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Hello all.

Here's what I know: HP is a measure of how fast the machine can go, while torque is a measure of how quickly it can get you there. Low-end torque is like a diesel truck, while high-end torque is like a Vette. I understand that.

But now that I am looking into a more powerful bike, I need to know how to compare apples with apples. I have a book that says that the displacement of your engine does not matter when it comes to power. In other words, the cc-size is not important. I don't get that.

Also,what does it mean that a machine has peak engine torque of, say, 82 ft-lbs. @ 8000 RPM? And how would that compare with 113.5 ft-lbs. at 7,500 RPM? Would the latter be faster at higher speeds, but the former be faster at slower speeds?

And what does this mean: Peak engine HP: 103 HP @6800 RPM?

And why don't all manufacturers list the peak enging torque and HP?

Wow, that is a lot of questions. Sorry about that. But inquiring minds want to know.

Thanks.
 

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OK. First of all engine size affects power, end of story. Its the volumetric efficency of the engine that will drastically change the amount of power made for that engine size. Which is basically saying the larger the explosion that can be created inside the engine for the amount of compression space avaliable the better.

Second. On the torque rating, all it is saying is that at that particular rpm the engine is making so much torque. Torque is the amount of force it takes to rotate an object. Which through a long complicated math equation translates into acceleration. So at 7500RPM you are going to have more acceleration capabilites than at 8000RPM.

Third horsepower is the amount of work done(torque in this case) over time(RPM in this case). So in other words its how fast something can be done. The higher the RPM, the faster the engine is working, so the faster you are able to go until you hit the HP peak point. Which means after you hit a certain RPM you are no longer able to go any faster in that particular gear.
 

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First of all engine size affects power, end of story.
That's not quite the end of the story. Engine configuration has a lot to do with it as well, especially when you are talking about motorcycles. A high-revving 600CC four cylinder will put out nearly twice the horsepower of a low revving 1200CC V-Twin. Torque can generally be estimated by the displacement, but engine RPMs and number of cylinders have a lot more to do with horsepower.
 

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I have a book that says that the displacement of your engine does not matter when it comes to power. In other words, the cc-size is not important. I don't get that.
CC size is important, but only part of the equation.

Here's my best advice:

Unless you are a professional racer, don't get hung up on the numbers. There are many factors which determine how a bike can get from point A to point B. Some are good in a straight line, while others are better through the twisties, and yet others are built for the track. Some are made for long cruises, some for hops around town, some for riding a trail in the mountains. The list goes on and on.

Determine what you will be using the bike for, and find the right tool for the job. There will be a lot of choices in every category. Just about all of the bigger machines will run circles around any of the traffic on the road and be plenty maneuverable.
 

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Ok slow down a bit. Torque is the twisting force produced by the engine. Horsepower is the rate that work is being done. These may seem like the same thing and they are closely related. Horsepower is directly proportional to the torque and engine rpm. What this means is that an engine that producing 100 ftlbs of torque at 8000rpm will be making twice as much power as an engine producing 100 ftlbs of torque at 4000rpm.

The formula for torque in ftlbs and power in HP is:
1 HP = 1ftlbs * rpm / 5252

The 5252 is a constant based on the size of the units. It only works for ftlbs and horsepower. All engines at 5252rpm will produce the same torque in ftlbs as they do power in horsepower.

Ok so in summary acceleration is based on power. The more power you're producing the faster you will accelerate. BUT if you've ever looked at a dyno chart you'll notice that power isn't constant across all rpm. If your engine is supposed to produce 120hp at 14,000rpm that is completely irrelevant if the engine is currently only turning at 5000rpm. At 5000rpm it might only be making 40hp. The momentary power (the power you're making RIGHT NOW) is based on torque (the torque at that rpm, not peak torque) and engine rpm.

Because of the way torque helps determine horsepower it is often given alongside the power number because if you know the amounts and rpm for both peak horsepower and torque you can get a good estimation of how the engine will perform across its rpm range. It isn't as good a picture as if you had the actual dyno chart but it's better than the peak power number alone. For example an engine that produces its peak torque at close to the same rpm as peak power (torque will ALWAYS peak first) will usually be a peakier engine that has a more narrow powerband and will need to be downshifted for decent acceleration. An engine with its peak torque at a low rpm will have a wide powerband and make more power at a lower rpm.
 

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While it may seem that torque is the more significant figure you have to keep in mind that torque is measured at the flywheel and is multiplied through gearing. If you have some gears set up that are geared down 2:1 and put 10ftlbs of torque in then the output will have 20ftlbs of torque.

So say you have an engine spinning at 4000rpm and producing 100 ftlbs off torque. When you plug the numbers into the formula you get:
100ftlbs * 4000rpm / 5252 = 76.16 hp

Now say you have another engine that produces 50 ftlbs of torque at 8000rpm. Plug the numbers in again:
50ftlbs * 8000rpm / 5252 = 76.16hp

If both vehicles are traveling at the same speed then the second one must be geared twice as low (say 4000rpm at 60mph vs 8000rpm at 60mph). Because gearing twice as low means multiplying the torque by 2 compared to the former this means that both vehicles are producing the same amount of power and the same amount of torque AT THE GROUND.

No matter what you plug in, given a certain hp figure you will always come out with the same torque to the ground at a given speed because horsepower is nothing more than torque adjusted for gearing.

So if you want to calculate acceleration (which is pretty much what we're all interested in after all) you can do so with either the torque function (torque curve) adjusting for gearing, or you can do the calculation more easily with the horsepower curve because gearing is already factored in.
 

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Hello all.

Here's what I know: HP is a measure of how fast the machine can go, while torque is a measure of how quickly it can get you there. Low-end torque is like a diesel truck, while high-end torque is like a Vette. I understand that.

But now that I am looking into a more powerful bike, I need to know how to compare apples with apples. I have a book that says that the displacement of your engine does not matter when it comes to power. In other words, the cc-size is not important. I don't get that.

Also,what does it mean that a machine has peak engine torque of, say, 82 ft-lbs. @ 8000 RPM? And how would that compare with 113.5 ft-lbs. at 7,500 RPM? Would the latter be faster at higher speeds, but the former be faster at slower speeds?

And what does this mean: Peak engine HP: 103 HP @6800 RPM?

And why don't all manufacturers list the peak enging torque and HP?

Wow, that is a lot of questions. Sorry about that. But inquiring minds want to know.

Thanks.
If you have a well with a crank turning a shaft making the bucket go up and down, you have a good demo of torque and horsepower. The twisting force you put on the shaft by trying to crank the bucket up is at any given instant the actual torque. Let us say the amount of torque required to make the bucket move is 1 foot/pound. That means that if the pump handle is 1 foot long, 1 pound of weight on it will move the bucket. If you put that 1 pound of torque on it and raise the bucket 10 feet in a certain amount of time, you have done a certain amount of work. That work is called horsepower. Of course my numbers are not the ones used, but that is the idea. So, if the bucket gets raised 10 feet in one minute, that would be using twice the power as raising the bucket 5 feet in the same amount of time. So an engine that can maintain a certain torque at a greater rpm would be doing more work, and therefore more rpm is greater hp. But torque is the same twisting force whether the object is moving fast, slow or not at all.. So they run an engine through it's rpm range and see where the most torque is generated. They run it again and see where the max torque is maintained at the highest rpm. That is the peak hp.
 

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Kevin Cameron wrote an article on the subject. It is searchable.
Torque or grunt, accelerates the bike. Horsepower overcomes the increasing wind resistance, especially over 90mph.
If it is a small engine like the 600 sport bikes, with a lot of HP like 120, it will not have a lot of torque, so will need a lot of gears to be used a lot. But okay for higher speeds of 120 plus.
If it is a bigger engine with a longer stroke ( stroke is the critical function ) it will accelerate well, and not require a lot of shifting, but may top out at say 102 mph.
Longer stroke creates more torque, but puts more strain on the engine at higher revs, say above 7000, as the piston speed gets too great, and the stopped and staring of the piston creates more strain on the rods and crank.
A 400 pound bike with 100 hp will be a good quick bike. Add a rider of 175 pounds and figure out how many pounds each horsepower has to push. 5.75. A good sport bike will be a lot less, a heavy tank a lot more. UK
 

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There is an old saying that goes something like this:

HP is how fast you get to the wall
Torque is how hard you hit it
 
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