The goal here is to explain the horsepower vs. torque situation as quickly and concisely as possible. If you do a search on Google you’re likely to find either a bunch of misinformation written in crayon or daunting passages from an astrophysics textbook. The following dialogue aims to take a straightforward path across the topic by focusing on the key concepts without ivory tower equations and jargon. Nothing wrong with the in-depth efforts, btw- I just think that the subject can be made more approachable. Please let me know if anything isn’t as clear as can be. Onward!
Each car’s motor boasts two numbers:
- Torque is rotational force. Grab your left wrist, twist it as far as it will go, and hold it. Freeze. Your left forearm is experiencing torque. Notice how no movement is taking place. You just kind of feel…a twisting tension…..until you let go. In a car, when the tires spin against the road, you feel the motor’s torque as a straight push against your back. Thus, the torque number for a motor basically indicates how strong that push can be.
- Horsepower describes how much work the motor can do in a given amount of time.
Ok. They sound very different. Aren’t they related somehow?
Yes. In fact, horsepower is torque applied over time. This may not make sense right away but here’s another way to look at it. Imagine you are under a car with a wrench. You are in the middle of tightening a big bolt. You have the wrench on the bolt and you pull and pull, but you can’t pull hard enough to make the bolt move. Torque is the twisting tension that you are putting on the bolt, right? Right. Now you reposition yourself so you have a better angle. When you pull the wrench this time, it’s difficult, but the bolt turns! Something was accomplished! Work was done! This work is described by horsepower. It happens when torque is applied AND turning takes place. A horsepower number is a “rate” that tells us how much forceful turning can be done in a given amount of time. Or, in fewer, less simple words: horsepower is torque applied over time. And here’s yet another another way to say it:
- Torque is twisting force.
- Horsepower is twisting force with turning going on.
Now before you say “hey but torque can take place while the motor is turning!” hear this: Yes. Yes it can. There can certainly be twisting tension while turning is happening. It’s not like once turning happens torque goes away. It’s just that the torque measurement is independent of turning. Nothing to do with it. Just completely unrelated to turning. Think about it like this: Torque is always and only about the twisting force that occurs when you try to turn a bolt that won’t move.
Torque is kind of like smell in that way. Smell is also independent of movement. When someone asks, “what is that smell?”, you don’t say “depends how fast I’m going.” You understand that smell just doesn’t have to do with movement.
Whew. I dunno if that fully sank in but I’ll come back to it if I have to. Is one more important than the other when comparing performance?
I’d say so. In straight-line performance terms:
- The horsepower number describes how powerful the motor can be, and thus plays into how quickly a car can reach a finish line.
- The torque number describes how strongly the car seat can push against your back, and nothing else. Zilch else.
Wait- doesn’t the amount of ‘push’ I feel against my back indicate how quickly the car is accelerating and therefore how fast the car is?
No, not necessarily. And that’s why there’s so much confusion surrounding this topic. Horsepower, even though it can’t be directly felt, is the number that has to do with how quickly the car accelerates, how fast it can go, etc.
What? So a car that pushes gently can beat a car that pushes hard in a race?
Absolutely it can. Assuming both cars’ weights and transmission types were similar, the gentler car’s motor would have to be spinning at higher RPMs and therefore be getting more work done in a given amount of time.
It doesn’t always happen that way, but I want to emphasize that the harder push against your back does not necessarily mean greater quickness.
Weird. So high RPMs can make more horsepower?
Yes. Spinning at higher RPMs than the competition is a huge advantage in generating horsepower. For the gentler-pushing but faster car, even though less work is done per turn of its motor, it wins the race by spinning more times per minute than the competing car.
Wait so you’re saying the harder pushing car does more work per turn of its motor?
Indeed. But in the scenario above, even though the harder pushing motor does more work per turn, it still loses the race because the gentler pushing motor can turn so much more quickly that it generates altogether more horsepower in a span of time and crosses the finish line first. Does that make sense?
Yes, I think so. Let’s see: Horsepower is work. Each turn of the motor does work. Higher torque = more work done per turn. RPMs = how many turns the motor makes per minute…
So horsepower is sort of like: torque x how fast the motor turns?
Yes, sort of. There’s a more precise mathematical definition but “torque x how fast the motor turns” is conceptually more clear.
Ok then is the quoted horsepower number for a car: ‘the torque number given by the manufacturer’ x ‘top RPM for the engine’?
No, because the torque number given by the manufacturer is the torque peak: the most torque the motor is capable of producing. The RPM at which the torque peak occurs isn’t necessarily the RPM where the horsepower peak occurs.
Whoa. Why not?
Because, like it or not, the horsepower and torque output of a motor change based on the RPM it’s spinning at. The way it changes vs. RPM can be shown as a curve on a graph, with the output falling off on either side of the peak. A dynograph shows a composite graph of the torque and horsepower power curves so you can see a motor’s output at a given RPM. I say composite because the two curves are plotted with same Y axis (RPM )but different X axes (HP, torque)- so it’s like two graphs in one. This is a dyno from the Mercedes-McLaren SLR:
Ok that graph helps a little I guess. I’m curious- What determines where the peaks occur?
For now, suffice it to say that the peaks come from the motor’s design and they vary from model to model based on a motor’s purpose and the philosophy of the manufacturer.
Fair enough. Let’s cut to the chase: Will the car with more peak horsepower always win in a race?
No, but having more horsepower relative to weight will almost always win. That’s why a 400 lb motorcycle with 170 horsepower is quicker than nearly any road car in a straight line.
You say “almost always”. Are there any other factors to consider in a straight line race?
Yes. Here’s are some of the factors that are usually secondary but still important:
- manual- stick shifts are good because there’s no power loss
- electronically controlled manual- basically a stick shift controlled by robotics; they tend to shift more quickly than manuals which definitely helps in a race
- automatic- some power is lost in these through a fluid connection, which is where the term “slushbox” comes from; usually designed for comfort rather than speed
- automatic with lockup- less power loss than a conventional automatic because the fluid coupling hands off to a solid coupling
- CVT- these tend to be roughly equivalent to automatics with lockup
- Think about how you can change gears on a bike to keep the pedalling easy. Same idea. Gearing between the motor and the wheels is designed to keep a motor in its power band; in other words: allow the motor to spin at optimal [easy] RPMs for whatever the car is asked to do. Usually the manufacturer will optimize gearing for their motor, so it’s not really a concern unless you’re looking at modified cars where power has been increased significantly.
- Usually a manufacturer will optimize the tires, suspension and gearing so that the power can be controlled enough to allow good traction, so once again this isn’t usually a huge concern unless the cars are modified. In any case, a car’s “layout”, aka which wheels transmit the motor’s output to the road, can make a difference. AWD, RWD, FWD are listed in order of traction potential with street tires but each have their upsides and downsides beyond traction. For what it’s worth, most focused performance cars are RWD.
- Not usually a concern until 100+ mph speeds, but eventually wind resistance will start to eat up horsepower. That’s when being aerodynamic can really start to pay off.
‘Peakiness’ or ‘Flatness’ of output
- Two cars could have the same peak horsepower number but very different looking dynograph curves. In such a situation, the flatter [less pointy] peak will tend to be an advantage because it means there’s more near the peak. When comparing the relative ‘peakiness’ of one motor vs. another, people might talk about the “area under the curve” on dynographs. More area under the curve= higher horsepower available through a broader range of RPMs.
Well that’s a lot to think about! I need a tidy “what this means to me” conclusion, now!
Ok. To sum up:
A car’s peak torque number, relative to weight, will give you a sense of how strong one car can push against your back when accelerating vs. another.
A car’s peak horsepower number, relative to weight, is the easiest way to size up the straight line performance of one car vs. another.
Why do car magazines and car experts sometimes talk about torque and horsepower as if they are rivals?
I don’t know. But it happens a lot. Here a misguided writer [formerly from Motortrend] cherry picks a flukey example to try to alter the definition of torque and scramble his [and his readers'] brains:
He says: ”It’s the stuff that rockets you away from the stoplight…” Not quite. Torque doesn’t rocket anything. It’s a measure of static twisting tension. Static, as in: stationary and non-rocketing. To rocket a car away from something requires work. Horsepower is a unit of work. So…yeah…
“But look at his opening example” you may say, “That’s compelling isn’t it?” Ok, well why doesn’t he mention that he’s comparing a car with a slick dual clutch electronic manual (fast shifting) to a car with a remarkably sluggish 4-speed automatic (slow shifting, lazy gearing)? Or that Motortrend itself pointed out that the 2000 I30t was puzzlingly slow?:
But even though the new horsepower output figure moves the I30 into first place in the class, something seems to get lost between spec sheet and stopwatch…interestingly, the new I30 is substantially slower than its predecessor, which [with 37 horsepower less] could hit 60 in 7.0 seconds and cover the quarter mile in 15.5 seconds.
The most amazing thing is that he uses that one absurd example and calls it a day. Sheesh. A few minutes of searching reduces it to absurdity, and the house of cards flutters down.
A better set of cars for a to look at would be the BMW 335i vs the 335d. The 335i has a gasoline motor which makes more horsepower. The 335d has a diesel motor which makes more torque. The cars are of fairly similar weight and the same transmission type, so it’s easy to compare the performance results and see that torque does not equal performance:
Note how, in the 335d review above, Car and Driver says:
Mashing the throttle unleashes a fusillade of torque down the driveshaft, and the nearly immediate availability of same allows the 335d to churn its way to 60 mph in 5.7 seconds and cover the quarter-mile in 14.2 seconds at 100 mph. We think the 335d feels even quicker.
Yep- it may feel quick, but torque is just a feeling.
Are dynographs & 1/4 mile drag racing time slips better ways to compare performance than using peak horsepower numbers?
Absolutely. A dynograph will give you a good idea how the motor offers up power before and after the manufacturer’s quoted peak, plus it factors in the transmission loss which is something like 15%-20% off the manufacturer’s quoted number (because the manufacturer records measurements with the motor out of the car).
A 1/4 mile drag racing slip created by a good driver is an even better indicator of real world straight line performance, assuming similar traction and atmospheric conditions between runs. And if traction from a dig is an issue then….
Then there’s the Vbox: a dash mounted device which uses precise GPS data to record any run in detail. Vbox runs can be compared and analyzed with great accuracy.
What is ‘low end torque’? Why do people like it?
It’s just the torque occuring at a low RPM. People like to say it because they like having a strong push against their back when moving away from a stop light. Any car that pushes strongly against their back in this situation is said to have ‘a lot of low end torque’.
Why doesn’t low end torque, or any torque number, really matter for comparing performance?
Because torque describes a twisting tension independent of any movement. Since movement takes time, and horsepower is torque applied over time, horsepower is the figure to look at for casual performance comparisons. Horsepower relative to weight, that is.
But what if I want to compare performance at low RPMs and the HP peak occurs at high RPM?
Good question – I’d say find some dynographs and compare the low RPM sections of the horsepower curves. There are dynographs all over the internet. Keep in mind that if you’re really trying to go fast, you’re going to be entering every gear from 2nd up at something over 3000 RPM, so low RPM performance is generally less important.
Gaahh! Why do you hate torque so much?
I don’t! Torque reflects how strong a motor feels. Nothing wrong with that. Torque feels good! All I’m saying is: that feeling shouldn’t be confused with what a motor can actually do, i.e performance. Is that so wrong?
What is the real formula for calculating horsepower?
Horsepower = (Torque x RPM)/5252
Where does this 5252 come from? Why do the horsepower and torque curves on the dynograph always cross at 5252 RPM?
Good question. 5252 is the artifact of unit conversions involved in the formula above. Here is a good rundown: http://www.howstuffworks.com/question622.htm