Don't Be Afraid to Ask Questions Part 6: Horsepower and Torque, I Do Not Think it Means What You Think it Means

biggest engine in the world
This'll do nicely, thank you
Imagine a pile of art critics debating the merits of a given painting without knowing the difference between blue and red. Imagine judges at a dog show not knowing their labradors from their spaniels. Silly, right? Yet you’ll find an infinitude of vitriol spilled over why only idiots would prefer low-end torque or high-end horsepower or vice-versa. The participants are typically those who’d fail their way out of Mad_Science_Sr’s Conceptual Physics class (that’s the easy one). So before can we start talking about VTEC (yo), for this week’s DBATAQ we’re rewinding to high school physics and the actual definitions of power and torque. Please focus on the task at hand, and not the girl who had a bit of a “growth spurt” between sophomore and junior year.
Push, you sissy!
We’ll start with torque. Torque is a measure of how much something wants to rotate or twist. Kind of like force, but going in a circle. Here in the US (sorry Mitch) we measure torque in pound-feet (lb-ft), which leads to an easy explanation: imagine a wrench that’s 1 foot long with 1 pound of weight hanging off the end of it. If you’re the bolt at the business end of the wrench, you’ve got 1lb-ft of torque applied to you. Want more torque? apply more than 1 pound of force out on the end of your wrench, or get a longer wrench. There’s more than one way to end up at the same amount of torque: both [2 lbs] X [1 ft] and [1 lb] X [2 ft] equal 2 lb-ft. They’re the same. We never talked about things actually moving as a result of that torque. Torque is independent of how fast something is twisting. When we combine torque and movement, we’re talking about power. Ignoring specific units for a second, power = torque X rotational velocity. Let’s go back to our 1-foot wrench. Say you’re putting 1lb-ft of torque on it, and spinning it at at 53 rpm. You’d be making 1/100th of a horsepower (hp). Not too impressive. So how do we get more power? Kinda like before, you’ve got two options: more torque or more rotational velocity (RPM). Both 2lb-ft at 53rpm and 1lb-ft at 106rpm equate to .02 hp. A key point to take away is that power is the result of torque at a given rpm. Actually getting into cars for bit, an engine that makes 100lb-ft of torque at 2500prm is by definition making 48hp. The formula t o convert torque to horsepower is HP = torque X rpm / 5252.
Power Vs Torque: How less torque at higher revs makes more power (click to embiggen)
Let’s say we rev the same engine up to 4000 rpm and it’s making 200lb-ft of torque, now we’re at 152hp. Taking it all the way up to redline at 6500rpm, torque output drops off (due to a number of complicated aspects of engine interals), so now we’re only at 170lb-ft. Oddly enough, even though we’re making less torque, we’re making more power: 210 hp. It’s twisting less hard, but somehow more powerful. Whaaa? The easiest way to wrap one’s head around the concept of less torque but more power is to use gearing. As a quick refresher, one can use gearing (or pulleys or chain-drives) to reduce rpm, but increase torque. For example if I run 20 lb-ft or torque at 100 rpm through a 2:1 gearbox, I get 40lb-ft at 50 rpm out the other side. For the moment, we’ll assume we’re working with idealized geartrains, meaning you don’t lose any power to friction or the like. Let’s say I pit our 170lb-ft at 6500 rpm motor up against a 2nd motor making 150 lb-ft at 7500 rpm, but motor #2 is running through a 1.15:1 gearbox. The output of that box will be running at 6500 rpm, but will be making 173 lb-ft of torque. Guess what? Motor #2 + a gearbox is now making more torque than motor #1.
The Mazda rotary engine: a high-revving, high power, torqueless wonder
In fact, one way to get more power out of most engines is to simply raise the redline. Chances are you’re gonna make more power at 7500 than 6500 rpm, provided your engine won’t explode. So why even worry about the much-lauded “low-end torque”? After all, you can just run higher revs and lower gearing, right? Maybe, if all you’re looking to do is blast around at full-throttle near redline. In a race car or dragster, this works great: just run ridiculously low gears and build an engine that’ll rev up to 10,000 rpm or more. Alas, all that power available at the top end does you no good if you pull away from a stoplight starting at idle. Even if you drop the clutch around 3000 rpm, you’ve got a long way to go before you’re up in the power band at the top-end. You can get to your powerband sooner with lower gears, but now you’re topping out 1st gear at 20mph and spending all your time shifting.
A notorious torque monster: the Cummins 6bt
Low-end torque makes a huge difference in a car feeling faster in every day driving. After all, if the motor pulls without needing to drop three gears when you stomp on it while cruising at 2500 rpm, it feels strong. If it launches away from the line right off idle, that’s low-end torque working for you. Obviously, this is a big plus if you’re towing/hauling. On the flipside, all the low-end torque in the world won’t help you once you’re actually up and running over 3000 rpm. If you’ve ever driven a diesel truck, you’ll notice that “hey this thing’s pretty quick” feeling fades rapidly as the revs climb. You’ll get the same effect from malaise-era engines like the Chevy 400c.i. smallblock. Lots of pull off the line, then nothing but more noise as the revs climb. So, to conclude: you now know what “horsepower” and “torque” actually mean. In ideal conditions (like in racing), you’d run a motor set up for maximum power and use gearing to get it down to whatever speed you need. Unfortunately in the real world, we need our engines to operate over a range of engine speeds, including down-low. Congratulations on reading the whole thing. We’re planning to use this primer as the basis for a range of DBATAQs about what makes a good engine for a good application and why it makes more sense to make a hybrid dump truck than a hybrid compact car, so stay tuned… Further Reading: Wikipedia entries for Torque and Horsepower Image Sources: 6btconversion.com; Wikimedia Commons

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