Horsepower and Torque: An Engineer's Guide

"DeWayneV8" (squirmish) 05/15/2015 at 14:56 • Filed to: None

6

24

A few days ago I made a !!!error: Indecipherable SUB-paragraph formatting!!! about the (incorrect) correlation that is often made between torque and vehicle performance. During my composition of said post, I assumed that the internet would join me in blasting the unwashed masses for their ignorance. Instead they tried to blast me.

×

!!! UNSUPPORTED LINE BREAK IN HEADER !!!

Every time I hear someone spout the old axiom “torque is what moves a car” I want to vomit out my… !!!error: Indecipherable SUB-paragraph formatting!!!

It became clear quite quickly that I hadn’t defined what I meant by “vehicle performance”, and that this definition needed some explanation. The combination of being told I’m an idiot with no experience and some much friendlier emails have prompted me to try and explain what I mean.

When I think of vehicle performance, I think of beating the car in the lane next to me. Sure, everyone likes to brag about 0-60 times or 0-100 times, but covering distance is what really matters. Race cars don’t compete with 0-60 times, they compete with time to distance. To move an object from point A to point B requires work . Work is, quite simply, the amount of average force put on something multiplied by the distance it travels.

Work = Force x Distance

In a vehicle application, the torque output of the engine is the “ Force ” in that equation. Work tells us nothing about vehicle performance, and here’s why. I can put a breaker bar on the front of an engine and transmit 150 lb-ft of torque through the drivetrain. I can crank that breaker bar around and around and eventually move that car a quarter of a mile. An engine generating 150 lb-ft of torque can also move that same vehicle a quarter of a mile. At the end of all that breaker bar cranking, the engine and I will have done the same amount of work. The difference is that the engine can do that work much faster, and it can do so because it has way more POWER.

Power is a measure of how quickly work can be done. In automotive terms, it literally is a measure of how quickly the engine can move that vehicle across a distance. This is the only metric that matters when evaluating engine performance from a numbers standpoint. I’m sure some people will respond with, “BUT WHAT ABOUT THE TORQUE CURVE?!?!”. Some commenters correctly pointed out that you cannot have horsepower without torque. While that’s true, you also wouldn’t have horsepower without RPM. Torque by itself will not move a vehicle, you need some velocity as well. The great thing about horsepower is that it tells us that the engine has BOTH! Amazing! It’s almost as if horsepower would be a really great way to deduce something about vehicle performance from a single number…..

Let’s address that torque “curve” for a minute. Again, some commenters correctly pointed out that a peak horsepower number could be generated by a laggy turbo engine that isn’t that fast in the real world. This is true. Before the turbo starts generating boost, the engine will not make much torque. Since:

Horsepower = (Torque x RPM)/5252

This also means that the engine will not make much power either! Do you see a trend? Any conclusions you draw from the torque curve could be drawn just as easily from the horsepower curve. A reduction in torque will result a reduction in horsepower, but keep in mind a reduction in RPM will also result in a reduction in horsepower.

So why are most automotive engines close to square in horsepower and torque? If torque didn’t matter, why don’t we have 30,000 RPM engines that only make 10 lb-ft? With less force the components wouldn’t have to be as beefy, and you could make the engine much smaller! The answer to that question is simple, cost and customer perception. Even the most hardcore auto enthusiast would have to admit that a motorcycle engine in a car would be a bad idea for the vast majority of automotive buyers. No one wants to sit there at 8000 rpm on the highway with your ears bleeding. No one wants to have the engine idling at 2500 rpm, and you’d never get the engine to pass emissions and fuel economy regulations anyway. The other factor is that extreme RPM means extreme inertial forces, which require extremely expensive parts. There’s a reason that the FIA started limiting engine RPM in F1, it’s cost.

Should the auto manufacturers stop publishing torque values? If they published horsepower curves for every engine, maybe. Even if they just gave horsepower numbers at a few different RPM points the torque number would become meaningless. I was presented with the example of an AMG E63 S’s 5.5L biturbo V8: it makes 577 horsepower @ 5500 RPM and 590 lb-ft of torque @2000-5000 RPM. I was asked what conclusions can be drawn from these numbers. First of all: that’s a lot of horsepower so that thing is going to be hella fast. The torque number is only interesting because they didn’t give a horsepower number at 2000-5000 RPM. You know that the horsepower at 2000 RPM will be relatively high because the torque number is high. Incidentally, 590 lb-ft at 2000 RPM is 224 horsepower, nothing to sniff at. If the torque number had been presented without an RPM number, it would be useless because it would tell you nothing about horsepower.

I’ll end with a question. If torque is more important than horsepower, why don’t we shift around the torque peak?

Arch Duke Maxyenko, Shit Talk Extraordinaire > DeWayneV8 05/15/2015 at 15:02

3

If torque is more important than horsepower, why don’t we shift around the torque peak?

You do in diesels.

themanwithsauce - has as many vehicles as job titles > Arch Duke Maxyenko, Shit Talk Extraordinaire 05/15/2015 at 15:06

1

And in small turbo engines. After about 5000-5250 rpm, the 1.6 in my FiST just runs out. Redline is further as is technically peak power but I’d rather shift a little short and hit the torque where the engine is happy.

RamblinRover Luxury-Yacht > Arch Duke Maxyenko, Shit Talk Extraordinaire 05/15/2015 at 15:14

2

If you’re malaise era US automakers, you bend the torque peak downward by increasing displacement.

230hp, 390 lb-ft @2000.

DeWayneV8 > themanwithsauce - has as many vehicles as job titles 05/15/2015 at 15:17

0

The 1.6 in your FiST is basically done at 5200 rpm. You’re still shifting around the power peak there.

HammerheadFistpunch > DeWayneV8 05/15/2015 at 15:17

1

I’ve been racking my brain for some time to come up with a good analogy to these two forces (torque and HP) and....I’m still waiting.

Its an interesting debate because you aren’t wrong when you say that torque alone is worthless, but I think you are doing the subject a disservice by ignoring the factor torque plays.

Torque is how much, and hp is how fast.

I like a really linear high strung NA motor...its the best...but having recently spent 4 days going very slowly moving a heavy load with a very high torque engine/low rpm NA motor I can attest to how awesome torque is too and why its important to tune for it.

Same story with my wife’s TDI. Its not as fast as a gas engine of similar displacement, but it feels much quicker everywhere in the torque curve, and when the curve is off it feels much slower, even though I am technically closer to peak HP.

EDIT: read the linked article and it was more thouroughly addressed, in the source and comments. I understand the relationship to torque and hp, but I didn’t think this article covered the merits of torque well, but I am satisfied having read both.

DeWayneV8 > Arch Duke Maxyenko, Shit Talk Extraordinaire 05/15/2015 at 15:18

0

I’m pretty sure that the fastest diesel race cars in the world (Audi) shift around the power peak.

hypnoticbutcher > DeWayneV8 05/15/2015 at 15:30

0

I wrote about 2 pages thoroughly explaining the difference between horsepower and torque, the derivations behind them, and what they all mean a few months back only to be cock blocked because I can’t post it anywhere. So it’s just sitting in a file.

DeWayneV8 > hypnoticbutcher 05/15/2015 at 15:37

0

I deliberately tried to keep the post low on math, energy is what it really all comes down to.

DeWayneV8 > HammerheadFistpunch 05/15/2015 at 15:39

1

Torque isn’t how much....it’s just a force. Force does not imply displacement. It becomes particularly meaningless when you factor in gearing.

You can have 100,000 lb-ft of torque but an engine that only makes 10 hp. It will be slow. More torque at a certain RPM point only means it will be faster because it raises the horsepower at that RPM point. It will not be faster than an engine that makes more horsepower.

HammerheadFistpunch > DeWayneV8 05/15/2015 at 15:41

0

When I say “how much” its an analogy. As in; its the meat of the driving force, where the hp is the rate at which it can be applied. I understand the relationship between the two...this is why coming up with an analogy has been so hard.

DeWayneV8 > HammerheadFistpunch 05/15/2015 at 15:48

0

Maybe....horsepower is how much damage you are doing overall, torque is how much each punch hurts.

HammerheadFistpunch > DeWayneV8 05/15/2015 at 15:50

0

yeah, something along those lines, a magnitude vs rate comparison.

KamikazePigeon > DeWayneV8 05/15/2015 at 15:55

1

This is how I’ve always interpreted it in a general sense. I could be wrong however.

I see it as the engine is a person going for a run. Torque is the force of each step. RPM is the amount of steps taken. Horsepower is the overall output of the amount of steps taken and the force of those steps.

DeWayneV8 > HammerheadFistpunch 05/15/2015 at 15:58

0

That’s exactly right. In fact, I’d be willing to bet that you could calculate the horsepower of a person that way!

DeWayneV8 > KamikazePigeon 05/15/2015 at 15:58

0

That’s exactly right. In fact, I’d be willing to bet that you could calculate the horsepower of a person that way!

themanwithsauce - has as many vehicles as job titles > DeWayneV8 05/15/2015 at 16:01

1

If all you’re trying to say is that power = speed, not torque, then I agree. Once you get that entire drivetrain moving, it’s a lot easier to keep it moving so having a strong instant output of force (torque) isn’t as important as you already overcame the force needed to get going and that amount needed to keep it moving is less and less thanks to momentum (objects in motion tend to stay in motion etc...). But this reads heavily like torque is a meaningless output and that isn’t the case. An engine with a 200hp peak and 150 lb/ft peak over a short curve is completely different in character and use than a 200hp engine with a 220 lb/ft peak.

If pure speed on flat, smooth ground is the only thing on your mind, then yes you don’t need that much torque but only if your car is also easy to move. See - motorcycle. No mass, some power = WEEEEEEEEEEEEEEEE. But with something like a truck towing a trailer, you’re gong to need that torque to move that drivetrain since your load is naturally going to add resistance to making your wheels turn. Going back to your analogy of the breaker bar on the driveshaft - if it takes 400 lb/ft of force to move that shaft at all and you don’t have it? Good luck. You’re winning nothing if you can’t even move. This is why the heavier touring car series rarely use low-torque motors. You need that force to get the damn thigns to start moving to overcome the forces trying to prevent those wheels from turning. And then you need the power to keep them accelerating. As you go up and down in speeds, you need both values. This is why your AM mercedes needs that advertised torque output. It’s a way of saying “Don’t worry about the mass, the engine can do it”.

DeWayneV8 > themanwithsauce - has as many vehicles as job titles 05/15/2015 at 16:18

1

Actually....power is still all the matters for moving anything. You’re right though, the character will be different. In the real world, no one wants to be towing something while spinning 7000 RPM the whole time, and an engine that produces more torque at a lower RPM would be beneficial for that. It is only beneficial because more torque means more power at that RPM.

In a vehicle drivetrain the clutch or torque converter take care of getting the mass moving. No ICE engine could get any vehicle moving without that slippage. Horsepower is still the measurement of how quickly an engine can move a mass. A higher revving engine could theoretically be geared up to move a bigger mass. Horsepower is what matters, but in the real world obviously NVH matters too so that’s not practical.

A 200 horsepower 1L superbike engine could move as much mass as quickly as a 200 horsepower FiST engine, it’s just the superbike engine would be SCREAMING.

bhardoin > themanwithsauce - has as many vehicles as job titles 05/15/2015 at 16:28

0

the torque is kinda meaningless - at the crank. if you wanted more torque at the wheel, you’d just have a shorter diff or shift later.

jariten1781 > DeWayneV8 05/15/2015 at 16:32

0

Meh, torque number is pretty important if you’re just trying to come up with a couple one point metrics on how a vehicle is going to feel. You can typically generalize the torque curve to being flat so a high torque engine is going to feel faster than a lower torque same output at any given normal RPM (until you redline the high torque motor). Yes, that’s because it has more horsepower at that rpm, but that’s not good for a simple metric. You’ll end up producing a graph, which is fine for us nerds, but doesn’t do a lot for someone who is only marginally interested and, frankly, it takes up more space and takes more time to digest...which again defeats the purpose of a simple metric. It’s not perfect, but simple metrics never are, and it is informative.

Also, a horsepower graph by itself doesn’t as easily, at a glance, inform as well how an engine behaves as a torque/horsepower graph. Engines with particular torque dips (looking at you flat 4s) feel like dogs through the range. Yeah, you can take a ruler out and look at the slope of the HP line, but why? If you’re already going to the trouble to make a graph it means you’re already measuring torque...throw it on there so it’s more easily digestible.

KamikazePigeon > DeWayneV8 05/15/2015 at 16:32

0

You probably could!!

Now would the Torque value be calculated by the lateral force exerted by the “foot” at distance from the buttocks (the pivot point)? Or do we have to include the knee somehow?

You now have me genuinely curious about this!

traderQAMobileTestAutomationMobileBoostOn > DeWayneV8 05/15/2015 at 16:40

0

And RPM is the punch rate?

TrustMeImAnEngineer, but I'd rather be an InternationalArtThief > DeWayneV8 05/18/2015 at 14:47

1

Excellent article, thank you for fighting to bring the right information to people!

DeWayneV8 > TrustMeImAnEngineer, but I'd rather be an InternationalArtThief 05/18/2015 at 14:52

0

Trust dem engineerzz

EX-engine-Neer > DeWayneV8 09/08/2017 at 13:17

1

Not directed at Mr. DewayneV8. But 90% of the comments below:

TORQUE IS A STATIC MEASUREMENT.

Horse Power is not.

Torque, in any dyno chart you have seen is back calculated from horsepower and rpm.

TOrque, with out some rate of application, is meaningless. Thus HP, or Watts in the rest of the world, is the best way to measure.