Difference between HP and Torque

[upthemaiden]

So I've tried to do as much research as I could about the subject, but I'm still confused about the big fuss over horsepower. I understand that HP is the torque x RPM divided by 5252, so the rpm plays a part in it, but shouldn't the length of the stroke have something to do with it if we're counting RPM??

Ever since I started looking into Buells everyone talks about how nice it is to have the torque instead of hp, but they're not too different things, one is just a part of the other. I've really only spent a lot of time on an i-4 600, so I understand how those engines feel, and someday when I hopefully find myself a buell(still in the market) I'll really get to feel the difference in the engines. From what I get, torque is just torque. If you're making 70ft lbs of torque at 5000rpm, or if you're making 70ft lbs of torque at 9000rpm, what is it that makes the bike faster at 9000rpm?

If the key to making a bike stronger is higher rpms, why didn't buell just put a shorter stroke on the sportsters engines so they could spin faster(I know it's not as simple as I put it, but still)?

Everyone always talks about how the xb engines run out of rpm so fast, you it's so easy to bounce off the rev limiter, but I've never had any problem bounding my last 14k rpm bike off the limiter, so I don't think that's anything to do with the RPMs. Given the xbs longer stroke, I feel like the longer stroke would mean it would take longer to get to the lower redline. If you took the numbers off of the tach and all you knew was that the needle could go 180 degrees before you hit the redline, would you even know how long the redline was?? I mean I fully grasp that the twin will have more power down low, and the i4 will have more power higher in the rpms, but anything beyond that is still above my head.

Anyone that can shed some light on this for me??u

 

[Alfatango1]

To over simplify torque is motivational force, the thing that moves objects. HP is just a type of measurement, like a pound or ounce is. The more torque the better. RPM is of course stated over time. So if Buell were to shorten the stroke on the V-twin sure you would get more revs but loose volume. So when an engine is a 4 cylinder as opposed to a 2 you not only get the fast spinning rpm's that you want but also the volume. As we know we loose low end grunt but allow the engine to build speed through the 14K or whatever range a multi has.

 

[browland]

I like to think of it as torque is shear balls out tear off the line front end wont see the ground snatching power... horse power is Jesus I'm going 130 miles an hour and this thing is still puling warp speed kinda power [cool]

 

[Alfatango1]

I just found this article, still reading it but it's a lot more technical then I am so I'll post it for you all to digest:
http://motorcyclebloggers.com/tech-talk-articles/twins-versus-inline-fours/

 

[konarider94]

so torque is a measure of work. hp is a measure of the rate of work. so even though an I4 doesn't do much work per cycle it has a lot of cycles, which gives it a high hp rating. a v-twin does a lot of work per cycle but it doesnt happen very fast.

so essentially riding around town and coming out of corners is much more enjoyable on a v twin because you feel the torque instantly. on an I4 your usually not cruising around at 10000 RPM which is where it makes its power. its all about where you want your power to be. its pretty hard to enjoy the peak power on an I4 bike at any legal speed. Whereas anytime you twist the throttle on your buell it should make you smile.

 

[theoctopus]

I hate to get technical but torque is not a measure of work. It's a measure of rotational force. Work would be utilizing that torque to rotate an object a certain distance.

 

[konarider94]

I hate to get technical but torque is not a measure of work. It's a measure of rotational force. Work would be utilizing that torque to rotate an object a certain distance.

let me rephrase then, torque is the measure of an engines ability to do work

 

[theoctopus]

Horsepower is the measure of the engine's ability to do work (i.e. energy).

Torque is simply a force acting on a lever arm a certain distance away from a point of rotation.

Simply, torque is the ability to accelerate. Horsepower is the rate at which the energy conferred to the system by torque is converted into kinetic energy (speed).

 

[briansiples]

The magnitude of torque depends on three quantities: First, the force applied; second, the length of the lever arm connecting the axis to the point of force application; and third, the angle between the two.

Someone correct me if i'm wrong but what i get is basically vtwins make more torque because basically with everything equal the stroke of the piston, the rod length which is the lever is longer, which will produce more torque on the force application (the crankshaft). I'm not very technical but i think this is about right. While i-4 have a short piston stroke (lever) to the force application (crankshaft?) Therefore producing less torque, but if you multiply these equations times rpms, i-4 eventaully catch up over time, which is horsepower.

So basically, if torque is work, and rpms are time, Power is the work/time. SO basically horsepower = torque/rpms, or should be something like that, but rpms isn't exactly time so i'm sure it is Horsepower = torque/ (some modified form of rpms).

 

[Bolt]

Upthemaiden-git yurself a friggin Buell& the point will be moot!
(up the who!?)

 

[NookieMonster]

hp is how fast you hit the wall, tq is how far you take the wall with you!

 

[browland]

hp is how fast you hit the wall, tq is how far you take the wall with you!

[up][up][up]

 

[konarider94]

Horsepower is the measure of the engine's ability to do work (i.e. energy).

Torque is simply a force acting on a lever arm a certain distance away from a point of rotation.

Simply, torque is the ability to accelerate. Horsepower is the rate at which the energy conferred to the system by torque is converted into kinetic energy (speed).

i will cite a definition if you want to check it at your local library. i do engine research and i have a couple I.C. engine text books.

You say power is energy and the rate at which energy is conferred to the system. so which is it? it cant be both. look at the units of work, power, energy, and torque in the SI system and its a little easier to follow.

"Note that torque is a measure of an engine's ability to do work; power is the rate at which work is done"
J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, 1988

 

[theoctopus]

I'm just going based on the classical physics that I've learned.

Power (J/s) is definitely a measure of the rate of energy conversion, which is why most people simply look at it in terms of energy.

Torque (N*m) is not a measure of an engine's ability to do work, as it is simply rotational force, and says nothing of the time period in which it can be done. I don't care what your textbook tells you. Work done by torque is calculated using a totally different equation (W=tau*theta) than the work on a linear scale (W=F*D). The confusion comes in the fact that 1 N*m = 1 J. This does not, however, make work equal to angular force.

 

[dave_xb12r]

"Note that torque is a measure of an engine's ability to do work; power is the rate at which work is done"
J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, 1988

That confuses me. The ability to do work and rate of work sounds like the same thing to me. The more work is done (rpms) the more hp increases. If it's doing the work, the ability should be the same. So why are torque values different from HP values as far as percentages go?


Buells have increasing HP, and almost a flat torque value. When i4's have no torque value and even faster increasing HP. (When I say faster, I mean by time, not RPM percentage)

 

[upthemaiden]

Wow, this thread really took off. That's what I get for taking a nap.

This is the usual scenario, the more I read about it, the more confused I become. The main issue to me still sound as though the power made is increased as rpms are increased, so if that's the case, why would anyone chose to make an engine with a long stroke, if limiting the rpms is only going to limit your power??

Is there much of a difference between the way the power feels between an inline twin compared to the v-twin in a buell?? I fixed up my wife's old gs500 last summer so I'll have something to ride this year until I get myself another bike. Maybe riding that one will give me a better idea of the situation. The red line is a little higher than an xb engine, but it's still a twin.

and Bolt: it's for Iron Maiden

 

[briansiples]

The main issue to me still sound as though the power made is increased as rpms are increased, so if that's the case, why would anyone chose to make an engine with a long stroke, if limiting the rpms is only going to limit your power??

Well...for one torque is lower with i-4's because the stroke is shorter, which is why you have to create greater rpms to get the power you want out of it. This also explains why there is a "power turn on point" or "sweet spot" with inline 4's, essentially the engine has reached a certain rpm for the stroke to turn out substantial power . With a longer stroke, more torque is created at half the rpm, which gives you acceleration throughout all rpms. I hope that simplifies it some. So while having a longer stroke does limit your high end power (horsepower) it gives you way more low-mid end power (torque).

 

[konarider94]

rate implies per unit time. torque is how much work is done per cycle not time. so a v twin does large amounts of work slowly but an i4 does small amounts of work very fast. eventually the i4 will win out in power because it can do work so fast.

the torque of any engine begins to fall off at higher rpms at some point due to cam profile and friction. any cam will produce a high peak in a small range of rpms or a lower peak across a broader range of rpms. cam design can dictate where the peak is. this is where the benefits of having variable valve timing come into play but little can be done to vary the amount of lift and duration. this is why power begins to fall off too. yes the speed is increasing which should increase power but torque continues to fall off due to flow limitations and increasing friction.

engine geometry limits the v twin to a certain rpm, the i4 geometry is better suited for higher rpm operation. designing engines is all about trade offs. you cant have everything. the type of riding that i do i prefer to have the low end grunt. other riding styles would prefer the high end hp of the jap bikes.

idk if i answered your question or not dave. ive completed one IC engines course, a senior level undergrad class. currently i am taking a graduate level course in IC engines. Additionally i am doing research on engines. I am always learning more and obtaining a better understanding of how this stuff works. There are a lot of things going on in an engine that i never knew about.

edit: brian is also pointing out the limitations of engine geometry. i would agree with the trends he is describing, but the sweet spot in an I4 is most likely due to the cam profile

 

[upthemaiden]

Well...for one torque is lower with i-4's because the stroke is shorter, which is why you have to create greater rpms to get the power you want out of it.

That's the one thing I do already get, and I also understand the general difference between the way a twin and an i-4 puts out power as far as the power curve, but I'm still confused as to why someone would chose to make a long stroke motorcycle engine when it's going to limit the hp? The long stroke sounds fine to me for cruising around, but obviously one of the biggest complaints you ever hear about buells is their lack of power. Erik eventually had the 1125 motors designed that are putting out 150hp, so he must've gone with a shorter stroke. I'm curious why he couldn't have just done that to the sportster motors considering they put out plenty of torque.

I think it's just one of those subjects I'll have to accept not understanding.

 

[briansiples]

well you have to remember buell tried, harley didn't let him...and whatever technology he made, harley got first dibs at it. And not all v-twins have that issue. Look at ducati. They don't use valve springs but "desmodromics" which allow for very high revs with high torque, and high horsepower, even though they still use a larger stroke. This of course is why ducati wins a lot of championships, tons of torque and hp. Hp is only good if you can use it i.e straightaways.