads, and up hills. Applying the same amount of effort on the pedals on a low gear makes it easier to go uphill, because you're not moving as far, but you're moving. A high gear going uphill would take way more force just to move.

What you're changing when you change gears is how long it takes to do how much work. A low gear takes less effort (less torque), but you it takes more RPM to go the same distance. A high gear takes more effort (higher torque), but you need fewer RPM to go as far. When you're on an incline or when you're towing, you're have to fight gravity as well as move your vehicle, so it already takes more torque to move. Dropping into a lower gear makes it easier to move.

Forumula 1 racecars have very high horsepower for their torque. Because they're so light, they don't need much torque to get moving. Their transmissions mean that for the amount of torque the engine is producing, they're cranking very high RPM (up to 18,000!) which gets them very high horsepower even with low torque. A bulldozer has very high torque (nearly 4,000 lb-ft), but not a lot of horsepower. It needs to be able to move a lot of heavy stuff, but it doesn't need the RPM to do its job that would get it high horsepower. It only needs about 1800 RPM to do its thing.

One (more) nice thing about Subaru's symmetrical all-wheel drive is that this power is given to all four wheels and distributed to the get the best possible traction. Front-wheel drive cars can slip because they are essentially dragging the weight of the car. Rear-wheel drive cars are typically set up to have lots of horsepower, but they too can slip if the RPMs get too high for the speed you're going (think every burnout/peel-out you've ever seen in a race movie). All-wheel drive helps make all that muscle actually do the work you want it to do. And that's how the Outback can tow so much.

Torque is the amount of force that can be applied.

Horsepower is a measurement of that torque over time given RPM.

The transmission, drive train, and differentials make that horsepower and torque do something.

So what does that actually mean for you and your car?

If you think about a highway on-ramp, torque is what gets you up to highway speed. Horsepower is what keeps you at highway speed while you're there.

Subaru makes vehicles with the torque and horsepower as near together as possible so that you have smooth, even, accessible power always. That's why the Outback has 174 lb-ft of torque at 4000 RPM and 175 horsepower at 5,800 RPM.

Categories: Technical Schmecnical