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.
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.
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.
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.
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
Don't Be Afraid to Ask Questions Part 6: Horsepower and Torque, I Do Not Think it Means What You Think it Means
Fantastic write-up. Can't wait to read the rest of the series.Loading…
More furtherer reading:
Very good article. Remember it like this: You and I can easily generate 1000 lb-ft. You and I can not push a car at 100 mph (or, frankly, 10 mph)Loading…
I can't wait to hear why my beloved straight six is perfectly balanced…Loading…
Maybe because you can put a mirror in the middle of your crancshaft and you see that both side are equal.Loading…
That would make sense, but is there more?Loading…
It is the main reason, because this makes all movements syncronized between frontt 3 cilinders and the back 3 cylinders. This makes the engine very easy to balance.Loading…
so is it related to a base 3? A six is easier to balance than an eight?Loading…
Truly. My old Suzuki GT550 two stroke triple was as smooth as a vacuum cleaner, and the number of power pulses per RPM is the same as in your Jeep six and my Ford six. Think of a peace sign (or Mercedes hood ornament), spinning around, that's the power stroke pattern of my old Suzuki. Same as the pattern in a straight six, but twice as many pushrods on the crank. I can't really articulate what I'm getting at, but that means perfect primary balance. As much mass going up as going down, with I dunno something balancing it at the same time. A 2 stroke twin or a four stroke four cylinder has only things going up and down at the same time, providing good balance, but the harmonics of a 2 stroke triple or a four stroke six are almost perfect.Loading…
sweet. Thanks guys!Loading…
Yes, there is more.
If I’m not mistaken:
Primary balance is the obvious sort– with one cylinder at TDC is there another at BDC, keeping their shared center of gravity in the same place? You can evaluate primary balance looking only at crankpins on the crankshaft. A parallel twin has this sort of balance, along with tons of other configurations. An imbalance here vibrates at the same frequency as the engine RPM.
Of course, you can have multiple, out of phase, imbalances.
Secondary balance is not as obvious. It has to do with the fact that an up moving piston has a different speed than a down moving piston. This means that, even though two pistons might balance each other at their extremes (TDC, BDC) and also at mid-stroke (where they’re both at the same height), they will be a bit off of each other at different parts of the stroke– when one is 1mm above mid-stroke, the other might be 1.1mm below. This sort of imbalance vibrates with a frequency 2x the engine RPM.
A flat twin has both primary and secondary balance. By extension, any matched horizontally opposed engine (barring weirdness like single pin cranks, but potentially even then) also does. I don’t recall how well an I6 does here off the top of my head.
In all this, I’m assuming no balance shafts. They come later, anyway.
Tertiary balance? I beleive I’ve read about tertiary imbalances, but I don’t recall if it was clear that there’s a consensus on what it refers to.Loading…
You should print this out and mail it to Honda. The VTEC Accord I had this summer was a torqueless wonder. It was downright scary when I had to pull out onto the highway in a hurry. Once the revs got above 4000 RPM or so it was fun to drive. I started manually shifting it to keep it in the power band as much as possible, which hurt fuel economy.
That's just an excuse to pretend you're drag-racing up the onramp, is all. Though it can be disappointing to bounce of the rev limiter through third gear and still be just barely keeping pace with traffic once you hit the top of the ramp.Loading…
I don't understand? do you have some kind of foot impairment that prevents you from getting it all the way to the floor? 😉
RPM is just a number. I honestly think small engined cars should have their tach's recalibrated so that at redline, the needle is pointing to 11:00, and the number reads 3. Obviously, full scale on the tach would then be at 4-5 o'clock, and the number would be 12. It would help north americans learn to put their foot down.Loading…
I think the trailer in that lead pic is at least as awesome as the engine it's carrying. I may be sick.
Suddenly I have axle envy.Loading…
So where is the Isetta that the engine on the trailer is going in?
My god, the snail on that thing up there at the top is the size of a small house. How much do you suppose it flows?
God, I love this. Keep 'em coming. Maybe I should've been a mechanic or an engineer. Oh well, gearhead marketing/tech writer will have to suffice.
I was taught, "Torque makes a stopped thing spin, power makes a spinning thing spin faster, or slower". Grossly oversimplified, but you don't want to fill a truck driver's mind with too many facts, he might forget where he's going. Thanks for explaining this in detail, yet in language anyone can understand, but…. um…. Can you tell me where we are?
That's how I choose to believe it is too, cuz I can actually remember it.
Great article, Mad_One, I will need to reread it to get it all through my thick skull, and twice more after that to make it stick.
What day is it?Loading…
Great write up, and being a die-hard rotary driver, I get more than my share of comments about torque.
In the HP/torque argument, I prefer to look at power. Torque requires too many mistake-inducing calculations.
Power makes it easy. Consider an imaginary diesel truck, driving 20mph, turning 2000RPM, and making 300 instant HP as soon as the injectors are opened up. Consider an otherwise identical gas truck alongside at 20mph, turning 5000RPM, and making 300 HP as soon as the throttle is opened. The drivers mash the load pedal at the same time, and they accelerate at the same rate.
For a given speed, equal power at the crank means equal torque at the wheels. In this example, the diesel makes 5/2 the torque of the gas motor, but the gas motor has 5/2 the reduction before it reaches the wheels.
Wait- now I'm confused. I didn't understand the part about the Honda with torque. I was under the impression that this wasn't fantasy.
So where's the trailer with the transmission for that thing?
It hooks to a generator. Thats what powers oil tankers and cargo ships. They are all diesel/electric
Yep, I was just fishing to see if anyone could postulate some amusing tranny combos.Loading…
'Horsepower dictates how fast you hit the wall, torque dictates how far you move it'
Let's go with great minds thinking alike.
The way I've learned to wrap my head around it is up to 5252rpm, torque is the shove you feel in your back. MBEP reached, the engine is no longer breathing as efficient, but it is gulping more to make up for it as things more often/happen very quickly at 9K. To me it has been torque pushes while horsepower pulls.
That thing definitely needs a lightweight flywheel 😀
Just imagine the shrapnel.
In physics, we had to make a Rupe Goldberg apparatus. I would have preferred to be in shop class.
These are sweet. Please cover spark knock and pre-ignition, making sure to point out that detonation does not happen in an internal combustion engine. It always irks me when I hear someone talking about detonation.
This was the most useful thing I've read in months. I go to one of the countries most expensive colleges. Interpret that how you may.
C'mon! Do you really go to college to learn something? I just went there for booze, beer and bull shit. In the process I managed to get a BS and an MS. Degrees, I mean.
Which school do you go to BTW? If you don't mind me asking.Loading…
100,000 hp at 102rpm. The polar opposite of my 13b rotary.
I don't believe these ships to be diesel-electric hybrids. The single screw is direct drive. No additional inefficiencies are required or tolerated.
Set sail into the sunset, 85 rpm for hours at a time….
Every ship I've ever worked on was Diesel/electric. Not to say that there aren't direct drive boats but the son of a bitch has to reverse at certain times and I don't see whatever this is going in as having a B&M shifter and a barefoot gas pedal in the wheel house. Also I believe on your larger boats the idea is to keep your screws in an as uninterrupted flow of water as possible. So they drop the screws down below the boat rather than behind which I don't think would lend itself well to direct drive.Loading…
[…] What is Horsepower and Torque? – Quite simple really. It’s (HP = torque X RPM/5252)…Yeah, we’re not sure we understand it either. How about more of both please! […]
Just to confuse everyone, the other way of looking at it is that horsepower is torque over time. Think of a two stroke one cylinder engine turning 1000 RPM. It’s going to push 1000 times per minute, however hard that little piston pushes. If another little one cylinder two stroke has a piston that pushes twice as hard, it only needs to turn at 500 rpm to make the same horsepower.
That didn’t help, did it.Loading…
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