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| 02-13-2008, 02:57 PM | #23 |
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Hm, I am frankly a little confused here, lot of valuable technical info, but I guess I am missing the background to fully understand it.
Maybe I can ask a simpler question which makes the answer a little more comprehensible to me. One of the funnest car I ever remember driving was a 1978 320i when I was a student. I remember the first time I sat behind the wheel I was completely taken over by the feel of that car. It had such a sense of urgency that it felt it wanted to go and it was always going to slip from under me. I am not sure what this quality needs to be attributed to. Was that the high reving concept or was this a torqy car for its time? I recently test drove the E39 M5 and though it's low end torque was mesmerizing it did not feel like that old 320i , it was more civil and controllable. I just want to get that feel back again and I am not sure which car is going to provide it. Hope I making some marginal sense  |
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| 02-13-2008, 02:59 PM | #24 |
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It could be attributed to many things, but weight must have had a lot to do with it as in power/weight and torque/weight although I must assume the M5 you test drove had higher ratios than the 320 you are referring to. Also, a lighter car will inherently be more nimble in handling regardless of how much power or torque its engine produces.
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| 02-13-2008, 04:07 PM | #27 | |
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http://www.m3post.com/forums/showthread.php?t=107178 "I am 35 years old and just had a baby as well." LOL, yeah, I guess I'd be a bit embarassed to admit that I still cared what people thought at age 50. Then again, maybe you are 12? Or is it 15? Ah, the internet - its usually not the truth but at least it wastes your time.  Ok, ok, edit: I reread - the car was a 1978, maybe it was just really old. So sorry bout that |
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| 02-13-2008, 04:17 PM | #28 | |
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 But yeah as I mentioned I was a student and the year was 1991 when I could barely afford the old 320, I really wouldn't lie on an anonymous forum but nice try  |
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| 02-13-2008, 04:24 PM | #29 | |
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| 02-14-2008, 09:52 AM | #31 | |
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I originally wrote that note in reply to a series of misconceptions in a string way back in 1993. It somehow got picked up and posted on the Vettenet site. It's now on around 100 sites or so, and has been translated into at least four languages. I've updated it over the years to make it of more general interest (instead of being applicable mainly to that original torque-centric string), and here's the current version, still in laymen's terms... Horsepower and Torque - a Primer There's been a certain amount of discussion, in this and other files, about the concepts of horsepower and torque, how they relate to each other, and how they apply in terms of automobile performance. I have observed that, although nearly everyone participating has a passion for automobiles, there is a huge variance in knowledge. It's clear that a bunch of folks have strong opinions (about this topic, and other things), but that has generally led to more heat than light, if you get my drift. This is meant to be a primer on the subject. OK. Here's the deal, in moderately plain English. Force, Work and Time If you have a one pound weight bolted to the floor, and try to lift it with one pound of force (or 10, or 50 pounds), you will have applied force and exerted energy, but no work will have been done. Work requires movement. If you unbolt the weight, and apply a force sufficient to lift the weight one foot, then one foot-pound of work will have been done. If that event takes a minute to accomplish, then you will be doing work at the rate of one foot-pound per minute. If it takes one second to accomplish the task, then work will be done at the rate of 60 pound feet per minute, and so on. In order to apply these measurements to automobiles and their performance (whether you're speaking of torque, horsepower, Newton meters, watts, or any other terms), you need to address the three variables of force, work and time. A while back, a gentleman by the name of Watt (the same gent who did all that neat stuff with steam engines) made some observations, and concluded that the average horse of the time could work at a rate that would lift a 550 pound weight one foot in one second, thereby performing work at the rate of 550 pound feet per second, or 33,000 pound feet per minute. He then published those observations, and stated that 33,000 pound feet per minute of work was equivalent to the power of one horse, or, one horsepower. Everybody else said okay. For purposes of this discussion, we need to measure units of force from rotating objects such as crankshafts, so we'll use terms that define a twisting force, such as torque. A foot pound of torque is the twisting force necessary to support a one pound weight on a weightless horizontal bar, one foot from the fulcrum. In fact, what standard engine dynamometers actually measure is torque (not horsepower) by using a resistance to hold the engine at a constant speed while at full throttle, and then measuring the resistance required to keep the engine from accelerating. Then we can calculate actual horsepower by converting the twisting force of torque into the work units of horsepower. Here’s how: Visualize that one pound weight we mentioned, one foot from the fulcrum on its weightless bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two foot circle), and, incidentally, we have done 6.2832 pound feet of work. Okay. Remember Watt? He said that 33,000 pound feet of work per minute was equivalent to one horsepower. If we divide the 6.2832 pound feet of work we've done per revolution of that weight into 33,000 pound feet, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 pound feet per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it's the equivalent of 1/2 horsepower (16,500 pound feet per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement: Torque * RPM ---------------- = Horsepower 5252 This is not a debatable item. It's the way it's done. Period. The Case for Torque Now, what does all this mean in car land? First of all, from a driver's perspective, torque rules, to use the vernacular. Any given car, in any given gear, will accelerate at a rate that exactly matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 pound feet of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower tends to not be particularly meaningful from a driver's “belt in the back” perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same. In contrast to a torque curve (and the matching push back into your seat), horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver feels. You don't believe all this? Fine. Take your non-turbo car (turbo lag muddles the results) to its torque peak in first gear, and punch it. Notice how the seat belted you in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Okay. Now that we're all on the same wavelength (and I hope you didn't get a ticket or anything), we can go on. Torque is What You Feel, but Horsepower Rules So if torque is so all-fired important (and feels so good), why do we care about horsepower? Because (to quote a friend), "It's better to make torque at high rpm than at low rpm, because you can take advantage of gearing." For an extreme example of this, I'll leave car land for a moment, and describe a waterwheel I got to watch a while ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft that was connected to the works inside a flour mill. Working some things out from what the people in the mill said, I was able to determine that the wheel typically generated about 2600(!) pound feet of torque. I had clocked its speed, and determined that it was rotating at about 12 rpm. If we hooked that wheel to, say, the drive wheels of a car, that car would go from zero to twelve rpm of its drive wheels in a flash, and the waterwheel would hardly notice. On the other hand, twelve rpm of the drive wheels is around one mile per hour for the average car, and, in order to go faster, we'd need to gear it up. If you remember your junior high school science classes and the topic of simple machines, you'll remember that to gear something up or down gives you linear increases in speed with linear decreases in force, or vice versa. To get to 60 miles per hour would require gearing the output from the wheel up by 60 times, enough so that it would be effectively making a little over 43 pound feet of torque at the output (one sixtieth of the wheel's direct torque). This is not only a relatively small amount; it's less than what the average car needs in order to actually get to 60. Applying the conversion formula gives us the facts on this. Twelve times twenty six hundred, over five thousand two hundred fifty two gives us: 6 horsepower. OOPS. Now we see the rest of the story. While it's clearly true that the water wheel can exert a bunch of force, its power (ability to do work over time) is severely limited. At the Drag Strip Now back to car land, and some examples of how horsepower makes a major difference in how fast a car can accelerate, in spite of what torque on your backside tells you. A very good example would be to compare the LT-1 Corvette (built from 1992 through 1996) with the last of the L98 Vettes, built in 1991. Figures as follows: Engine--------------Peak HP @ RPM-------------Peak Torque @ RPM L98---------------------250 @ 4000----------------------340 @ 3200 LT-1--------------------300 @ 5000----------------------340 @ 3600 The cars are essentially identical (drive trains, tires, etc.) except for the engine change, so it's an excellent comparison. From a driver's perspective, each car will push you back in the seat (the fun factor) with the same authority - at least at or near peak torque in each gear. One will tend to feel about as fast as the other to the driver, but the LT-1 will actually be significantly faster than the L98, even though it won't pull any harder. If we mess about with the formula, we can begin to discover exactly why the LT-1 is faster. Here's another slice at that torque and horsepower calculation: Horsepower * 5252 ------------------- = Torque RPM Plugging some numbers in, we can see that the L98 is making 328 pound feet of torque at its power peak (250 hp @ 4000). We can also infer that it cannot be making any more than 263 pound feet of torque at 5000 rpm, or it would be making more than 250 hp at that engine speed, and would be so rated. In actuality, the L98 is probably making no more than around 210 pound feet or so at 5000 rpm, and anybody who owns one would shift it at around 46-4700 rpm, because more torque is available at the drive wheels in the next gear at that point. On the other hand, the LT-1 is fairly happy making 315 pound feet at 5000 rpm (300 hp times 5252, over 5000), and is happy right up to its mid 5s red line. So, in a drag race, the cars would launch more or less together. The L98 might have a slight advantage due to its peak torque occurring a little earlier in the rev range, but that is debatable, since the LT-1 has a wider, flatter curve (again pretty much by definition, looking at the figures). From somewhere in the mid-range and up, however, the LT-1 would begin to pull away. Where the L98 has to shift to second (and give up some torque multiplication for speed, a la the waterwheel), the LT-1 still has around another 1000 rpm to go in first, and thus begins to widen its lead, more and more as the speeds climb. As long as the revs are high, the LT-1, by definition, has an advantage. As a practical matter, a typical L98 6-speed car might cover a quarter mile with an ET of around 14 seconds at around 99 or 100 mph, while the equivalent LT-1 will generally be at least a half second faster, at 104 – 105 mph. Mind you, as I’ve mentioned, the LT1 doesn’t pull any harder – just longer. There are numerous examples of this phenomenon. The Acura RSX Type S, for instance, is faster than the garden variety RSX, not because it pulls particularly harder (it doesn't), but because it too pulls longer in each gear. It doesn't feel particularly faster, but it is. A final example of this requires your imagination. Figure that we can tweak an LT-1 engine so that it still makes peak torque of 340 pound feet at 3600 rpm, but, instead of the curve dropping off to 315 pound feet at 5000, we extend the torque curve so much that it doesn't fall off to 315 pound feet until 15000 rpm. Okay, so we'd need to have virtually all the moving parts made out of unobtanium, and some sort of turbo charging on demand that would make enough high-rpm boost to keep the curve from falling, but hey, bear with me. If you raced a stock LT-1 with this car, they would launch together, but, somewhere around the 60-foot point, the stocker would begin to fade, and would have to grab second gear shortly thereafter. Not long after that, you'd see in your mirror that the stocker has grabbed third, and not too long after that, it would get fourth, but you wouldn't be able to see that due to the distance between you as you crossed the line, still in first gear, and pulling like crazy. I've got a computer simulation that models an LT-1 Vette in a quarter mile pass, and it predicts a 13.38 second ET, at 104.5 mph. That's pretty close (actually a bit conservative) to what a stock LT-1 can do at 100% air density at a high traction drag strip, being power shifted. However, our modified car, while belting the driver in the back no harder than the stocker (at peak torque) does an 11.96, at 135.1 mph - all in first gear, naturally. It doesn't pull any harder, but it sure as heck pulls longer. It's also making 900 hp, at 15,000 rpm. Of course, looking at top speeds, it's a simpler story... At the Bonneville Salt Flats Looking at top speed, horsepower wins again, in the sense that making more torque at high rpm means you can use a stiffer gear for any given car speed, and have more effective torque (and thus more thrust) at the drive wheels. In fact, operating at the power peak means you are accelerating the absolute best you can at any given car speed, measuring torque at the drive wheels. I know I said that acceleration follows the torque curve in any given gear, but at any given car speed, horsepower is the absolute governor of how fast you can accelerate. In fact, horsepower is a kind of shorthand in this context. No matter what gear you’re in or what the final drive ratio is, more power at that speed means more acceleration because you’ll have more torque at the drive wheels. I'll use a BMW example to illustrate this: At the 4250 rpm torque peak, a 3-liter E36 M3 is doing about 57 mph in third gear, and, as mentioned previously, it will pull the hardest in that gear at that speed when you floor it, discounting wind and rolling resistance. In point of fact (and ignoring both drive train power losses and rotational inertia), the rear wheels are getting 1177 pound feet of torque thrown at them at 57 mph (225 pound feet, times the third gear ratio of 1.66:1, times the final drive ratio of 3.15:1), so the car will bang you back very nicely at that point, thank you very much. However, if you were to re-gear the car so that it is at its power peak at 57 mph, you’d have substantially more torque at the drive wheels. You'd have to change the final drive ratio to approximately 4.45:1 in order to do this, but with that final drive ratio installed, you'd be at 6000 rpm in third gear at 57 mph, where the engine is making 240 hp. Going back to our trusty formula, you can ascertain that the engine is down to 210 pound feet of torque at that point (240 times 5252, divided by 6000). However, doing the arithmetic (210 pound feet, times 1.66, times 4.45), you can see that you are now getting 1551 pound feet of torque at the rear wheels, making for a nearly 32% more satisfying belt in the back. Any other rpm (other than the power peak) at a given car speed will net you a lower torque value at the drive wheels. This would be true of any car on the planet, so, you get the best possible acceleration at any given vehicle speed when the engine is at its power peak, and, theoretical "best" top speed will always occur when a given vehicle is operating at its power peak. Force, Work and Time At this point, if you're getting the idea that work over time is synonymous with speed, and as speed increases, so does the need for power, you've got it. Think about this. Early on, we made the point that 300 pound feet of torque at 2000 rpm will belt the driver in the back just as hard as 300 pound feet at 4000 rpm in the same gear - yet horsepower will be double at 4000. Now we need to look at it the other way: We NEED double the horsepower if we want to be belted in the back just as hard at twice the speed. As soon as we factor speed into the equation, horsepower is the thing we need to use as a measurement. It's a direct measure of the work being done, as opposed to a direct measure of force. Although torque and horsepower are obviously related (and each in a sense a function of the other), a good way to think about this is that torque determines the belt in the back capability, and horsepower determines the speed at which you can enjoy that capability. Do you want to be belted in the back when you step on the loud pedal from a dead stop? That's torque. The water wheel will deliver that, in spades. Do you want to be belted in the back in fourth gear at 100 down the pit straight at Watkins Glen? You need horsepower. In fact, ignoring wind and rolling resistance, you'll need exactly 100 times the horsepower if you want to be belted in the back just as hard at 100 miles per hour as that water wheel belted you up to one mile per hour. Of course, speed isn't everything. Horsepower can be fun at antique velocities, as well... "Modernizing" The 18th Century Okay. For the final-final point (Really. I Promise.), what if we ditched that water wheel, and bolted a 3 liter E36 M3 engine in its place? Now, no 3-liter BMW is going to be making over 2600 pound feet of torque (except possibly for a single, glorious instant, running on nitromethane). However, assuming we needed 12 rpm for an input to the mill, we could run the BMW engine at 6000 rpm (where it's making 210 pound feet of torque), and gear it down to a 12 rpm output, using a 500:1 gear set. Result? We'd have *105,000* pound feet of torque to play with. We could probably twist the entire flour mill around the input shaft, if we needed to. The Only Thing You Really Need to Know For any given level of torque, making it at a higher rpm means we increase horsepower - and now we all know just exactly what that can do for us, don't we? Repeat after me: "It's better to make torque at high rpm than at low rpm, because you can take advantage of gearing." Thanks for your time. Bruce Last edited by bruce.augenstein@comcast.; 03-27-2009 at 04:24 PM.. |
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| 02-14-2008, 11:39 AM | #32 |
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Nice to meet you Bruce! I always liked your write up on Torque and Horsepower. Very informative and easy to understand. I actually read this about 10 years ago online somewhere and thought it was the best explanation in laymen's terms.
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| 02-15-2008, 12:23 AM | #34 |
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Bruce, thanks to others on thist forum I do fully understand HP and torque. But you did the best job of describing it of anyone. Ever. In the history of the world. A very enjoyable read. Thanks for taking the time!!!
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| 02-15-2008, 12:34 AM | #35 |
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very nice write up. bravo!
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| 02-15-2008, 07:01 PM | #36 | |
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First of all, there are many factors that go into rating a really good engine, but a key one (in my opinion) is that it should offer "balanced" power. That is to say, the peak horsepower and peak torque numbers should not be too far apart, and ideally (again according to me), the horsepower number should be a bit higher than the torque number. Engines that make a lot of power compared to their torque numbers are often considered "peaky", meaning you have to wind them up in order to get any real performance out of them. (As an aside, these engines are rarely "peaky", in that they will typically have very broad torque curves. It's just that in order to extract a lot of performance out of them you have to use the gears to their utmost.) An example of a power-laden, relatively torque-deficient engine is the one in the Honda Civic Si. It makes 197 horsepower at 7800 rpm, and only 139 pound feet of torque at a very high 6200 rpm. When driving behind such an engine, you'll tend to feel that the last half of accelerator travel is the good half, while the first half won't do a bunch for you. In addition, from rest, you can't get anywhere close to that torque peak unless you are completely out of your mind, so the car will be a bit lethargic at low speeds, and especially off the line. Engines that offer a large surplus of torque over horsepower will offer a good deal of lunge off the line, but then die when really pressed. When driving behind such an engine, you'll tend to feel that the first half of accelerator travel is the good half, and the second half won't do much more for you, since you'll need to shift before anything really cool happens in terms of "winding it out" acceleration. Think diesels, including the upcoming spate of turbo diesels that we'll be getting to drive soon in the U.S. (now available elsewhere). "Balanced" engines tend to have a really good first half, followed by an even better second half. In my opinion, these are the most rewarding to drive. There are exceptions to this "balanced power rule", but only if the engine in question makes a good deal of whatever it's comparatively short of. As a for instance, the latest Ferrari (the 599) makes a bunch more power than torque, but it makes a *lot* of torque (448 pound feet, I think), combined with really excessive horsepower.  Another example is any one of the current "65" Mercs. They make 738 pound feet of torque, which is pretty much a river of torque in any chassis they make - but they're way down on power - at only a little over 600. How much torque is enough? Good question. My personal opinion is that, for a good and lively drive (not the same as flat out racing), you'll want no more than about 15 pounds of vehicle weight for each foot pound of torque, or something in that vicinity. Somewhere in that neighborhood is where cars can be "lazy fast". That is to say, such cars can be herded down the road with alacrity without having to wind them up using a bunch of throttle along with the resultant noise and unwanted attention. The driver will get a sense of effortless speed, which is never a bad thing. Consider that Civic Si: Since it has to haul over 20 pounds around for every foot pound of torque, it'll never be lazy fast. Not to say that it isn't a quick car when you really want it to be, but you'll need to wind it way the hell up in order to get it to accelerate really well. How does the new M3 stack up? Well, it's pretty horsepower centric, but on the other hand, it makes just enough torque to make it pretty fast in everyday driving. It only suffers a bit when you compare it to the 335 (or RS4) in that arena. Otherwise, good to go. A note about torque rich, lower powered cars: Unless I'm overlooking something (quite possible),we only have one in the U.S. right now, which is the Mercedes Bluetec diesel. It's a fun drive with its seven-speed automatic, but I believe such cars will work a lot better with an auto than with a stick. With a stick, you'll get a series of truncated lunges at full power, and the driver will feel as if he's frantically batting the tachometer down with the shift lever. Just driving around, however, all that torque again makes these cars effortlessly fast. We're about to get a bunch of such cars starting next year, and it will be interesting to see how it works out. Bruce |
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| 02-15-2008, 08:34 PM | #37 |
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torque is an instantaneous force
power is a measure of the rate of work (force over a distance) over time, time-distance in the case of rotary motion is rev/min 1 HP = moving 550 lbs 1 foot in 1 sec or work/time T = HP x 5252/rpm 5252 = 550 ft lb/sec x 60 sec/min / 2 Pi unit check: ft lb/sec x sec/min x 1 rev x 1/rev/min = ft lb cancelling units...or torque... it's best not looking at these like seperate variables, they are measures of different things, but related... only 2 things effect engine torque, displacement, and pressure (ratio and boost, mean effective) T = (V/p)/4Pi, that simple T = 2 Pi P n 2 Pi n = rottational speed w... so T = w P |
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| 02-15-2008, 11:00 PM | #38 | |
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I've cited to that over on the e90post board, and it was well-received. Nice writing -- probably the best exposition of HP/TQ I've read anywhere.
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| 07-20-2011, 11:52 AM | #40 |
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Thanks Bruce I truly enjoyed your write up
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