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  #1  
Old 7th April 2009
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aswracing aswracing is offline
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Default Torque, rpm, and power ...

I know we've been over this a bazillion times, but I have a new analogy that I believe conceptually illustrates the relationship between torque, rpm, and power.

Let's say you're digging a ditch. What affects how fast you get it done?

The size of your shovel scoop has an effect.

How rapidly you're scooping dirt has an effect.

You might have a great big shovel, but only taking one scoop every minute. You're riding a Harley. Lots of torque, not much rpm.

On the other hand, you might only have a hand trowel, but you're taking a scoop once per second. You have a 600 sportbike. Not much torque, but lots of rpm.

Ultimately, if you're racing your buddy next to you who's also digging a ditch, what matters is not how big the shovel scoop is, or how how rapidly you're scooping. What matters is how fast the ditch is getting dug. If you can get it done faster with a smaller scoop moving faster, you win. So it's not the size of the scoop that matters, or the rate you're scooping that matters, it's the combination of the two that matters.

You can predict who will get it done faster if you take the size of the scoop (torque) and multiply it by the rate you're scooping (rpm). The result is a number that represents how fast the ditch gets dug (power).

People love to generalize about torque and power, saying things like "torque is what accelerates" and "power is what maintains the speed". These kinds of statements are, at a minimum, misleading, and only serve to confuse people.

People, especially Harley people, also love to talk about the torque number like it's a measure of the performance. But it's not, it's only half the equation. RPM is every bit as important when it comes to evaluating performance. And when you talk about torque and rpm in combination, we use the word "power" for shorthand. That's literally what power means.

The other thing, too, is that with an engine, it's very very simple to change the scoop size into the rate or vice-versa. Gear something down, you raise the torque (scoop size) and reduce the rpm (scoop rate). That's why you can wheelie in first gear but you can't go very fast. Likewise, gear something up, you reduce the torque (scoop size) and increase the rpm (scoop rate). That's why you can't wheelie in top gear, but you can move down the freeway at 75mph. Power is the same in either case, it's just got a different makeup.

Nevermind me, I'm just bored.
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Old 7th April 2009
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Great analogy....I just stuck this....I think people should see it!

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  #3  
Old 7th April 2009
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that's a very good analogy Aaron.......I'm sure I'll use that in the future as that subject seems to come up a lot and is usually mis-understood.......

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Old 7th April 2009
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So what size shovel do you recommend for maximum scoop rate ?

J/K

Great analogy and should help some people understand what their talking about.
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Old 7th April 2009
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Default Power

What a Great word picture, Best simple explanation Ive ever heard! I believe instead of being a Torque addict I must just like my Power low in the RPM range on the street.
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Old 7th April 2009
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Cantolina is right about stickying your explanation, it's the best one ever.
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Old 7th April 2009
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Quote:
People love to generalize about torque and power, saying things like "torque is what accelerates" and "power is what maintains the speed". These kinds of statements are, at a minimum, misleading, and only serve to confuse people.

People, especially Harley people, also love to talk about the torque number like it's a measure of the performance. But it's not, it's only half the equation. RPM is every bit as important when it comes to evaluating performance. And when you talk about torque and rpm in combination, we use the word "power" for shorthand. That's literally what power means.
Going back to shovels and scoops. You don't think a bigger shovel has advantages in certain cases? For example, the initial scoop. At first scoop the bigger shovel will scoop more dirt and put it in the lead and the smaller scoop won't catch up until later.

It seems that with the bigger scoop you will always start in the lead immediately until the smaller scoop catches up an finally exceeds the larger scoop because of digging faster.

If you consider the dirt as the distance you need to cover. What if the distance is short? So the big shovel will be able to scoop all the dirt in one scoop and be done while the shovel with smaller scoops will need to play catch up. For smaller piles it seems in certain scenarios the bigger scoop will have an advantage.

Then there is the problem that unfortunately the rate isn't static, it is variable. So the larger scoop appears to have an advantage up to a certain speed until the smaller scoop can catch up and exceed the bigger scoop.
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Old 7th April 2009
NRHS Sales NRHS Sales is offline
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Cbass,
You are assuming that the big scoop is done quickly which is wrong. The big scoop takes a very long time to dig into the ground, then pick it up and dump it. Until it dumps it you have nothing. During the same time the small scoop is moving lightning fast and has scooped many many loads.
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Old 21st April 2009
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Default

This is a very nice writeup by aswracing about torque, rpms, and power.

I'd like to add something about these three functions of an engine.

Torque is a function of an engine's Piston Displacement. If you increase the displacement by 20% you will increase it's torque by 20%. This assumes that you built both engines out of the same quality materials and used the same engineering designs.

Max rpm's is a function of 1/Stroke. If the stroke goes up 10% the rpms go down 10%. This is because the piston's acceleration is what determines how high you can rev an engine and the usual substitute for piston acceleration is the average piston speed. The piston speed is generally 4,000 ft/minute for an engine build from stock parts and with a longevity of many 10's of thousands of miles. For drag racing, 5,000 ft/min can be used because the longevity isn't anywhere near as long as for a street engine. At 4,000 ft/min a 4" stroke can rev to 6,000 rpms and a 2" stroke can rev to 12,000 rpms.

Max rpm's is also a function of 1/(Power Bandwidth). Power bandwidth is the low rpm power divided by the max power so the higher your low speed power is the higher your power bandwidth is for the same max power. Power bandwidth is very important to street drivers, especially with heavy bikes. Anyone driving on the street in stop and go traffic with an 800 lb. touring Harley would rather have a 80 horsepower, 1600cc engine than a 125 hp, 600cc engine because the 1600cc engine will rev lower and therefor have a higher power bandwidth making it easier to drive around town.

The power of the engine is a function of the engine's Piston Area. If you increase the engine's piston area by 30% you will increase the power by 30%. Now this assumes that you also increase the valve size (which you can't do if you increase just the engine's stroke), increase the port size, increase the throttle body and muffler sizes. The engine’s displacement has nothing to do with it’s power, only it’s piston area has to do with it’s power.

To sum things up. The important parts of an engine are it's Piston Area and it's Stoke used separately to determine the engine's Power and Power Bandwidth. When multiplied together, Piston Area and Stoke give Piston Displacement but that looses the information about Power and Power Bandwidth and only gives you Torque, which has nothing to do with acceleration. Torque times Gear Ration has to do with acceleration but Torque times Gear Ratio IS Power.
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Old 21st April 2009
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Digging a hole, hmmmm... interesting analogy... something bothering you?
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