Topic: torque vs. HP

[paulages]

got in an interesting argument tonight with a guy i tech i know who runs a dyno part of the time at the show he works at. he maintained that torque is all that matters on a dyno readout, while i argued that peak HP is just as important. to begin with, he wouldn't acknowledge that the two are essentially describing the same thing, or at least two parts of the same equation (torque X RPM / 5252), but maintained that once torque begins falling off on the dyno, you stop producing usable power. mind you, he's primarily works with twins.

now... anyone with an inline four knows that the torque curve can begin to wane, but when the RPMs kick in it really begins to be used more efficiently. really... by his assertion, we should expect nothing out of a high performance SOHC4 above 6K RPM or so.  

so my question is: to you racers, what power band would you prefer to be in on an average track (not too short, not too long), with a CR transmission allowing you to stay in that sweet spot as much as possible? peak torque or peak HP?

[turboguzzi]

rubbish, let the guy continue working on v-twins

on the other hand, there's an interesting thing called thrust curves, basically the torque curves multiplied by the gear ratios and it shows you when its best to shift to maintain best thrust,

with the stock box, I try to never fall bellow 6500 when reopening throttle exiting curves, otherwise you can really feel that the engine is not in the powerband.

If you'll look at my old dyno curve, you'll see that the torque peak is at 8000 or something, more than one person told me its too low for a race engine, but its people who are used to modern four valvers,

any peakier than that and I would need a CR box (which I don't have money for...)

TG

 

[Joksa]

http://www.v8914.com/Horsepower-v-torque.htm

It's better to make torque at high rpm rather than low rpm, so we can take advantage of gearing.

[paulages]

rubbish, let the guy continue working on v-twins

  

that's what i told him once it got a bit heated, but he just told me to "stick to the vintage stuff," as if the laws of physics have changed over the last 30 years.   my engine builds peak torque at about 8000 RPM... to suggest that "it makes no more power after that point" is ludicrous.

[mlinder]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

This guy thinks he's right because most american (read: harley) v-twins don't do well over 5252 RPM. They become inefficient at higher RPM, so their dyno sheets show crap above that, so they make the assumption that 'torque' is more important than 'hp'.
Easy mistake for people who don't like to think very hard.

[bucky katt]

the long stroke motors (like the harley v-twin) is more efficient at lower rpms. the shorter the stroke is, compared to the bore, the higher your powerband is. this is how i understood it anyways. this is why a 327 chevy motor will spin faster than the 350, same bore shorter stroke.

[Joel]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

This guy thinks he's right because most american (read: harley) v-twins don't do well over 5252 RPM. They become inefficient at higher RPM, so their dyno sheets show crap above that, so they make the assumption that 'torque' is more important than 'hp'.
Easy mistake for people who don't like to think very hard.

Torque and HP are different but related.  Torque is a force.  Horsepower is power.  Torque over distance produces work.  Work over time is power.

A force over a given distance always does the same amount of work.  However, the power produced depends on the time required to do that work.

[MJL]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

This guy thinks he's right because most american (read: harley) v-twins don't do well over 5252 RPM. They become inefficient at higher RPM, so their dyno sheets show crap above that, so they make the assumption that 'torque' is more important than 'hp'.
Easy mistake for people who don't like to think very hard.

Torque and HP are different but related.  Torque is a force.  Horsepower is power.  Torque over distance produces work.  Work over time is power.

A force over a given distance always does the same amount of work.  However, the power produced depends on the time required to do that work.

Torque is work, and hp is how fast you can do the work.  (simply)

[mlinder]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

This guy thinks he's right because most american (read: harley) v-twins don't do well over 5252 RPM. They become inefficient at higher RPM, so their dyno sheets show crap above that, so they make the assumption that 'torque' is more important than 'hp'.
Easy mistake for people who don't like to think very hard.

Torque and HP are different but related.  Torque is a force.  Horsepower is power.  Torque over distance produces work.  Work over time is power.

A force over a given distance always does the same amount of work.  However, the power produced depends on the time required to do that work.

That's what I said, only more simply than you did......

[mec]

too much hp makes you crash into a wall.
torque is related to how hard the impact is.

mec

[754]

He is probably right as far as big V twins are concerned, but it is not necessarily true on other engines.

 I am having trouble with your last question (first post).
 I am thinking it depends on WHICH racing..

 Flattrack seems to be won by torque, at at least it seem to matter more.

 When they ran the single gear classes at the drags, I dont think a four stands a chance.

  But in a lot of other racing, horsepower seems to be king..

[TDIpowered]

horsepower is how fast you are going when you hit the wall. torque is how far you take the wall with you after you hit it

[paulages]

He is probably right as far as big V twins are concerned, but it is not necessarily true on other engines.

 I am having trouble with your last question (first post).
 I am thinking it depends on WHICH racing..

 Flattrack seems to be won by torque, at at least it seem to matter more.

 When they ran the single gear classes at the drags, I dont think a four stands a chance.

  But in a lot of other racing, horsepower seems to be king..

i wouldn't argue with this at all. in fact, in the conversation i said that what most people call "torque," (low end torque) is more usable for normal riding... the longer the torque plateau, the more range you have to use the torque. in any case, even if a v-twin produces torque in a lower range- and feels like it has more "grunt," by definition peak torque cannot happen before peak power. this is an irrefutable law of physics, and was the main gist of the argument.

even in road racing, a person might prefer to have a loooooong strong torque curve that a higher peak HP, given the right track. even with a CR transmission, the ability to use the torque seems critical. in any case, this was the main feedback in wanted from racers, and i guess by "racing" i meant roadracing. great to hear feedback on other forms of the sport though!

[Joel]

He is probably right as far as big V twins are concerned, but it is not necessarily true on other engines.

 I am having trouble with your last question (first post).
 I am thinking it depends on WHICH racing..

 Flattrack seems to be won by torque, at at least it seem to matter more.

 When they ran the single gear classes at the drags, I dont think a four stands a chance.

  But in a lot of other racing, horsepower seems to be king..

i wouldn't argue with this at all. in fact, in the conversation i said that what most people call "torque," (low end torque) is more usable for normal riding... the longer the torque plateau, the more range you have to use the torque. in any case, even if a v-twin produces torque in a lower range- and feels like it has more "grunt," by definition peak torque cannot happen before peak power. this is an irrefutable law of physics, and was the main gist of the argument.

even in road racing, a person might prefer to have a loooooong strong torque curve that a higher peak HP, given the right track. even with a CR transmission, the ability to use the torque seems critical. in any case, this was the main feedback in wanted from racers, and i guess by "racing" i meant roadracing. great to hear feedback on other forms of the sport though!

Why can't peak torque happen before peak horsepower?

[paulages]

i'm not engineer nor a physicist (i just remember it as a known law), so i'll have to think about that a little to explain it better. i just did a google search and came up with a few articles though, and this wiki entry explains a lot.

http://en.wikipedia.org/wiki/Torque

[Rod]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

Do you mean this thread Mark?

http://forums.sohc4.net/index.php?topic=34599.0

maybe this is one of the many threads that should be migrated to hp/racing section?

[paulages]

yeah, i remember that one. i stayed out of it. the math speaks for itself. however; different from that thread, i asked which you'd rather tune for: peak torque or peak HP, which is very specific to the application. the duc mark talked about in that thread is a good example though. you can have an engine with a well balanced torque curve that will get its ass handed to it by a bike with a peaky engine with a higher peak HP, that is able to stay within that powerband with gearing. in other words, more horsepower will ultimately make you go faster, but broader torque will make up for any inability to stay in that sweet spot. on a street bike, that is a very nice thing.

so... you strike a balance. but which would you prefer?

[754]

I am going for torque..
 Few reasons.
 Get to use it more of your riding time..

 Its fun to have great midrange grunt & be able to shortshift..
 
 You can be riding legally, and still get the use out of your motor.

 Nowadays they want to take your bike away for streetracing..

[mlinder]

OK, reason 1:

Think about shifting:
If you 'stopped' making power after 5252 rpm, think about where you'd have to shift.

On a cb550, if you stopped making torque at 4250, to get to 65 mph as fast as you can you'd have to change gears 4 times (5 grand in 5th is about 65mph)
If you use your horsepower (this is misleading, because, again, these are two different ways of expressing the same thing), you shift only once, as 65mph is about 9.5k in 2nd.)

Why can't peak torque happen before peak horsepower?

It usually does (in larger engines. Larger engines tend to lose efficiency more quickly as engine speed increases than smaller engines do) . Remember, again, they are different ways of measuring and expressing the same thing.
Horsepower is a function of torque. It is not a seperate entity. It's a way of asking, and  showing, how much work this engine can do with the torque provided at a given engine speed.
The efficiency of the engine (amount of torque made) does decrease as the engine speeds up, but the FREQUENCY at which it makes power increases.
So, let's say theres a decrease of effiiciency of 5% from 6krpm to 7krpm, but the number of work cycles has increased 16%, which means you have 11% more power availabe to you at 7k than 6k.
Or with real numbers for the same as above, imagine you made 30 lbft at 6krpm with the same loss as stated above. That means 28.5 lbft at 7k.

That gives us HP (work available) = (30*6000)/5252, which comes to 34.27hp
That gives us HP (work available) = (28.5*7000)/5252, which comes to 38hp.

Sure it's been posted before, but please read:
http://www.vettenet.org/torquehp.html

[MJL]

I googled some HD dyno charts, and with one exception they all stopped at 5500-6000 rpm.

[paulages]

that link is a good read. it's funny, because as i read the first article i got a bit confused as it seemed like he was explaining several things wrong, though i wouldn't have been able to articulate it as well as the fellow who wrote the article following and critiquing it. if you want to cut to the chase, skip to third one where plato and aristotle and hashing it out.

[paulages]

here are a couple exerpts that pertain particularly to what we are discussing, without quoting the physics and math that backs it up (but is also in the article):

"Plato:  Well, there are still certain benefits to emphasizing torque in lieu of power, aren’t there?

Socrates:  Certain effects, such as improved acceleration from a full stop and less frequent shifting, are the result of a comparatively flat, uniform spread of engine output, starting at comparatively low engine speed.  It makes perfect sense to attribute such effects to a de-emphasis on peak power.  However, logically speaking, torque and power are not opposites, and it does not follow from the fact that you have de-emphasized peak power, that you have emphasized torque.  Of course, if there exists some other justification for the practice of equating the engine’s low-speed performance to torque, that will also constitute justification for equating a de-emphasis on peak power to an emphasis on torque, never mind that torque and power are not opposites.  At the wheel, the affinity between low rotational speed and torque is quite genuine, owing to the fact that the transmission is used to exchange rotational speed for torque.  But this effect does not apply to the engine.  The practice, of equating engine performance at low and moderate engine speed exclusively to torque, seems to derive essentially from the fact that the peak engine torque occurs at a lower engine speed than does the peak power.  This seems a weak justification when you consider that the peak engine torque reveals the engine performance accurately at only a single engine speed.  That engine speed is often above the midpoint of the engine’s operating range, and no matter how low the actual engine speed, the actual performance depends partly on the engine speed, and is fully revealed by the actual power.

Plato:  I need to go get measured for a new toga, but before I run along, I’d like to know what you think about the various claims that engine torque is the true indicator of engine performance.

Socrates:  Those sorts of claims have to be interpreted to mean that you are always supposed to get the same acceleration for a given amount of engine torque, no matter the engine speed at which that much engine torque is delivered.  There simply is no other meaningful, tangible interpretation of those claims.  Yet, as we have already seen, wheel torque depends just as much on engine speed as it does on engine torque.  Anyone who is not convinced of that, need only discover for themselves that at any of the various vehicle speeds where the transmission will permit you to choose between two equal-torque points on opposite sides of the torque peak, the acceleration will be dramatically greater in the lower of the two gears.  It is logically dubious to infer, from the fact that the peak power does a poor job of revealing the engine’s performance at low and moderate engine speeds, that torque is the true indicator of engine performance."

[kos]

Torque is:


When you get up in the mourning with a "piss hard on" and when you go to aim it down towards the toilet... your heels raise off if the floor.


KOS

[Joksa]

what power band would you prefer to be in on an average track (not too short, not too long), with a CR transmission allowing you to stay in that sweet spot as much as possible? peak torque or peak HP?

As much HP as possible without narrowing the torque curve too much. Quite optimal would be if HP peak rpm would match peak torque rpm with every upshift.

[sangyo soichiro]

I remember having this argument on this board before.

Few people understood that torque and horspeower are different words for ways of expressing the same thing.
Power, and power over time.

This guy thinks he's right because most american (read: harley) v-twins don't do well over 5252 RPM. They become inefficient at higher RPM, so their dyno sheets show crap above that, so they make the assumption that 'torque' is more important than 'hp'.
Easy mistake for people who don't like to think very hard.

Torque and HP are different but related.  Torque is a force.  Horsepower is power.  Torque over distance produces work.  Work over time is power.

A force over a given distance always does the same amount of work.  However, the power produced depends on the time required to do that work.

Torque is work, and hp is how fast you can do the work.  (simply)

I'm not exactly sure what you guys were trying to say, but I don't entirely agree with the parts I marked in red.

Torque is not exactly a force, it is the normal component of a force multiplied by the distance from the point of rotation to the point where the force is applied.  The torque acts in such a way so as to produce a rotation.

The statements in red about torque's relation to work are not entirely correct.  Torque is not work.  Torque and work have the same units (Force * distance), but in the case of work, the force is applied in the direction of distance, while in the case of torque, the force is applied perpendicular to the 'distance'.  Since they have the same units they are easy to confuse.  






As far as the original question (torque vs. hp), I would think that it's the power that gets you down the road.  Since motorcycles are wheel-driven, torque plays a role.  But, for example, a jet's capacity to 'go' is thought of in terms of thrust, which is a force.  For the jet (or rocket),

horsepower = thrust (lb) * speed (ft/sec) / 550

In the case of the jet, torque doesn't factor in.

It is the power that is dominant in enabling you to 'go'.  In other words, your bike has the greatest capacity for acceleration when the horsepower is peaked, not when the torque alone is peaked.  For a wheel-driven vehicle, it's the combination of torque and RPM that gives you that power.



Let's try this with your friend Paulages...
Going strictly from definitions:

acceleration = force/mass   (Newton's second law)

Since the mass of your bike doesn't change (neglecting fuel consumption), we see that you get the greatest acceleration when the force is greatest.  But force is related to work which is related to power via P=W/t=F*d/t.  Using this, force can be written in terms of power like so; F=P*t/d.  

So acceleration = power*time/(distance*mass).  In terms of mathematics, "time" is 'unit time' and "distance" is 'unit distance,' mass doesn't change, so it's the power that governs the acceleration of the bike.  Sure, power is proportional to torque*RPM, but it has to be the combination of torque and RPM (i.e., power) that governs your acceleration.  (Your friend can have all the torque in the universe, but your friend ain't goin' nowhere if his RPM is zero!  Thus, it's the combination that counts, i.e., power.)


(To be fair, at low RPMs, well... it's the high-torque engine that will produce the better acceleration, but only because the combination of high torque at the low RPM produces more power than the lower torque engine at the same RPM.)




The following is for those who are curious where this equation comes from.
******************************************************************
The following equation (which was previously mentioned) relates hp to torque:

hp = torque*RPM/5252


Here is the derivation:

First some definitions: (* means 'times', or in other words, multiply,  / means divide.)

work = force*distance : (the force in the direction of the distance)
torque = force*radius : (the force perpendicular to the radius)
power = work/time  
hp = 550 lb*ft /s : (1 hp is moving 550 lbs 1 foot in 1 second)
circumference of a circle = 2*pi*radius
RPM = 60*RPS : (conversion between rounds per minute and rounds per second)

Here we go...

power = work/time                          : start with the definition
         = force*distance/time              : replace work with its definition
         = force*(2*pi*radius)*RPS        : distance is the circumference of our wheel, time in RPS
         = force*(2*pi*radius)*(RPM/60) : convert RPS to RPM
         = force*radius*RPM*(2*pi/60)   : rearrange
         = torque*RPM*(2*pi/60)           : torque=force*radius

We are almost done.  The above equation has the units [ft*lb/time].  But horsepower is 550 ft*lb/sec.  So we must divide what we have above by 550, and we get

hp = torque*RPM*(2*pi/60*550) ≈ torque*RPM/5252
******************************************************************