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Driving a petrol like a daysul

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  • 09-11-2012 2:23pm
    #1
    Registered Users Posts: 2,921 ✭✭✭


    I was thinking about this over the week and wonder if a boardsie could inject some knowledge to it.

    How much of a contributing factor is low rpm on the increased mpg on diesels compared to petrols?

    An example would be if you had two cars that were identical except for the engine. One would be a 2.0 litre petrol and the other would be a 2.0 litre diesel without a turbo (does such a thing exist?)

    If your driving habits with regard to the rev range were the same with the petrol as the diesel (i.e drove the petrol within the confines of a diesel powerband) would there still be much difference in mpg ?

    There is probably something glaringly obvious that I am missing :D


«1

Comments

  • Closed Accounts Posts: 12,985 ✭✭✭✭dgt


    Remmy wrote: »
    I was thinking about this over the week and wonder if a boardsie could inject some knowledge to it.

    How much of a contributing factor is low rpm on the increased mpg on diesels compared to petrols?

    An example would be if you had two cars that were identical except for the engine. One would be a 2.0 litre petrol and the other would be a 2.0 litre diesel without a turbo (does such a thing exist?)

    If your driving habits with regard to the rev range were the same with the petrol as the diesel (i.e drove the petrol within the confines of a diesel powerband) would there still be much difference in mpg ?

    There is probably something glaringly obvious that I am missing :D

    My friend had an 05 Golf SDI that was a 2.0

    Diesels have a better thermal efficiency to begin with. Low rpms ain't good for a petrol....


  • Registered Users Posts: 9,208 ✭✭✭keithclancy


    Easy way to explain it.

    The Petrol wins against the Diesel unladen (Power)

    The Diesel wins if towing a heavy trailer up a hill. (Torque)

    The small rev range on the Diesel is the rev range where the Turbo is spinning at maximum.


  • Registered Users Posts: 16,067 ✭✭✭✭CiniO



    The Petrol wins against the Diesel unladen (Power)

    The Diesel wins if towing a heavy trailer up a hill. (Torque)

    That means you completely don't understand idea behind torque and power.


  • Registered Users Posts: 1,446 ✭✭✭Anjobe


    Diesel engine have higher thermal efficiency as dgt said, this is due their higher compression ratio relative to petrol engines. Also diesel fuel is considerably denser than petrol so contains more usable energy in the same volume.


  • Registered Users Posts: 9,208 ✭✭✭keithclancy


    CiniO wrote: »
    That means you completely don't understand idea behind torque and power.

    http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/horsepower-versus-torque4.htm
    The power at low-end rpm provided by high levels of torque lets you move huge loads without much effort.

    Low RPM a Petrol Engine will provide compartively very little torque compared to a Diesel.

    Same principal applies to boats:
    Speedboat is always driven with Petrol because its so light
    A Tug is always powered by diesel as I provides a very large amount of torque with low rpm.


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  • Registered Users Posts: 7,009 ✭✭✭Wossack


    Anjobe wrote: »
    Diesel engine have higher thermal efficiency as dgt said, this is due their higher compression ratio relative to petrol engines. Also diesel fuel is considerably denser than petrol so contains more usable energy in the same volume.

    at the same time, the diesel engine will be heavier then its petrol equivalent - offsetting some of this efficency


  • Registered Users Posts: 4,373 ✭✭✭Dartz


    Torque is the force of the engine turning. It's the force that goes through the tyres. Power is the rate at which the engine generates this force... it determines how fast you go.

    The acceleration of a car is directly proportional to this force.

    (The force through the tyres is determined by other things like gearing and such, but for now let's assume both cars are equal)


    My diesel produces 200 torques
    My petrol produces 100 torques

    (In this case, 'Torques' is a catch all term for whatever your preferred method of measuring torque is. Imperial or metric, the physical principals remains the same)

    Both are geared the same. What happens when they drag race?

    The diesel sprints off the line, spins up to all of 4500 rpm, then has to change gear.
    The petrol lags a little.... runs up to a much higher RPM, then shifts

    Once speeds start getting up high, the extra power produced by the low-torque petrol will start to show... it'll pass the diesel and keep going while the diesel will run out of gears.

    Gearing ads a new dimension. Diesel engines produce a lot of torque but they don't spin very fast, so they run through the gears very quickly, or need wide gear-ratios in order to go anywhere at speed.

    With the longer rev-range of a petrol, you can gear it shorter and gain more mechanical advantage at the tyres. That's what power can do... it's basically the product of the force moving a thing, and how fast it's moving.

    Torque is golden when you want to get things to start moving. That's why big torque engines can feel so effortless.... especially moving things at slow speed.
    Power is golden when you want them to move quickly.Which is why F1 engines go up to 18,000rpm, despite making almost the same torque as a road-going 2.4 litre car.


  • Registered Users Posts: 8,718 ✭✭✭Matt Simis


    Remmy wrote: »

    There is probably something glaringly obvious that I am missing :D

    The obvious thing you are missing is that in the scenario the petrol would be producing lower HP than the diesel, making it a moot comparison, you cannot "drive a petrol like a diesel".


  • Registered Users Posts: 9,590 ✭✭✭tossy


    Matt Simis wrote: »
    you cannot "drive a petrol like a diesel".

    And you cannot "drive a diesel like a petrol" :D


  • Registered Users Posts: 1,446 ✭✭✭Anjobe


    Dartz wrote: »
    Torque is the force of the engine turning. It's the force that goes through the tyres. Power is the rate at which the engine generates this force... it determines how fast you go.

    The acceleration of a car is directly proportional to this force.

    (The force through the tyres is determined by other things like gearing and such, but for now let's assume both cars are equal)


    My diesel produces 200 torques
    My petrol produces 100 torques

    Both are geared the same. What happens when they drag race?

    The diesel sprints off the line, spins up to all of 4500 rpm, then has to change gear.
    The petrol lags a little.... runs up to a much higher RPM, then shifts

    Once speeds start getting up high, the extra power produced by the low-torque petrol will start to show... it'll pass the diesel and keep going while the diesel will run out of gears.

    Gearing ads a new dimension. Diesel engines produce a lot of torque but they don't spin very fast, so they run through the gears very quickly, or need wide gear-ratios in order to go anywhere at speed.

    With the longer rev-range of a petrol, you can gear it shorter and gain more mechanical advantage at the tyres. That's what power can do... it's basically the product of the force moving a thing, and how fast it's moving.

    Torque is golden when you want to get things to start moving. That's why big torque engines can feel so effortless.... especially moving things at slow speed.
    Power is golden when you want them to move quickly.Which is why F1 engines go up to 18,000rpm, despite making almost the same torque as a road-going 2.4 litre car.

    You can't separate power and torque like that, for an internal combustion engine power output at a given engine speed is proportional to torque multiplied by the engine speed.

    For example, a diesel engine generating 200 torques at 3000 rpm outputs exactly the same power as a petrol engine generating 100 torques at 6000 rpm.


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  • Registered Users Posts: 2,921 ✭✭✭Remmy


    Matt Simis wrote: »
    The obvious thing you are missing is that in the scenario the petrol would be producing lower HP than the diesel, making it a moot comparison, you cannot "drive a petrol like a diesel".

    Well I'm showing up how little I know about cars here because I would have assumed that if everything was equal a petrol in the same size would have more hp than a diesel.:o


  • Registered Users Posts: 8,718 ✭✭✭Matt Simis


    Remmy wrote: »
    Well I'm showing up how little I know about cars here because I would have assumed that if everything was equal a petrol in the same size would have more hp than a diesel.:o
    But engines dont output (max) horsepower statically, thats what RPM is for! In its most simplest terms a petrol at 2000rpm (with power to redline of 8000RPM) would be expected to develop a lot less HP than a diesel at 2000rpm (with its power to 4500rpm).
    Anjobe wrote: »
    For example, a diesel engine generating 200 torques at 3000 rpm outputs exactly the same power as a petrol engine generating 100 torques at 6000 rpm.
    Dartz wrote: »
    My diesel produces 200 torques
    My petrol produces 100 torques
    WTF are "torques".
    Do you two have some sort of deathly fear of using actual metrics, like Newton Metre or Ft/lbs? Nm is even shorter to write than "torques", I dont get it, it undermines everything you are saying.


  • Registered Users Posts: 16,686 ✭✭✭✭Zubeneschamali


    Dartz wrote: »
    Both are geared the same.

    There's your problem.


  • Registered Users Posts: 7,009 ✭✭✭Wossack


    Remmy wrote: »
    Well I'm showing up how little I know about cars here because I would have assumed that if everything was equal a petrol in the same size would have more hp than a diesel.:o

    typically would, think you're picking Matt up wrong

    the diesel will peak sooner then the petrol engine, so at the low end of the rpm range the diesel has the edge and at that point probably creating more power then the petrol. The petrol however may have another 6000rpms still to go..


  • Registered Users Posts: 9,208 ✭✭✭keithclancy


    Dartz wrote: »
    Torque is golden when you want to get things to start moving. That's why big torque engines can feel so effortless.... especially moving things at slow speed.
    Power is golden when you want them to move quickly.Which is why F1 engines go up to 18,000rpm, despite making almost the same torque as a road-going 2.4 litre car.

    I had a 250CC inline 4 Kawasaki ZR ... redline on that was 19,000 rpm.

    Its not pleasant or comfortable :D


  • Registered Users Posts: 1,446 ✭✭✭Anjobe


    Matt Simis wrote: »
    WTF are "torques".
    Do you two have some sort of deathly fear of using actual metrics, like Newton Metre or Ft/lbs? Nm is even shorter to write than "torques", I dont get it, it undermines everything you are saying.

    No it doesn't - "torques" is clearly just an arbitrary unit of torque here, the argument depends on the proportions of the torque outputs of the 2 engines, not the absolute values. Most people aren't going to understand N m or Ft lbs either, unless they know some physics.


  • Registered Users Posts: 1,446 ✭✭✭Anjobe


    Dartz wrote: »
    Torque is golden when you want to get things to start moving. That's why big torque engines can feel so effortless.... especially moving things at slow speed.
    Power is golden when you want them to move quickly.Which is why F1 engines go up to 18,000rpm, despite making almost the same torque as a road-going 2.4 litre car.

    Its not really clear which is best for a race car engine though, is it? The race series where petrol and diesel cars race against each other (Le Mans Prototypes) have been dominated by diesels in recent years.


  • Closed Accounts Posts: 14,983 ✭✭✭✭tuxy


    If you drive the petrol in high gear at low RPM won't you be labouring the engine. And get even worse MPG than the petrol car normally gets?


  • Registered Users Posts: 1,636 ✭✭✭b318isp


    @op: The other factor at play for diesels (in addition to the fuel energy density and the high compression ratio) is the efficiency that a turbo gives - it recovers heat and converts it into mechanical energy.

    The high compression ratio then works against the engine as the revs rise, relative to the petrol.

    So, at low speeds, the diesel will produce the same power as petrol but use less fuel.

    At higher speeds, this advantage is reduced, and may be even bettered by a petrol engine, particularly at top sppeds.


  • Registered Users Posts: 4,373 ✭✭✭Dartz


    Matt Simis wrote: »
    But engines dont output (max) horsepower statically, thats what RPM is for! In its most simplest terms a petrol at 2000rpm (with power to redline of 8000RPM) would be expected to develop a lot less HP than a diesel at 2000rpm (with its power to 4500rpm).

    WTF are "torques".
    Do you two have some sort of deathly fear of using actual metrics, like Newton Metre or Ft/lbs? Nm is even shorter to write than "torques", I dont get it, it undermines everything you are saying.

    Honestly..... because I thought it didn't matter what metric was used. Nm of Ft/lbs. Was meant to be a way of saying 'whatever' units of torque you happen to like.


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  • Registered Users Posts: 4,373 ✭✭✭Dartz


    Anjobe wrote: »
    Its not really clear which is best for a race car engine though, is it? The race series where petrol and diesel cars race against each other (Le Mans Prototypes) have been dominated by diesels in recent years.

    I'd argue that this is as much because the rules are weighted in favour of diesels as anything else.


  • Registered Users Posts: 22,080 ✭✭✭✭Big Nasty


    Remmy wrote: »
    I was thinking about this over the week and wonder if a boardsie could inject some knowledge to it.

    How much of a contributing factor is low rpm on the increased mpg on diesels compared to petrols?

    An example would be if you had two cars that were identical except for the engine. One would be a 2.0 litre petrol and the other would be a 2.0 litre diesel without a turbo (does such a thing exist?)

    If your driving habits with regard to the rev range were the same with the petrol as the diesel (i.e drove the petrol within the confines of a diesel powerband) would there still be much difference in mpg ?

    There is probably something glaringly obvious that I am missing :D

    I think what Remmy is asking is if you drove a petrol and changed up at similar revs to what you would in a similarly powered diesel would you achieve closer mpg figures.

    I guess you probably would to a certain extent, there has to be a reason taxi drivers change gear at 1500rpm! There are other factors involved however in the differences between petrol and diesel engines and their delivery of power and torque.


  • Registered Users Posts: 1,636 ✭✭✭b318isp


    MCMLXXV wrote: »
    I think what Remmy is asking is if you drove a petrol and changed up at similar revs to what you would in a similarly powered diesel would you achieve closer mpg figures.

    I guess you probably would to a certain extent, there has to be a reason taxi drivers change gear at 1500rpm! There are other factors involved however in the differences between petrol and diesel engines and their delivery of power and torque.

    No, the diesel is inherently more efficient at lower rpm.


  • Registered Users Posts: 16,067 ✭✭✭✭CiniO


    Matt Simis wrote: »
    WTF are "torques".
    Do you two have some sort of deathly fear of using actual metrics, like Newton Metre or Ft/lbs? Nm is even shorter to write than "torques", I dont get it, it undermines everything you are saying.

    He used torques to visualise what's it about, as most people are not familiar with untis, while "torques" is really clear here and everyone know what he means.
    You are the best example as Ft/lbs is definitely not the unit of torque.


  • Registered Users Posts: 16,067 ✭✭✭✭CiniO


    Dartz wrote: »
    Torque is the force of the engine turning. It's the force that goes through the tyres. Power is the rate at which the engine generates this force... it determines how fast you go.

    The acceleration of a car is directly proportional to this force.

    (The force through the tyres is determined by other things like gearing and such, but for now let's assume both cars are equal)


    My diesel produces 200 torques
    My petrol produces 100 torques

    Both are geared the same. What happens when they drag race?

    The diesel sprints off the line, spins up to all of 4500 rpm, then has to change gear.
    The petrol lags a little.... runs up to a much higher RPM, then shifts

    Once speeds start getting up high, the extra power produced by the low-torque petrol will start to show... it'll pass the diesel and keep going while the diesel will run out of gears.

    Gearing ads a new dimension. Diesel engines produce a lot of torque but they don't spin very fast, so they run through the gears very quickly, or need wide gear-ratios in order to go anywhere at speed.

    With the longer rev-range of a petrol, you can gear it shorter and gain more mechanical advantage at the tyres. That's what power can do... it's basically the product of the force moving a thing, and how fast it's moving.

    Torque is golden when you want to get things to start moving. That's why big torque engines can feel so effortless.... especially moving things at slow speed.
    Power is golden when you want them to move quickly.Which is why F1 engines go up to 18,000rpm, despite making almost the same torque as a road-going 2.4 litre car.


    That's all correct.

    If someone has any doubts what torque and power means I've got great text on my harddisk which explains it. (it's quite long buy if someone has some time to learn something it's worth to read it).
    I paste it below:
    Bruce Augensteina:

    Torque and Horsepower - 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 . I've posted a subset of this note in another string, but felt it deserved to be dealt with as a separate topic. This is meant to be a primer on the subject, which may lead to serious discussion that fleshes out this and other subtopics that will inevitably need to be addressed.
    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. 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 foot pounds 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.

    Awhile 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 lift a 550 pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight hour shift, more or less. He then published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to the power of one horse, or, one horsepower. Everybody else said OK.

    For purposes of this discussion, we need to measure units of force from rotating objects such as crankshafts, so we'll use terms which define a *twisting* force, such as foot pounds of 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.

    Now, it's important to understand that nobody on the planet ever actually measures horsepower from a running engine. What we actually measure (on a dynomometer) is torque, expressed in foot pounds (in the U.S.), and then we *calculate* actual horsepower by converting the twisting force of torque into the work units of horsepower.

    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, incidently, we have done 6.2832 foot pounds of work.

    OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds 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 foot pounds per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement:

    KOD
    Torque * RPM

    Horsepower =

    5252

    This is not a debatable item. It's the way it's done. Period. Uwaga: oczywiście działanie jest prawidłowe tylko jeśli moment podamy w lbs/ft. Dla Nm współczynnik w mianowniku wynosi 7023.5 i takiego należy używać również w we wzorach pojawiających się dalej w tekście

    The Case For Torque

    Now, what does all this mean in carland?
    First of all, from a driver's perspective, torque, to use the vernacular, RULES . 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 foot pounds 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 isn't particularly meaningful from a driver's 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 pushback 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 the belt in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Fine. Can we go on, now?


    The Case For Horsepower
    OK. If torque is so all-fired important, why do we care about horsepower?
    Because (to quote a friend), "It is 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 carland for a moment, and describe a waterwheel I got to watch awhile ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft which 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(!) foot pounds 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 drivewheels of a car, that car would go from zero to twelve rpm in a flash, and the waterwheel would hardly notice .

    On the other hand, twelve rpm of the drivewheels is around one mph for the average car, and, in order to go faster, we'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot pounds of torque at the output, which is not only a relatively small amount, it's less than what the average car would need 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 HP.

    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 Dragstrip

    OK. Back to carland, 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 current LT1 Corvette with the last of the L98 Vettes, built in 1991. Figures as follows:


    KOD
    Engine Peak HP @ RPM Peak lbs/ft @ RPM




    L98 250 @ 4000 340 @ 3200

    LT1 300 @ 5000 340 @ 3600


    The cars are geared identically, and car weights are within a few pounds, so it's a good comparison.
    First, 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 LT1 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 LT1 is faster. Here's another slice at that formula:


    KOD
    Horsepower * 5252

    Torque =

    RPM



    If we plug some numbers in, we can see that the L98 is making 328 foot pounds of torque at its power peak (250 hp @ 4000), and we can 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 LT1 is fairly happy making 315 pound feet at 5000 rpm, and is happy right up to its mid 5s redline.

    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 occuring a little earlier in the rev range, but that is debatable, since the LT1 has a wider, flatter curve (again pretty much by definition, looking at the figures). From somewhere in the mid range and up, however, the LT1 would begin to pull away. Where the L98 has to shift to second (and throw away torque multiplication for speed), the LT1 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 LT1, by definition, has an advantage.

    Another example would be the LT1 against the ZR-1. Same deal, only in reverse. The ZR-1 actually pulls a little harder than the LT1, although its torque advantage is softened somewhat by its extra weight. The real advantage, however, is that the ZR-1 has another 1500 rpm in hand at the point where the LT1 has to shift.

    There are numerous examples of this phenomenon. The Integra GS-R, for instance, is faster than the garden variety Integra, not because it pulls particularly harder (it doesn't), but because it pulls *longer*. It doesn't feel particularly faster, but it is.

    A final example of this requires your imagination. Figure that we can tweak an LT1 engine so that it still makes peak torque of 340 foot pounds 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. OK, so we'd need to have virtually all the moving parts made out of unobtanium , and some sort of turbocharging 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 LT1 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'd 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 LT1 Vette in a quarter mile pass, and it predicts a 13.38 second ET, at 104.5 mph. That's pretty close (actually a tiny bit conservative) to what a stock LT1 can do at 100% air density at a high traction drag strip, being powershifted. 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, of course. It doesn't pull any harder, but it sure as hell pulls longer . It's also making *900* hp, at 15,000 rpm.

    Of course, folks who are knowledgeable about drag racing are now openly snickering, because they've read the preceeding paragraph, and it occurs to them that any self respecting car that can get to 135 mph in a quarter mile will just naturally be doing this in less than ten seconds. Of course that's true, but I remind these same folks that any self-respecting engine that propels a Vette into the nines is also making a whole bunch more than 340 foot pounds of torque.

    That does bring up another point, though. Essentially, a more "real" Corvette running 135 mph in a quarter mile (maybe a mega big block) might be making 700-800 foot pounds of torque, and thus it would pull a whole bunch harder than my paper tiger would. It would need slicks and other modifications in order to turn that torque into forward motion, but it would also get from here to way over there a bunch quicker.

    On the other hand, as long as we're making quarter mile passes with fantasy engines, if we put a 10.35:1 final-drive gear (3.45 is stock) in our fantasy LT1, with slicks and other chassis mods, we'd be in the nines just as easily as the big block would, and thus save face . The mechanical advantage of such a nonsensical rear gear would allow our combination to pull just as hard as the big block, plus we'd get to do all that gear banging and such that real racers do, and finish in fourth gear, as God intends. icon_smile.gif

    The only modification to the preceeding paragraph would be the polar moments of inertia (flywheel effect) argument brought about by such a stiff rear gear, and that argument is outside of the scope of this already massive document. Another time, maybe, if you can stand it icon_smile.gif.


    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 thus have more effective torque *at the drive wheels*.
    Finally, operating at the power peak means you are doing 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 if you factor in gearing vs car speed, the power peak is *it*. An example, yet again, of the LT1 Vette will illustrate this. If you take it up to its torque peak (3600 rpm) in a gear, it will generate some level of torque (340 foot pounds times whatever overall gearing) at the drive wheels, which is the best it will do in that gear (meaning, that's where it is pulling hardest in that gear).

    However, if you re-gear the car so it is operating at the power peak (5000 rpm) *at the same car speed*, it will deliver more torque to the drive wheels, because you'll need to gear it up by nearly 39% (5000/3600), while engine torque has only dropped by a little over 7% (315/340). You'll net a 29% gain in drive wheel torque at the power peak vs the torque peak, at a given car speed.

    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, theoretical "best" top speed will always occur when a given vehicle is operating at its power peak.

    "Modernizing" The 18th Century

    OK. For the final-final point (Really. I Promise.), what if we ditched that water wheel, and bolted an LT1 in its place? Now, no LT1 is going to be making over 2600 foot pounds of torque (except possibly for a single, glorious instant, running on nitromethane), but, assuming we needed 12 rpm for an input to the mill, we could run the LT1 at 5000 rpm (where it's making 315 foot pounds of torque), and gear it down to a 12 rpm output. Result? We'd have over *131,000* foot pounds of torque to play with. We could probably twist the whole flour mill around the input shaft, if we needed to icon_smile.gif.

    The Only Thing You Really Need to Know

    Repeat after me. "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*." icon_smile.gif


  • Registered Users Posts: 8,718 ✭✭✭Matt Simis


    CiniO wrote: »
    He used torques to visualise what's it about, as most people are not familiar with untis, while "torques" is really clear here and everyone know what he means.
    You are the best example as Ft/lbs is definitely not the unit of torque.

    It looks retarded and at best it promotes newbs going around using made up metrics too without knowing better. Lets call them icecreams while we are at it, it would have the same arbitrary representative power.

    I dont know what Im the best example of, but Ft/lbs (pound foot which is commonly interchanged with foot pound as its "nicer" English) is the/a common unit for measuring torque:
    http://en.wikipedia.org/wiki/Pound-foot_(torque)


    While I would prefer Nm, Ft/lbs (or Lb/ft if you want) is a whole lot better than this Clarkson-esque "torques" nonsense.

    PS: Foot Pounds, Pound Foot and lb/ft are used repeatedly in that bible sized quote you just put up...?


  • Registered Users Posts: 1,636 ✭✭✭b318isp


    Interesting article, but I don't agree with his idea of "the case for", and there is no a case to be won. Torque describes a force, and power describes the rate of application of force (over a distance). They are both intricately wound up with each other via rpm. In practice roughly speaking, torque describes the instantaneous force for acceleration while power indicates the force over a period of time.

    Torque metrics lack a time function, while power includes it. Because of this, if you want to consider how a car accelerated over time, power gives you the better indication.

    Here's a poor analogy that ignores gearing and torque/power curves but may help:

    I want to hammer a nail into a block of wood and I have two choices in how to do so:

    1) I can hit the nail hard at a low speed

    or

    2) I can hit the nail with low force but at a fast rate

    In both cases the same work will be done if the end result is the nail being knocked in in the same time. The power exerted is the same.

    The important point is that knowing how hard the nail is hit (equivalent to torque) is meaningless without knowing how fast it was repeatdly hit (equivalent to rpm).

    So, yes the nail will "feel" the acceleration of hammer blow, but knowing how long it takes to have the nail fully driven home requires the rate that it gets hit. Power, torque and rpm are all part of the same process of doing motive work.


  • Registered Users Posts: 16,067 ✭✭✭✭CiniO


    Matt Simis wrote: »
    It looks retarded and at best it promotes newbs going around using made up metrics too without know better. Lets call them icecreams while we are at it.

    I dont know what Im the best example of, but Ft/lbs (pound foot which is commonly interchanged with foot pound as its "nicer" English) is the/a common unit for measuring torque:
    http://en.wikipedia.org/wiki/Pound-foot_(torque)


    While I would prefer Nm, its a whole lot better than the Clarkson-esque "torques" nonsense.

    "Pound foot" or "foot pound" obviously yes.
    The only detail is that it's ft * lbs not ft/lbs

    It makes huge difference is you multiply or divide.


  • Registered Users Posts: 1,636 ✭✭✭b318isp


    Matt Simis wrote: »
    I dont know what Im the best example of, but Ft/lbs (pound foot which is commonly interchanged with foot pound as its "nicer" English) is the/a common unit for measuring torque.

    Actually, torque is a MULPLICANT of force and distance, so it is ft-lbs, or lbs-ft if you prefer! :D

    Using the slash (i.e. ft/lb) implies a division or rate which is wrong.


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  • Registered Users Posts: 16,067 ✭✭✭✭CiniO


    b318isp wrote: »
    Interesting article, but I don't agree with his idea of "the case for", and there is no a case to be won. Torque describes a force, and speedpower describes the rate of application of force (over a distance). They are both intricately wound up with each other via rpm. In practice roughly speaking, torque describes the instantaneous force for acceleration while power indicates the force over a period of time.
    I think you meant "power" there.

    One think which is cruicial here, is that torque as an engine parameter doesn't really tell us anything until we know the gear ratios.
    It's the torque on driving wheels which we are interested it, as the bigger this torque is, the faster car acclererates, and obviously can pull heavier load / climbs steeper inclince, etc..

    Torque on the wheel can be counted by multiplying engine toque and gear ratio.


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