over revving?

Discussion in 'Gran Turismo 4' started by chameleon, Mar 21, 2005.

  1. chameleon2

    chameleon2

    Messages:
    99
    I looked but I don't seem to be able to find an answer to my q.
    What actually is the point in over revving all the way up the red line until it hits te electronic limiter? Besides stuff like perfect shift point and such, I can only come up with one answer that would make sense: to get into a meatier portion of torque when the next gear hits? I can't think of anything else, and I'm still confused about torque and force. I believe because of more revs, more force is generated. But peak torque isn't usually all the way into the redline, it seems to be dropping on some cars when over a certain amount of revs. For example, with the BMW M5 I seem to be accelerating faster when I drive in it's peak torque (2000 - 4000 rpm) instead of completely overreving it. It might produce less sound so it looks as if you accelerate slower, but when looking at the speedometer it's looks as if it's faster??

    Basically I suppose I wanna know which is faster? Always go all the way till the electronic limiter? Or should I drive in the peak torque? Guess every car being quite different is another factor making it a bit confusing, but I have a feeling that peak torque delivers the greatest times? arrggg confused confused :dunce:
     
  2. Duke

    Duke Staff Emeritus

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    It's hard to tell without accurate torque curves and complete gearing information for each car... which would make it easy (if tedious) to calculate.

    But typically, it's worthwhile to go past the torque peak in a lower gear in order to be closer to the peak in the next gear up. This usually means shifting at or above redline, but before hitting the rev limiter (obviously). Here's a good essay on the subject:
     
  3. Jmac279

    Jmac279

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    If you don't want to read all that, the basics are ...

    You'll produce the most force in any single gear at peak torque ... However, you'll produce the most force at any given SPEED at peak horsepower ...

    Therefore, you should keep horsepower as high as possible to achieve maximum acceleration ...
     
  4. HoWheels

    HoWheels

    Messages:
    38
    Different cars in the game have different powerbands. If you want to find out if your car makes more (or less) power AFTER redline, try racing with a ghost. Find a long straight (Las Vegas track works well), and try driving it shifting at redline. Then try it shifting at the fuel cutoff. See how you do against your own ghost.

    -a
     
  5. chameleon2

    chameleon2

    Messages:
    99
    Thanks for all the info.....so basically I was right about getting the best performance shifting at the redline. Not in all cars though but I'm getting the concept here. Crazy calculations though....how long did you study for that?
     
  6. Soyfu

    Soyfu

    Messages:
    405
    That was a great excerpt from the Ford guy. Very interesting, but I have a problem with the recommendation to always shift at the redline.

    The trouble is with the Transmission output torque chart (Figure 1). The example engine has peak torque at a fairly high RPM. The case when a car should shift before redline is when the engine has peak torque at a fairly low RPM. I believe that if the chart were made on that basis (ie. peak torque at 3000 RPM), it would show that it's possible to get more torque in the next higher gear than at the redline in the current gear.

    Figure 1
    Code:
        Engine     Transmission output torque (ft-lb):
               Torque      1st     2nd     3rd     4th     5th
         RPM  (ft-lb)     3.54    2.13    1.36    1.03    0.72  <- gear ratio
        ----    -----     ----    ----    ----    ----    ----
        1000       50      177     107      68      52      36
        1500       65      230     138      88      67      47
        2000       80      283     170     109      82      58
        2500       92      326     196     125      95      66
        3000      104      368     222     141     107      75
        3500      114      404     243     155     117      82
        4000      120      425     256     163     124      86
        4500      125      443     266     170     129      90
        5000      130      460     277     177     134      94
        [B]5500      133      471     283     181     137      96[/B]
        6000      130      460     277     177     134      94
        6500      122      432     260     166     126      88
        7000      110      389     234     150     113      79
    
        (note: peak torque is at 5500 RPM, peak horsepower is at 6500 RPM)
    Here is my completely made up data to illustrate how I think some engines opperate:

    Figure 2
    Code:
        Engine     Transmission output torque (ft-lb):
               Torque      1st     2nd     3rd     4th     5th
         RPM  (ft-lb)     3.54    2.13    1.36    1.03    0.72  <- gear ratio
        ----    -----     ----    ----    ----    ----    ----
        1000       50      177     107      68      52      36
        1500       65      230     138      88      67      47
        2000       80      283     170     109      82      58
        2500       92      326     196     125      95      66
        [B]3000      133      471     283     181     137      96[/B]
        3500      130      460     277     177     134      94
        4000      122      389     260     166     126      88
        4500      110      311     234     150     113      79
        5000      100      252     201     139     102      71
        5500       91      215     187     121      89      62
        6000       84      189     165     104      81      54
        6500       78      164     146      88      75      49
        7000       71      149     133      79      68      42
    An example of an engine like this is the Mazda Autozam AZ-1. The best performance comes from keeping the revs between 4k and 6k RPM (peak torque is just after 4k RPM, redline is at 9500 RPM).

    If you let the AZ-1 engine rev all the way to redline, it barely has enough power to maintain its speed. But as soon as you shift up to the next gear, acceleration resumes.
     
  7. gumpy

    gumpy

    Messages:
    1,119
    Forget all the detail.

    Test a few cars.

    The license tests include a few mini experiments.

    Viper likes changing below the redline - that is how I golded the accelerate and brake license test.

    The lower powered cars prefer to be revved past the redline to just before the point of no more acceleration. That is how I golded those acclerate and brake tests.

    In general I have performed better when holding gears until right at the limit and I believe this is for 2 reasons.

    1. You may get 1 or more less gear change(s) which saves time. It does on the mini at Tsukuba B license test and the accelerate and brake tests.
    2. The car accelerates better in the next gear. The Viper is the only car I have found where this is not the case.

    I prefer to do the test on the track than read the graphs.
     
  8. Jmac279

    Jmac279

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  9. Chris Criswell

    Chris Criswell

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    Say for example, And I'm just throwing some numbers out here:

    A cars max torque is at 4500 RPM's, does it mean for every single gear? or just for certain gears? Would a car have say a Max torque of 4500 RPM's in First Gear, but say 5000 for Second gear?
     
  10. Jmac279

    Jmac279

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    Location:
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    In each gear, the peak force will be at peak torque ... However, at any given speed, whichever gear is making the most power will be making the most force ...

    Let's say your engine makes 150 lb-ft @ 4500 RPM but only 125 lb-ft @ 6000 RPM ... In 3rd gear, your engine is at 6000 RPM but it would be at 4500 RPM in 4th gear at the same speed ... Which gear should you be in ?

    Force (lbs) = Wheel Torque (lb-ft) * Gear Ratio * Final Drive Ratio *24 / Tire Diameter (in)

    Final Drive = 4:1
    Tire Diameter = 24"
    3rd gear ratio = 4:3 (1.333)
    4th gear ratio = 1:1

    3rd gear Power @ 6000 RPM = 142.8 hp
    4th gear Power @ 4500 RPM = 128.52 hp
    3rd gear @ 6000 RPM makes 11.111% more Power than 4th gear @ 4500 RPM

    3rd gear Force @ 6000 RPM = 666.667 lbs
    4th gear Force @ 4500 RPM = 600 lbs
    3rd gear @ 6000 RPM makes 11.111% more Force than 4th gear @ 4500 RPM

    The speed (though irrelevent) is 80.357 MPH ...

    Anyways, the point is that the difference in Force at a given speed will always be proportional to the difference in Power at that speed (Power1/Power2 = Force1/Force2) ...
     
  11. vladimir

    vladimir

    Messages:
    1,165
    apart from getting into peak torque again in the next gear by shifting late, you also stay longer in the lower gear where you can accelerate faster even with less torque due to the shorter ratio.
     
  12. somebody

    somebody

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    Location:
    Australia
    And there is always gear change inertia to consider.

    After your RPM rises above peak power in any gear the rate of acceleration in that gear will drop off. If the peak power is reached well before the redline, by the time you get to the redline the car has stopped accelerating quickly.

    This is the problem with the Mazda Autozam AZ1 mentioned a few posts before. This is also where having the upgraded clutch comes in handy for the quicker shift so the engine inertia is not lost during gear changes.
     
  13. oldskul

    oldskul

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    Location:
    Philippines
    oh my, the numbers are making me dizzy...
    applying all the numbers and all that mumbo jumbo stuff while targeting the apex will be quite a challenge resulting to data overload to the driver. i have a less technical approach that will just require your talent- raise the TV volume a bit so that you can hear the engine revs, then wait for the "brrrr" sound indicating overrev then shift to the next higher gear and quickly glance at the revmeter to see where that overreving happened. On the next gear try to anticipate and shift before it overrevs. After a few laps your brain will get the hang of it. Talent will be the key, not charts. Downshifting properly will then present an even greater challenge (and an exciting one once you get it). Now where is that downshifting thread......
     
  14. Jmac279

    Jmac279

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    So instead of just looking at the power graph to determine your gear ratios and shift points and doing a few very basic calculations, you should just listen for a "brr" sound that supposedly indicates that the power is dropping off substantially ?

    How do you set up your gear ratios ?
     
  15. Ikari_San

    Ikari_San

    Messages:
    182
    Wow, you guys have really gone all out with this, good stuff :D

    Can I just throw a wet blanket on this and say most of it is probably best applied to drag racing, or high speed racing only. On circuit-style tracks I find myself often short-shifting through some corners, while other times revving way too high in some gears just to be in a better position for "grabbing" the next gear. This is mostly because of the difficulty turning the steering wheel with one hand while shifting with the other (try it on the DFP with 100% FF).


    Buy "Fully customizable transmisson" or whatever it's called.
     
  16. ZeratulSG

    ZeratulSG

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    917
    Actually that's a good question for the mathematicians and engineers on this thread: downshifting. I would imagine that optimal engine braking is achieved in a similar way to optimal acceleration, only in reverse. In other words, downshifting at the point where the RPMs in next lowest gear would be at peak power provides the best engine braking. Would this be correct?
     
  17. Jmac279

    Jmac279

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    You misinterpreted my question ...

    I was asking him how he goes about setting his gear ratios ...

    I don't see how this isn't applicable to circuit racing ...
     
  18. Jmac279

    Jmac279

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    It doesn't really work in reverse ... The faster the engine spins, the more resistance, so you want to keep your revs as high as possible without hitting the rev-limiter when downshifting (theoretically) ...
     
  19. Skant

    Skant

    Messages:
    146
    Downshifting is not for stopping the car. That's what the brakes are for.

    You downshift so that you'll be in the proper gear for turn exit.

    The greatest engine drag occurs at the highest rpm (not the peak torque rpm), and you want to _avoid_ this because it tends to snatch up the drive wheels and change the effective brake bias of the car, possibly causing a spin or other bad effects. Also, if you're shooting for downshift to max rpm and you mistime it a little, you will blow up your engine, and the rev limitter can not save you (of course, that's not a consideration in GT).

    - Skant
     
  20. JTSnooks

    JTSnooks

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    1,436
    In GT, you do want to downshift to help slow down. I haven't ever had a problem with it spinning me out, it just slows me down faster. Also, you obviously don't have to worry about blowing up your engine, so that's not a concern. Only thing is, don't downshift so fast that your engine bounces off the rev limiter, because that seems to basically do nothing. In real life, you obviously wouldn't want to downshift to near redline to slow your car down, but it's ok to use it in lower rpms.
     
  21. Jmac279

    Jmac279

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    In previous GT games, when you bounced it off the rev-limiter (downshift too early), your tires would lose traction, greatly increasing your braking distances ...

    Not sure about GT4 ...
     
  22. wfooshee

    wfooshee Premium

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    4,421
    This brings up memory of an article I saw YEARS ago in Road & Track about finding shift points.

    Torque should be thought of as the force available for acceleration. Horsepower should be thought of as the force available to displace the air the car moves through. The two are mathematically related, but have different relationships to performance. High torque gives great acceleration. High HP gives great top speed.

    Now to find shift points- - - -

    Take your engine's torque curve (this is a totally fictitious curve, RPM on the X-axis, torque on the Y):

    [​IMG]

    I'm not showing units because I just want to show a concept. Higher RPM is to the right, higher torque is to the top.


    Now multiply the curve by each gear ratio.

    [​IMG]

    More torque multiplication in lower gears.


    Now plot these six curves against vehicle speed instead of engine speed.

    [​IMG]

    Shift when the lines cross. That's it. If you carry more engine speed, the torque for that gear is actually lower than the torque in the next gear, so you're not getting maximum acceleration.

    This graph is called a cascade, and that's what the magazine article was talking about. Obviously, there's a significant fudge factor for frictional losses in the drivetrain, but if you can dyno the drive wheel torque in each gear, you can get this graph.

    As for GT4, you don't have access to this data, so all you can really do is drag race the ghost car on the test course.
     
  23. Soyfu

    Soyfu

    Messages:
    405
    That's how I've always thought about it, but some people are saying you want to focus on "power" (HP).
    I can't tell if you guys are in agreement or not. Which is it?

    Also, thanks to both of you for trying to explain this stuff.
     
  24. Skant

    Skant

    Messages:
    146
    Here's an article I've been working on about this subject. Still a little rough yet, but hopefully the points are clear.


    FORWARD

    There is a lot of confusion when it comes to measuring how powerful an
    engine is. Horsepower? Torque? Which is the more important figure? Why
    are there two? What does it mean?

    It's a subject that bewilders a lot of people. Even worse, there are many
    who think they understand but don't. It has been my observation that most
    people fall into one of three camps:

    A) They know the equations. They try to explain it to other people in terms
    of complex mathematics and a big list of quoted laws and theorems from
    physics books. I have long suspected that most of these folks have just
    memorized the equations but don't actually understand why they work.

    B) They know an oversimplified (and fairly erroneous) saying and think that
    covers it. The one most commonly quoted seems to be "Horsepower gives you
    top speed, torque gives you acceleration". These folks are generally
    bewildered when the real world doesn't behave anything like how they thought
    it would.

    C) They know that they don't understand horsepower and torque. And they
    just figure bigger numbers are better numbers. Understanding more than that
    looks to be pretty difficult. Especially when the type A guys above talk
    about it.

    It's not all that difficult really. This article seeks to make sense of
    this mess by explaining it in clear terms without resorting to deep math
    equations or oversimplifications.


    WHAT'S THE POINT?

    Lets start by defining our goal here.

    What we're really interested in is how the engines makes the car move. At
    the end of the day, it's all about understanding how horsepower and torque
    play into 0-60 times and top speeds.

    It must be understood that the engine output is only one of the factors that
    determine these end results. Because discussing the engine in vacuum isn't
    all that useful, this article will cover all of the elements which have a
    strong effect on the final results.


    TORQUE

    Torque at the drive wheels is what really matters. Greater torque means
    both faster acceleration and higher top speed.

    The amount of torque applied at the wheels directly translates into forward
    thrust against the mass of the car. So the rate at which the car actually
    accelerates is affected by both the torque and the weight being moved.
    These relationships are linear. That is, if the car is half as heavy, it
    will accelerate twice as fast. If the car has twice the torque, it will
    accelerate twice as fast.


    HORSEPOWER

    Torque is what actually makes the car go. The problem is already solved.
    So why does horsepower even matter?

    Lets start by getting one thing clear. Horsepower and torque at the engine
    are not the same as horsepower and torque at the wheels. This is where a
    lot of the confusion comes from.

    So far I've said that torque at the wheels is the most important figure.
    But at the engine, horsepower is generally the most important figure. The
    torque figure at the engine is often ignorable.

    Why is that? Because there is gearing inbetween the engine and the wheels.
    The torque at the engine is multiplied by the gearing before it becomes the
    torque at the wheels. You can have as much torque as you like just by
    choosing the gear ratio. So a drive train with half the torque at the
    engine but twice the gear ratio turns the wheels just as hard.

    Well, that's just great then. We can take an engine of any strength and get
    as much acceleration as we want out of it just by using higher and higher
    gear ratios in the drive line. But wait... there's a catch, isn't there?

    Of course there is. The problem is that any engine can only spin so fast.
    It has an rpm limit. A red line. Higher gear ratios make the car
    accelerate faster, but they also reduce the speed at which the engine
    reaches its rpm limit and therefore can not accelerate further.

    That is... just by using a very high gear ratio, you could make an economy
    car with a tiny engine accelerate like a Porsche... but only up to 5 mph.

    This is why transmissions have multiple gear selections. In lower gears,
    you have higher torque multiplication (and therefore greater torque at the
    wheels) but more limitted maximum possible speed.

    And that finally brings us to explaining horsepower. Horsepower is simply
    the torque multiplied by the rpm. Unlike the torque figure, horsepower is
    _not_ changed by gear ratios because it takes into account both the torque
    and rpm. When gearing doubles the torque, it halves the rpm... so the
    horsepower ends up the same regardless.

    This means that the level of horsepower at the engine determines what can be
    done with gearing to produce torque at the wheels. Greater horsepower at
    the engine means you can have greater torque at the wheels and/or higher
    possible top speed without compromising one for the other.


    POWER CURVES

    So far, we've talked about engines as if they produced a fixed amount of
    torque. They actually don't. They produce different amounts of torque at
    different rpm speeds.

    Normally, engines will achieve their peak torque somewhere in the middle of
    the rpm range. However, they don't usually achieve their peak horsepower at
    the same rpm... horsepower comes from multiplying torque by the rpm, and it
    turns out that in most engines, the rpm wins out over the torque as the
    dominant part of the peak horsepower determination. So the peak horsepower
    will be found at a higher rpm.. usually at or near the redline where the
    torque has fallen down lower, but the rpm is at its highest.

    There are a lot of people that seem to think that increasing the torque in
    the engine will accelerate the car faster. While increasing the horsepower
    will only increase the top speed and not acceleration.

    This is sortof true. But it's an oversimplification, and it depends on a
    lot of assumptions.

    The horsepower and torque figures advertised for an engine reveal some
    information about what the torque curve looks like. But they can be very
    deceptive. The peak horsepower and torque are usually reported along with
    the rpm speeds at which they occur. Essentually, you are only being told
    where two points are on the power curve graph. And that's not enough.

    If you had two different engines with completely identical ratings, one of
    them could accelerate the car much faster than the other. Why? Because the
    ratings only show the power curve at two points. All of the other points
    could be entirely different.

    Similarly, you could make a modification to the engine which increases it's
    power strongly at the low end of the rpm range while decreasing it slightly
    at the high end of the range. The peak torque figure will probably improve
    because it's at a point toward the low end of the range. And the peak
    horsepower figure will go down because it's at the high end of the range.
    But overall, the car accelerates faster because there's more total area
    under the entire torque curve. So you have a car that accelerates faster
    despite reduced peak horsepower.

    That's where the saying 'torque gives you acceleration, horsepower gives you
    top speed' comes from. There are a lot of modifications that do this.

    The funny thing is, the reverse is also true. You could make a modification
    to an engine that reduces the power slightly at low rpms but increases the
    power strongly at high rpms. The peak torque figure will be reduced and the
    peak horsepower figure will be increased. But again the total area under
    the power curve is greater overall and so again, the car accelerates faster.

    So it becomes apparent how the situation gets oversimplified. No
    manufacturer advertises the _average_ torque output of the engine. Now
    there would be a useful figure...

    The upshot of all this is that the peak horsepower and torque figures can
    lie about the engine. Many modern small engines have very peaky power
    curves that make them look like they produce as much power as larger,
    beefier engines because the figures at the peaks are the same or even
    higher. But compared to a wide, flat power curve, they actually produce
    much less power overall. The peaks are high, but the average isn't. Sorry
    folks, but all else being equal, a bigger engine still produces more power
    than a smaller engine. Manipulating the tuning to advertise bigger peak
    values doesn't change that.


    SHIFTING GEARS

    So now that we understand more about the relationship between horsepower and
    torque, lets look at the transmission a little closer to see what effects it
    has on 0-60 times.

    We know we can produce pretty much any amount of torque we want at the
    wheels just by gear selection... but we pay the price for it in rpm limit
    (and therefore speed limit).

    The most ideal transmission would have an infinite number of gear ratios so
    that it could always keep the engine operating at its peak torque rpm. Most
    transmissions, however, have only a handful of gear ratios available. So
    they can only approximate this ideal.

    The more ratios a transmission has, the closer to the ideal the transmission
    becomes. They can keep the engine in the beefiest part of its power band
    more of the time _and_ they can more accurately make the ideal trade off at
    any particular speed between torque multiplication and max rpm. Very often,
    a transmission upgrade will improve a car's performance more than engine
    upgrades would.


    DRAG

    So far, we've ignored the effect of drag on the car. Drag is force that
    pushes against the weight of the car opposite the direction it is moving.
    It acts to slow it down.

    There are multiple kinds of drag, but the one that's most interesting to the
    automotive world is wind resistance.

    Wind resistance comes from the force of air pushing against the car as it
    moves. At low speeds, wind resistance is quite minor and the car can
    happily drift at the same speed with the engine in neutral for quite a long
    time. But at high speeds, wind resistance becomes enormous. In fact, wind
    resistance increases as the square of speed. That is, if the vehicle moves
    twice as fast, the wind resistance will be four times as great!


    TOP SPEED

    In order for a car to accelerate, it must generate more forward thrust from
    the drive wheels than reverse thrust from drag.

    As the car goes faster and faster, the drag increases exponentially (because
    of wind resistance). Eventually, it must overcome the torque at the drive
    wheels. Even worse, the torque at the drive wheels decreases because less
    torque multiplication is available in high gears.

    It's a losing battle. And the point at which the torque at the wheels and
    the drag become equal is where the vehicle reaches its top speed.

    Peak engine horsepower is king here. Additional top speed can not be
    attained by trading off rpm for more torque multiplication. So it will just
    have to be brute force and lots of it. Horsepower would have to be
    quadrupled in order to double the top speed.

    Aerodynamics have a dramatic effect on top speed. A car with half the wind
    resistance will have twice the top speed.

    One element that is unexpectedly _not_ part of top speed is the vehicle
    weight. Why? Because in order to accelerate, the thrust force must be
    greater than the drag force. Weight has nothing to do with that.

    The difference between those forces works against the weight of the car.
    And so the weight affects how quickly the car will accelerate but not
    whether it will accelerate.

    If the drag force is greater than the force at the wheels, the car will
    decelerate. And greater weight will reduce the rate of deceleration, too.


    ROTATING MASS AND DRIVETRAIN FRICTION

    A 2:1 ratio gearing does double the torque running through it. However, it
    also increases the mass that torque must accelerate. The gears themselves
    have mass. And the greater the ratio, the more mass they will have. They
    also have friction. And that eats part of the torque.

    High rotating mass is a big penalty. While it may seem strange to be so
    concerned about as little as a pound or two here and there in the drive
    train, it makes a big difference. The mass of the drive train has to be
    accelerated to much higher speeds than the rest of the car. And a
    significant portion of it has to be spun up to speed multiple times... once
    for each gear! So it's often been said that each pound removed from the
    rotating mass is the equivalent of 10 pounds removed from the rest of the
    car.

    This is why light weight flywheels are often a serious performance upgrade
    for any car.

    It's also another reason why cars with high peak horsepower but very low
    torque are often not as fast as their horsepower ratings would lead you to
    believe. These cars need substantial torque multiplication and all that
    gearing adds to the rotating mass. In addition, much of the drive train has
    to spin up to much higher speeds, so the penalty for that mass is even
    greater.

    That's not to say that high rev, low torque engines are inferior. It's an
    engineering trade off. The rotating mass issues are weaknesses that must be
    overcome by superiority in other areas. For instance, these engines tend to
    be smaller and lighter than high torque engines, and that has obvious perks.


    FINAL WORD

    By now, you should understand why a light car with low horsepower but a high
    power to weight ratio still doesn't have a very high top speed. Or why a
    300hp high torque V8 car can often outrun a 300hp low torque rice rocket
    which is lighter. Or why it could be the other way around.

    It's just not as simple as comparing the horsepower... or even the power to
    weight ratios.
     
  25. Scaff

    Scaff Staff Emeritus

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    23,537
    Location:
    United Kingdom
    Damn right Skant, the only way you should be stopping/slowing a car on the track is with the brakes. GT4 makes high speed threshold braking a very, very easy task when compaired to the real world (its good don't get me wrong, better than GT3, but still a way to go). Engine braking is the surest way to unbalance the car, screw up your brake bias and lock the wheels.

    The downshifts on GT4 help massively, with almost perfect heel and toe downshifts that help keep the revs in the right range and you never mess them up (now that is different from real life).

    BTW nice piece above, have just given it a scan, will read it fully later.

    Regards

    Scaff
     
  26. Jmac279

    Jmac279

    Messages:
    1,012
    Location:
    Canada
    We are in agreement, to some degree ... High torque does not necessarily equate to high acceleration ...

    The Force vs. Speed (or Output Torque vs. Speed) graph is entirely correct ... If you actually figure out the Power made in both gears at the transition point (place where they intersect), you'll see that they're both making the same amount of Power at that speed. After that point, the lower gear is making less Power (thus less Force).

    Force = Power/Speed
    Acceleration = Force/Mass
    Acceleration = Power/(Speed*Mass)

    From this, you can understand that, at a given speed and mass, the vehicle making more power will be accelerating faster. This means that you could take an extremely low torque engine and use very high and close gear ratios to keep power high and you'd out-accelerate a high-torque diesel engine, even off the line. This is because, even at 5 MPH, the vehicle making more Power will be accelerating faster (assuming equal mass). Of course, that low torque vehicle would probably have to sacrifice top speed a lot unless it revved insanely high ...

    You also have to remember, though, that more Torque at a given RPM = more Power at that RPM.

    Plotting Power vs. Speed is a much easier way to find shift points (much less math). However, you can integrate Aerodynamics into Force vs. Speed, but you can't for Power vs. Speed. This allows you to find your theoretical top speed and makes it easier to compare multiple vehicles with differing aerodynamic properties.
     
  27. Jmac279

    Jmac279

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    There are some parts of that article where he may confuse the reader ...

    Then he goes on to say ...

    Then, he confuses the reader by putting 2 paragraphs down ...

    Then he confuses the reader yet again with ...

    And, finally, ...

    Area under the Power curve is what determines acceleration and he said that many times, although he seemed to contradict himself several times as well ...
     
  28. Skant

    Skant

    Messages:
    146
    I'm afraid I don't see where I'm contradicting myself. The statements you have quoted are all correct. If I'm confusing you, I'd like to know exactly how so that I can clarify the article. Your post doesn't make it very clear to me what you think is actually wrong.

    - Skant
     
  29. Jmac279

    Jmac279

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    Oh, didn't read the first sentence ... :p I thought you got it from somewhere else thus "he"

    Anyways, when you said

    This would only be true if the area under the power curve were greater as a result of an increase in the area under the torque curve. Also, it should be noted that, unless you were using wide gear ratios, the decrease in high-end power may well result in slower acceleration beyond 1st gear.

    Also, when you say "The torque figure at the engine is often ignorable." then you keep referencing torque (I assume it's engine torque being referenced in sentences such as "No manufacturer advertises the _average_ torque output of the engine. Now there would be a useful figure..." It may lead to confusion and gives the impression of contradiction ... Though, it's not difficult to get the point of the article and is pretty easy to understand ...
     
  30. Skant

    Skant

    Messages:
    146
    Actually, that statement would always be true. It's an excerpt from a paragraph which is talking about engine alteration, not gearing changes. This article was actually written for RL and most people can not change the gearing in their cars (except the differential ratio). So its examining a change in engine output only.

    You are correct in that it's technically possible to increase the area under the power curve while decreasing the area under the torque curve, and gearing may be able to take advantage of this. But then again, it might not.

    If the increased torque is below the usable range, yes. However, I don't think any engine alterations on the market purposefully increase torque below that range. (And by 'usable', I mean the rpm range that it rolls through as you shift from one gear to the next)

    There were a number of strange theoretical scenarios which are technically possible which I didn't discuss in the article because they only serve to obfuscate the real issues. I wanted to tackle only the real points that an RL racer needed to understand about modifying his car.

    In short, the article was not written for engineers designing aftermarket parts. It was written for racers trying to understand which to buy and/or normal people just trying to make heads or tails of the whole mess.


    Manufacturers advertise the peak torque figure... which isn't the same thing as the torque curve. I've tried to make it clear the differences between peak engine torque, engine torque curve, and the torque at the wheels... which are all surprisingly divergent from eachother. Because all of these are referred to as 'torque', that's where a lot of the confusion comes from. I've tried to explain that in the article, but it sounds like I may not have hit on that point well enough.

    - Skant