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Old 08-02-2001, 05:45 PM   #1
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Horsepower vs. Torque

Definition of HP
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In Watt's judgement, one horse can do 33,000 foot-pounds of work every minute. So imagine a horse raising coal out of a coal mine as shown above. A horse exerting one horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1000 feet in one minute, or 1,000 pounds 33 feet in one minute, etc. You can make up whatever combination of feet and pounds you like - as long as the product is 33,000 in one minute and you have a horsepower. You can probably imagine that you would not want to load 33,000 pounds of coal in the bucket and ask the horse to move it one foot in a minute because the horse couldn't budge that big a load. You can probably also imagine that you would not want to put one pound of coal in the bucket and ask the horse to run 33,000 feet in one minute, since that translates into 375 miles per hour and most horses can't run that fast
Definition of Torque
Quote:
Imagine that you have a big socket wrench with a 2-foot-long handle on it and you apply 50 pounds of force to that 2-foot handle. What you are doing is applying a torque, or turning force, of 100 foot-pounds (50 pounds to a 2 foot long handle) to the bolt. You could get the same 100 foot-pounds of torque by applying one pound of force to the end of a 100-foot handle or 100 pounds of force to a one-foot-long handle.
Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft. A dynomometer measures this torque. You can easily convert torque to horsepower by multiplying torque by RPM / 5252

hp= torque x RPM/5250

The torque produced by the engine is what is moving your car, and horsepower how fast its being produced.
If you see the equation, you have to increase either torque or rpm to make more power.


Every type of power delivery has advantages and disvantages , but for STREET USE, I found that torque is more useful than HP....

The S2000 is fast yes, but it's also a car that has to be revved like mad to get the 240hp, and to keep the engine at their peak operational rpm you need a close ratio gearbox, so you have to shift a lot...

The Viper is also fast, but with ample reserves of torque on tap, you can drive around easely and you can use longer gears that gives you better fuel economy.....
For example the S2000 vs M roadster:
S2000: a fast car but for you to enjoy the 240hp you need to reach 8500RPM, someting that you cant enjoy all day, and with such a small powerband you need a close-ratio gearbox, and with less than 160lb-ft torque you better have a light car like the S2000.
On the other hand we have the 240hp M roadster that produces 240hp@6000 but with 236 lb-ft @3800 RPM, both cars are similary fast off-line(even when the M is 300lb heavier) but really what car do you think is more easy and enjoyable to drive around(not the whole car just the engine), thatīs why Im asking if it worth something making all this sacrifices in order to get incredible hp/liter figures......fuel ecconomy is not an excuse since usually the big-displacement/lower revving engines sucks as much gas as the small displacement/high revving, in this case the S2000's 2.0 gets 20/26 mpg and the M roadster's 3.2 gets like 19/26 mpg.....so again why to bother???
Dont get my wrong hight-rpm hp low torque cars can be very fast , look at the Modena for example, but most of these cars weights at or less than 3000lb, so what happens when you have a 4000lb car, hey im the kind of guy who likes hot performance , a lot of comfort and a lot of luxury and this doesnt come in a 3000lb car so again for me torque is king!!!!

p.s. Besides you can make a hell of burnouts with massive torque!!!!
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Old 08-03-2001, 02:49 AM   #2
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As my siginature says.

'Horsepower sells parts, but Torque wins races!'
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Old 08-05-2001, 09:21 AM   #3
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and thus, I'll never own an S2000
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Old 08-05-2001, 09:26 AM   #4
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"Most Horses can't run 375 Miles per hour"


Thanks for that conversion forula for HP-TRQ and TRQ-HP.
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Any car built by "Dr. Technology" is probably not worth $5000
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Old 08-07-2001, 09:14 PM   #5
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Interesting descriptions there. Thanks, I know some people who'll enjoy those analogies.













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Old 08-09-2001, 11:47 PM   #6
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they're from www.howstuffworks.com if you guys want to look at them some more...
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Old 08-17-2001, 11:25 AM   #7
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To echo carman's sentiments...
Peak acceleration in any given gear will occur at peak torque, while top speed in any given gear will occur at peak horsepower. The point here is that what matters most is the "power band", or the gap between when the peak torque occurs and when the peak hp occurs. If you look at a dyno chart you'll see two curves: the first is the torque curve that peaks usually in the range of 3000rpm, and the second is the hp curve that peaks later in the range of 5000rpm. To get the fastest 0-60 time, a driver would rev the engine to 3000 (the peak torque for 1st gear), drop the clutch and start accelerating. The revs will climb until 5000rpm where the peak horsepower occurs, and this is where the driver will want to change gears to second (dropping back to around 3000rpm and climbing again to 5000rpm). If the driver decided not to change gears yet, but continued to climb in revs, he will find that his 0-60 time will be less, since he is not using the engine to its full capacity.
Comparing the Honda s2000 (240hp@8300, 153 ft/lb@7500) and the BMW Mcoupe (240hp@6000rpm, 236 ft/lb@3800) you will notice that the Mcoupe has a much wider power band than the S2000 (2200rpm vs. 800rpm). This is an incredible difference. What this means is that the driver in the Mcoupe can stay in each gear LONGER than the driver in the S2000. Generally this results in faster 0-60 times even though the horsepower is the same (Mcoupe does it in 5.1 seconds, S2000 does it in 5.5 seconds).
But more importantly, having a wider power band is simply more fun to drive. You just stick it in a gear, gas it, and the engine will pull pull pull for a quite some time before you have to shift gears. You shift much less often and just focus on the driving. The guy in the S2000 has to shift and rev like mad to keep up.
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Old 08-18-2001, 04:46 AM   #8
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This is a good topic, it's been discussed on every automotive message board in existence and now I guess it's our time to flesh it out. So here's my thoughts...

First of all, the definitions of torque and horsepower given above are very accurate. But to add just a bit here, think of torque as Energy (rotational energy). Energy is defined as force x distance here, such as the 50 lbs of force applied to a 2ft lever arm in the earlier example. Further, think of horsepower as Power, which for this discussion is energy x time. 1 horsepower is equal to 550 lb/ft of torque per second, or simply "torque over time".

The most important thing in this discussion is examining why horsepower is the single most important factor to a vehicle's accelerative ability (or more truly, the vehicle weight to horsepower output ratio), yet has no direct application to the physics of acceleration. To understand this, you must first understand what makes a car accelerate.

Acceleration is a time based event, but the force (energy) behind it is NOT. The energy of acceleration is not just torque either, it's effective torque to the wheels, which is found by multiplying engine torque at a specific RPM by the gear ratio you are in and the final drive ratio (shown here in this example)....
Example A:
first gear transmission ratio: 3.42:1
final drive ratio (differential reduction): 4.27:1
engine torque at 5000 RPM: 150 lb/ft
150 x 3.42 x 4.27 = 2190.5 lb/ft effective torque

So in that example, our meager torque output is greatly magnified to about 14.6 times it's original amount of energy. Just as the 2ft lever arm in the earlier example multiplied the force input by 2, the gear reduction here multiplied the torque output by 14.6. Now if you know gearing, you also know that the final drive ratio is fixed for a given differential, but the higher the gear you select, the lower (numercially speaking) the transmission gearing becomes. This is why acceleration always feels weaker in the next higher gear; torque multiplication through gearing is reduced, thus reducing the energy available to accelerate the car.

So why is any of this important? Because, if you understand how important gearing is to the equation, you understand why torque output, taken by itself, is meaningless in estimating accelerative ability. A typical deisel engine in a car outputs near to (or over) 300 lb/ft of torque, yet is almost never as fast as gasoline engines making 100 lb/ft less torque at peak. This is because deisel engines cannot run at high RPM, and therefore cannot take full advantage of torque multiplication through gearing. Since most cars are geared to redline at similar speeds in a given gear (usually around 35-40 mph in first, 60-70 in second, etc...), the engine's redline and power output at high RPM are actually indicative of it's gearing. So let's say we have two engines, Engine A makes 300 lb/ft of torque at all RPM and 300hp at redline, while Engine B makes 150 lb/ft of torque at all RPM and 300hp at peak. Given the earlier math that hp = torque x 5252/ RPM, one can see that engine B must be turning twice the RPM as Engine A to make the same amount of horsepower with only half as much torque to work with. And this is extremely important to realize because, if Engine B's drivetrain is geared exactly twice as short (numerically higher), the effective torque to the wheels will be precisely the same at any speed relative to the Engine A drivetrain. Which means that, given the vehicles weigh the same and we throw out variables like drag and traction, they will both have EXACTLY the same accelerative energy available to move the car forward. Which means they will both accelerate at exactly the same rate, even though the first motor makes twice the energy of the second.

And this is why cars like the S2000, what with it's meager torque output, can run side by side a car of around the same horsepower but with much more torque. To state this simply, horsepower ratings are like a clean summed up guesstimate of a car's accelerative ability, because they nicely package in torque and gearing into one number. And while this is sligthly oversimplified (given I only have so much room to type), that really is how it works. Look at any two cars with about the same horsepower output and you will see they both have about the same acceleration curves... regardless of torque output. Although the S2000 only has about 150 lb/ft of torque to work with at peak, it's gearing makes full use of this torque to motivate the car. Which is why it still feels quick at 4000 RPM, when it's a full 3500 RPM away from it's torque peak.



So to sum up, torque is the only thing an engine produces. It is the direct measure of instantaneous energy the engine makes to provide acceleration, and is the only thing directly important to the physics of acceleration. And yet it is also the most useless single number to look at when judging accelerative ability of a car, since 1000 lb/ft of torque means nothing unless you've got gear multiplication (read: RPM ability) behind it.

Conversely, horsepower output is a direct derivative of torque, and has no direct bearing or application to the physics behind acceleration. Yet it is the single most important number in judging the accelerative ability of a vehicle, because it so nicely packages torque output, RPM ability and gearing into one roundabout number.

And this strange dichotomy is the reason why manufacturers always give both figures, and why people will continue to post excellently informative threads like this on every atuomotive message board across the world. So after reading this, all I hope is that you come away with a newfound respect for both horsepower and torque numbers, and do not immediately dismiss any engine for focusing upon healthy production of one over the other. After all, everything has it's application, the S2000 motor is a gem because it packages 240hp into a 320 lb engine/transmission combo, while the BMW 3.2L I6 weighs much more but has a wonderfully broad and muscular torque curve. Hope this helps explain things, peace.


Ps-As one last thing, I want to say that increasing torque output over a large RPM range continues to be the most effective way to increase a vehicle's accelerative ability. You can increase horsepower output without chaning maximum or even average torque levels (by moving the powerband to a higher RPM range), but this will never be as effective in cost or extra speed as simply making more torque. And that's because you have to adjust gearing to take advantage of this newfound RPM potential, which isn't cheap. This is why forced induction setups are so effective at improving acceleration with minimal cash outlays, and why making an NA car go as fast will take MUCH more time, effort and money to acheive.
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