Can someone please explain

[dmoff1698]

I'm confused in the head over horsepower versus torque!!

 

I have wondered about this for years and never got a satisfactory answer.

 

I have read and believe I understand the definition of horsepower and what it means.

 

I also know what torque is, how it's calculated, and what it does.

 

My problem is when I mix these two pieces of info together in my puny little brain I get the idea that horsepower and torque should follow each other. That is..more horsepower should produce more torque. Conversely, if you need more torque, make more horsepower. Just seems reasonable to me.

 

However, if you look at dyno results you see I am all wrong here. Typically HP increases to a certain point then starts to drop off, while torque continues to increase.

 

This makes no sense to me at all. Can someone please explain the real relationship between the two (if any), and where my thinking goes south on me?

 

Lucy!!!!Gu got som' 'splanin' to do!

[SilvrT]

Torque refers to the force required to move an object..eg. the more torque, the heavier the object that can be moved.

 

Horsepower refers to the ability to move something along once it gets going.

 

I guess that would be "my" interpretation

 

I guess from your post you already know that... so I have no idea why I posted this ... :think:

[scotty]

Torque is a twisting force. Speed is not a factor. An example is a torque wrench.

Rotating speed is measured in RPM.

Horsepower is (rpm x torque) divided by 5252

Horse power is what makes you go.

So if you had 1000 lbft of torque and 0 RPM you have 0 horse power and you aint gonna get anywhere.

Dose it make sense?

[dingy]

posted at http://www.largiader.com/articles/torque.html

See red highlited text towards bottom for probable answer to your question.

 

 

See link below for various torque vs. Horsepower graphs

http://www.dynoperformance.com

 

Torque vs. Horsepower

 

If you've been around motorized vehicles for any length of time, you have probably been exposed to the great torque vs. power debate at some point. If not, it goes like this:

"Torque is what makes a bike accelerate, not power."

 

 

"Wrong."

 

 

Torque and power are inescapably linked by the fact that horsepower equals torque (in ft-pounds) times RPM divided by 5250, so people who talk as if they are independent are full of it. If you have a given torque curve for an engine, you have the horsepower curve also. Knowing how these two numbers work with each other lets you can poke through some of the BS you might read.

First, as usual, a few definitions.

Torque is a twisting force applied to an object, like a wheel or a crankshaft. Note that motion is not required for torque to exist! If you stand on a lug wrench that is on a frozen lug bolt, you are applying a torque to that bolt even though there may be no movement. For our purposes, we will consider that torque is measured in pounds-force feet (lbf-ft) meaning the equivalent of a given force, in pounds, acting on the end of a lever of length in feet. For example, standing with 180 pounds body weight on a lug wrench one foot long yields 180 lbf-ft of torque. A child of 90 pounds standing on a two-foot lug wrench applies the same torque.

Work is the application of force over a distance. Unfortunately, the units used are the same (pounds times feet) but we write this as ft-lb just to distinguish it. The real difference is that in this case, the "feet" part means feet of movement. If you push on a car with 100 pounds of force and maintain that for 30 feet, you have done 3000 ft-lb of work. An easier example is lifting a weight (in pounds) a given distance (in feet). If you use some sort of mechanical advantage, like a winch, you will do the same amount of work because by halving the effort required, you will have to double the distance through which you apply the force to achive the same objective.

 

 

Power is the application of work within a finite time. 550 ft-lb of work in one second is one horsepower.

 

 

So, let's first go through the numbers to get from torque to horsepower. Pushing with 87.5 pounds (force) on the end of our 1-foot lug wrench applies a torque of 87.5 lbf-ft. No motion yet, so no work and no power. But now let's say the lug bolt loosens slightly and starts to turn, but that same 87.5 pounds of force is needed to keep the wrench turning. For every revolution of the wrench, you are applying 87.5 pounds of force over a distance of (2 * pi * 1 foot) or 6.28 feet, the circumference of the circle that your hand is making, for a total of 550 ft-lb of work. It's only when this system is actually moving that work is being performed. From here, it's a quick step to say that if you work fast enough to turn that wrench once per second, then you are doing 550 ft-lb of work per second, which means you are applying one horsepower.

 

 

By the definitions we can see that HP is directly proportional to torque and RPM. "Directly proportional" means there may be a multiplyer involved, so let's find it using our example numbers, remembering that 1 revolution per second is 60 RPM:

torque * RPM * constant = hp 87.5 lbf-ft * 60 rev/min * X = 1 hp X = 1 / (60 * 87.5) = 1/5250 torque * RPM * 1/5250 = hp hp = (torque * RPM) / 5250 For internal combustion engines, torque is always given at a certain RPM because they can't generate any torque when they aren't moving. Once they are running fast enough to sustain their own operation, the force that they are exerting against a load can be measured, and the speed at which they are turning can be measured, so the torque (and therefore power) numbers become known.

 

 

So, if there is such a fixed relationship between torque and power, why do some people say that a certain engine has lots of power, but no torque? Remember that the connection between torque and power is rotational speed. A sportbike motor might generate 150hp at 14,000 RPM but the torque at that RPM is very small; about 53 ft-lbs. In comparison, a large-displacement twin might peak at 100 hp at 7000 RPM. The torque applied at the twin's 7000 rpm, 75 ft-lbs, is greater than the torque applied at the sport bike's 14,000 rpm but the sport bike makes up for it with a lot more engine speed and ends up with more horsepower.

 

 

The street, though, complicates things because the sport bike will probably not be ridden at 14,000 RPM. At 5000 RPM, the twin would likely have more power. This is an artificial handicap; the sport bike wasn't meant to be ridden at that speed since it generates its power by sending the RPM part of the equation sky-high. For street riding, the twin is easier to ride, less prone to stalling as you pull away from a light, and you get that satisfying "oomph" when you twist the throttle. But as the RPM increases, the twin runs out of breath and the race bike, although the torque is low and probably getting lower, continues to make more and more power until it hits its peak at 14000.

 

 

Engines are designed for their intended use. Our twins are designed to yield fairly high torque values at low RPM, because this makes them easy to ride in day-to-day life, and Harley-Davidsons have their torque concentrated even lower in the RPM range than BMWs do. Low-end torque is accomplished by several design traits, one being small valves and intake tubes which create high air velocity into the cylinder for good fuel mix at low speed.

 

 

Those effects tend to become a restriction at high RPM, which means that engines intended for high RPM end up with larger valves, larger air intakes, smaller cylinders and other things that let them continue to breathe when other engines start to gasp. Race bike engines have fairly small displacement, which limits the torque that can be produced at the crank. They apply that torque at much higher speeds to get high horsepower (and who can argue that those bikes don't accelerate quickly?).

 

 

To a lesser extent, BMW varies these techniques for different bikes. The GS series has narrower intake tubes to give a faster intake charge, giving better fuel/air mixing and better torque at low RPM. Since this becomes a bottleneck at higher RPM, the "power" engine in the RS and RT bikes have larger intake tubes. Swapping the GS tubes into an RS or RT is a common retrofit, as it makes the bike torquier at low RPM where most of us ride. Newer technology in cars, like variable valve timing and variable intake tract length, can give motors the best of both worlds by increasing torque at higher RPM without giving it up at low RPM. Incidentally, Honda has variable valve timing on a motorcycle now.

But to get back to the main point, it is power that moves our bikes down the road. Yes, torque provides the pushing force through the drivetrain, but it needs to happen at some given speed, and those two factors define "power."

 

 

Why does torque drop after a certain RPM?

Torque starts to decrease because the engine cannot breathe as well. Due to the speed, the cylinder does not fill with air as well. A designer can get around this problem with "tuned intake" which sets up a resonance to pack the cylinder with air, but it only happens at a certain RPM. The next evolution of design is to make a variable system which packs the cylinders with air at all RPM; this is usually called "variable tuned intake runners" or something like that and involves valves which open and close to create a different size for the airbox and manifold.

 

 

Why does power continue to increase after torque decreases?

Remember that the power is essentially the product of the RPM and the torque. At first, decrease in torque is small and is not enough to offset the increasing RPM, so the overall product still increases. Eventually the decrease in torque becomes large enough that it outweighs the increase in RPM and we see the power start to drop. Because of this, the power peak will always be after the torque peak.

 

 

A few other well-writen articles illustrate these concepts, although many can be found by entering "torque horsepower" into Google:

[MiCarl]

HP=Torque*RPM/5250

So all your torque curves have HP=Torque at 5250RPM.

 

Think of it as a spinning shaft lifting a weight on a rope. Forgetting the units a torque of torque (T) turning the shaft will lift the weight (W) a distance (X) in a revolution. The work done is W*X

 

If you keep the torque the same but double the speed the weight will go 2*X in the same amount of time. Your work is W*(2*X).

 

If you keep the shaft speed the same but double the torque you can lift twice the weight (2*W) * distance. So your work is (2*W)*X.

 

W*(2*X)=(2*W)*X

 

HP measures the ability to do work. You can get equivalent work done by either varying the Torque or the speed (RPM).

 

The 5250 is a constant to make HP roughly equivalent to what a standard horse can do.

 

If you used 4000 it might be mule power (MP) instead of HP. Of Course a constant of 500 would be GP (Gerbil Power).

[MiCarl]

As usual Gary types faster'n me:doh:

[MikeWa]

Horsepower is the speed at which work is done. You can move a 1 pound weight very quickly or 100 lbs very slowly and develop the same horsepower. It is in the math, distance, weight and time. But there must be movement or no work is done and no horsepower developed. Torque on the other hand is a measurement of the force applied. No actual movement is required to develop or measure torque. It is torque that breaks things. So horsepower helps you go fast. Torque gets you up to speed.

[dmoff1698]

This is going to take some digestion for me.

 

Let me reread a few times, I'll get it soon enough.

 

Again, thanks for giving me something to do for the rest of the evening.

[pegscraper]

That is..more horsepower should produce more torque. Conversely, if you need more torque, make more horsepower.

 

Think of this the other way around. More torque will produce more horsepower. If you need more horsepower, make more torque.

[MikeWa]

The design of the engine will determine at what rpm horsepower and torque will reach their maximums. A long stroke and small bore will usually make for a lower rpm engine that develops good low speed torque. A short stroke and large bore engine is more suitable for high rpm horsepower. And of course there is a middle ground where most of us live. Then camshaft, manifolds, airflow, valves and all of the other engineering marvels seek to maximize the characteristics the engineers want for that bikes weight and gearing. These things determine at what full throttle engine speeds torque and horsepower will peak out.

[jlh3rd]

dmoff

 

i read your post carefully and there is a misunderstanding ......torque does not continue to increase...both torque and horsepower reach a max and then decrease as RPM continues to increase....this max occurs at different rpms for each value and is different for each type of engine...torque peaks early then declines...horsepower continues to increase ,levels off and declines a little....but as in the previous posts...cross at 5250 rpm's....they are related but not equal ,as the math formulas show...

torque is a measure of how much work your trying to or are doing in a rotational direction ( pounds/feet ) ....W= Force times Distance.....Force = Mass times Acceleration.....so work = mass times acceleration times distance........no work is done unless a distance is involved....notice , i didn't say torque = force.....and torque doesn't equal work...so torque will reach a peak and then decline.

 

horsepower is a measure of how fast you want to do the work...notice in the above simple definitions...there is no mention of time in the torque definition ( lbs/ft)...so the difference between torque and horsepower is a time factor....at a certain point in the rpms your acceleration rate will decrease (not speed, it is still increasing ,just at a slower rate because your rpms are increasing and the piston can't fill as completely as at lower rpms, so the "push" declines)...so torque begins to drop... and sooner than horsepower......so, you do your hardest work in the middle rpm range..moving a set weight over a short measured distance per so many seconds ( high acceleration,high torque, horsepower increasing.rpm increasing, ).....as rpms increase the rate of your acceleration declines ( your still going faster so horsepower is still increasing, but your torque is decreasing because the " push" is decreasing )so your moving the mass faster, but not pushing as hard as you were because of piston speed and the ability of the piston to fill and make power..then throw in friction and drag at the higher speeds and you've reached your horsepower limit as you can't go any faster (horsepower = work fast ) and your "push" has been decreasing .

 

 

got it ! right!.......LOL....hey, all these other posts are correct....i just posted mine to make a point...and to correct your minor discrepancy....the point is is that horsepower/ torque definitions go from single sentences to pages of explanations....

maybe i helped a little....

Edited January 20, 2010 by jlh3rd

[MiCarl]

dmoff

 

...so horsepower begins to drop... and sooner than torque......

 

It's the other way around. Horsepower doesn't drop unless either torque or RPM drop.

 

In most torque curves torque peaks and begins to fall as RPMs continue to rise. Horsepower generally continues to rise until the torque drops faster than the RPM increases.

 

Generally speaking torque ~ acceleration. Horsepower ~ ability to overcome drag (wind, tires etc.). HP limits top speed, torque is how fast you get to top speed.

[jlh3rd]

micarl

 

your right..i looked at my dyno run wrong..so reverse my hp /tq...but my torque dropped even as horse power increased...my torque was 314 at 4700rpm...hp 280,and rising.......hp 303 at 5800rpm, torque 260,and dropping

Edited January 20, 2010 by jlh3rd

[BuddyRich]

I always think in simple terms. Torque is how much force needed to get the weight moving and HP is how fast can you make it go.

[jlh3rd]

buddy rich

 

exactly !

 

micarl

 

i changed my post.....do "I" understand it any better ??

[Yammer Dan]

Now my head hurts!!!

 

I know I twist my right hand and "UGLY" passes just about anything in sight!!!

[jlh3rd]

hey

 

lets throw tranmissions and gearing into this !

[Guest tx2sturgis]

Dale, you got a bunch of textbook answers but I'm thinking thats not what you asked. You can read that same stuff on thousands of websites.

 

I'm thinking you wanted a laymans explanation that helps you wrap your brain around the concept. I will give it a shot here.

 

This is BASIC, folks...and slightly bends a few rules:

 

Ok, Torque is how MUCH work can get done. Horsepower is how FAST it gets done.

 

Picture a simple analogy here. Fill a wheelbarrow full of dirt. Pick it up and start pushing it slowly. The force you use to push it along at a steady speed, is the (more or less) equivalent of torque. If you decided you needed to push it twice as fast, the time it takes you to speed up your pace to that new speed, is a measurement of Horsepower.

 

 

Lets say your buddy is twice as athletic as you. You and he can both push identically loaded wheelbarrows along at the same walking pace. And as an observer, I see that you are both working at the same rate, both pushing the load at the same speed, both producing the same level of 'torque'. But when you both decide to instantly start running while pushing the wheelbarrows, your buddy is able to quickly reach a higher running speed, than you were able to. He is able to apply his 'torque' at a faster 'rate' and the result is, he can produce more (horse)power that is easy to see and measure. At the end of the day, your athletic friend is able to move more mass ( the piles of dirt) from one area to another, because of the difference in 'horsepower'.

 

Or put another way, if the job was to move 100 piles of dirt for each of you, he got his done in half the time it took you, because of his higher level of horsepower. You both did the same AMOUNT of work ( torque), but he produced more HORSEPOWER because he could do it twice as fast.

 

The techies on here will probably find ways to pick this simple analogy apart. But if it helps you understand the difference, then it was worth posting.

 

 

 

 

 

Edited January 20, 2010 by tx2sturgis

[SilvrT]

Dale, you got a bunch of textbook answers but I'm thinking thats not what you asked. You can read that same stuff on thousands of websites.

 

I'm thinking you wanted a laymans explanation that helps you wrap your brain around the concept. I will give it a shot here.

 

This is BASIC, folks...and slightly bends a few rules:

 

Ok, Torque is how MUCH work can get done. Horsepower is how FAST it gets done.

 

Picture a simple analogy here. Fill a wheelbarrow full of dirt. Pick it up and start pushing it slowly. The force you use to push it along at a steady speed, is the (more or less) equivalent of torque. If you decided you needed to push it twice as fast, the time it takes you to speed up your pace to that new speed, is a measurement of Horsepower.

 

 

Lets say your buddy is twice as athletic as you. You and he can both push identically loaded wheelbarrows along at the same walking pace. And as an observer, I see that you are both working at the same rate, both pushing the load at the same speed, both producing the same level of 'torque'. But when you both decide to instantly start running while pushing the wheelbarrows, your buddy is able to quickly reach a higher running speed, than you were able to. He is able to apply his 'torque' at a faster 'rate' and the result is, he can produce more (horse)power that is easy to see and measure. At the end of the day, your athletic friend is able to move more mass ( the piles of dirt) from one area to another, because of the difference in 'horsepower'.

 

Or put another way, if the job was to move 100 piles of dirt for each of you, he got his done in half the time it took you, because of his higher level of horsepower. You both did the same AMOUNT of work ( torque), but he produced more HORSEPOWER because he could do it twice as fast.

 

The techies on here will probably find ways to pick this simple analogy apart. But if it helps you understand the difference, then it was worth posting.

 

 

 

 

 

 

So what you're really saying is, if you want to get some yard work done, hire an athlete...

[jlh3rd]

and at the point where they were both producing the same torque and horsepower.. they ran into each other !

[dmoff1698]

I think I got this.

 

What I was NOT getting before is the concept that there has to be motion before Work is being done (my wife told me this the other day, but that was on a different subject). I was not getting past the idea that force did not = work. (She noticed it right away)

 

So last night I put a wrench on a stuck bolt...put a weight on a stick and picked it up....and filled my wheel barrow with sand and pushed it across the yard. Now I understand what work is.

 

Thanks guys

You're the best.

[SilvrT]

Now I understand what work is.

 

Now don't be tellin that to your wife... :rotf: