over revving?

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

  1. rev

    rev

    Messages:
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    back to the engine braking...

    The higher the RPM, the harder the engine brakes. Of course you do not want to hit the rev limiter...

    In reality, if you were to downshift to quickly, the tires would squeal. In GT3, i do remember losing traction if I downshift to quickly. In GT4, if you downshift to early, the RPMs bounce off the rev limiter but nothing happens. It is as if it goes into neutral. This of course increases braking distances.
     
  2. wfooshee

    wfooshee Premium

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    I have a couple of issues with or questions about Skant's post. I'm not trying to degenerate into arguments, or "I know more than you," because I probably don't. Maybe I do. Who knows? Discussion is healthy, and how we learn. I feel, though, that Skant's post is inconsistent in places.

    First, an agreement:

    Exactly what I said when I talked about being able to dyno the torque at the wheels in each gear. OK.

    I'm not bewildered by this concept, it's exactly what was tought to me, drilled into my skull with heavy tools.


    Don't know. I've been taught by them what's s'posed to know, faster acceleration, but nothing to do with top speed.

    This is where inconsistency begins for me. You can't just throw away RPM. Horsepower includes RPM, torque doesn't. We'll get to that momentarily.

    How is that different from the "Horsepower gives you top speed" part of my statement? And technically, force is torque, horsepower is power. So by brute force I assume you mean brute power. Picky, yes, but it seems words for force and words for power are being used interchangably, which doesn't help an explanation of the difference.

    Absolutely. Let's actually prove the mathematical relationship between horsepower and torque, which we can do without actually bothering with units (I'm told I like to over-simplify) :) :


    In physics, power is the amount of work done per unit of time. Work is force times displacement. (Displacement means how far something is moved, not how big an engine is. If the force and the displacement are not the same direction, there is a factor to account for the difference in directions of the force vector and the displacement vector.) Of course we all know that velocity is displacement per unit of time. Hmm...

    Power = work / time

    Work = force * displacement

    So Power = force * displacement / time

    Since velocity = displacement / time, then reduce to:

    power = force * velocity.

    Our force and velocity are rotational, so power = torque * RPM.

    By now the reader may be asking so what? What has this to do with acceleration? Well, absolutely nothing, which is my point.

    In his section "POWER CURVES" Skant has these statements, describing a "torque" car and a "horsepower" car:

    then

    Why did we change curves? You can't prove a possibly incorrect point by using incorrect data. Where this section is misleading is when Skant talks about increasing power at low RPMs. If you have increased the power at low RPM, you've increased the torque, because HP = torque * RPM. If HP at an RPM is greater, torque has to be greater. It's the torque that improves acceleration.

    As I think back on that magazine article, there is another curve that can be plotted, speed vs. force required to maintain that speed. That would be a curve beginning very low at the left rising very slowly, but curving upwards as speed builds, as a function of the drag increasing as the velocity squared. Where that curve meets the cascades, you have terminal velocity. It would look like this:

    [​IMG]

    The force available from the powertrain is equal to the force of wind resistance. Now, since that happens on a torque graph, it would seem that if I had more torque, I could go faster. Well, yes I could, but as soon as you stop accelerating, you're not talking about just force (torque) any more. You're talking about work, because you're using that force to displace something (air). To maintain a velocity (as opposed to reaching it in the first place) you're talking about power. That's how it's been shown to me, and that's how I understand it. I will admit that it's been a while since I've used any of this math, since my work building servers and PC networks doesn't need any of this kind of engineering. It was made clear to me at the time, though, that once velocity becomes constant, you're looking at a different set of variables, and you're looking at power (horsepower, watts, whatever), not force (as in weight or torque).
     
  3. Zardoz

    Zardoz

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    2,208
    I browsed through the replies, and I don't think you ever got an answer.

    Are you aware of the old "tranny trick"? If not, here it is. It dates back to GT3. It only works on the fully customizable transmission:

    Slide the Final adjuster full right to 5.000 (or 5.500 on some cars). Now slide the Autoset full right to 25, then back full left to 1 and leave it there. Your six-speed (or five-speed) now has a nice set of evenly-spaced close-ratio gears, and you didn't have to tediously set each individual gear ratio.

    Now go back and set the Final ratio for each track. You want to redline in sixth gear just before your braking point on the longest straight, of course.

    Sliding the Autoset full right to 25 is a holdover from GT3. The story was that some cars needed that to "clear" some presets or something. Its probably unnecessary in GT4, but I do it out of habit.

    Sure, it does not result in the theoretical ideal for each gear on every car, but with 700 cars in the game, who has time to set each gear on each one?
     
  4. Soyfu

    Soyfu

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    405
    Zardoz, don't be such a happy idiot, posting "answers" to non-existent "questions." If you actually read the conversation you'd understand that Jmac wasn't actually asking how to set gear ratios. It was part of a wise crack on another happy idiot who was posting "answers."

    Sorry to snap, but I don't think anyone ever has to post another guide on using the misfortunately-named "tranny trick." I've only been reading these forums for a couple of weeks, but it seems that every other post is a suggestion to use the "tranny trick." Please, just keep the secret to yourself.

    Now, back to discussing this torque business...
     
  5. Jmac279

    Jmac279

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    I know how to set up gear ratios, I know the "tranny trick," I was simply asking how he personally goes about setting gear ratios for his cars (as in his techniques).

    To clear up any confusion, I'm not asking for help.
     
  6. Skant

    Skant

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    146
    Well, the discussion certainly is making me aware that I'm not communicating some of the fine distinctions well. So far, nobody has pointed out anything that was actually wrong... but clearly they're misunderstood, and it seems to be because of similar sounding terms used for things that are actually different.

    The quote is referring to torque at the drive wheels. Torque at the drive wheels really does determine both acceleration and top speed.

    The saying 'Torque is acceleration, horsepower is speed' is referring to _peak_ torque and _peak_ horsepower. The figures that manufacturers advertise. An increase in peak torque will not increase top speed. But an increase in peak horsepower will.

    It sounds like I'm conflicting myself, doesn't it? But I'm not. An increase in _peak_ torque does not increase the top speed of the car because it's not increasing the torque at the wheels when the car is pushing its top speed... that additional torque is only available at a lower rpm.. and it's gone at the higher rpm.

    Conversely, an increase in torque at the rpms the car is running at while it's pushing its top speed _will_ increase its top speed. And since this increases the torque at the peak horsepower rpm, the peak horsepower itself will be increased.

    So there is some truth to the saying.

    Oh yes you can. If you want to be technical about units, the rpm component of horsepower is cancelled out when you divide it by the rpm of the drive wheels. The end result is the torque at the drive wheels.

    To be sure, we're not talking about peak horsepower at this point. We're talking about the entire horsepower curve. Greater horsepower means you can have more torque at the drive wheels without having the top speed limit of a shorter gear. This isn't referring to the final top speed of the car, but the top speed of the gear in question... which is limitted by the rev limits of the engine.

    I think I did a very poor job of making various pieces of that clear in the original paragraph. The way I said it tends to confuse the reader as to which things I'm referring to.

    Horsepower does improve top speed. That is true. What I'm trying to attack in the saying is the notion many people have that torque has nothing to do with top speed (and peak torque normally doesn't, but other torque properties do) and horsepower does not affect acceleration.

    You are correct that I'm using force and power interchangably. Because of how they are related, they often can be used interchangably. But I'm not helping the reader at all by doing that. I'll need to edit the article to be consistent in its usage. Thanks.

    Ummm... to be honest, what you describe is a pretty scrambled way of looking at it. It's definately derived from a mathematician's point of view as opposed to looking at what's really happening. It's not like the laws of physics suddenly shift to a different mode when the car stop accelerating. But that does seem to be the point where your teacher switches from one set of equations to another. This type of thing is why I find a purely mathematical approach to understanding physics to be... ummm... poor and often misleading in the extreme.

    In reality, whether the car is accelerating or not, you can model it mathematically as horsepower or as torque. It's not two 'modes'. It's two sets of equations... two different ways of modelling the real world events using math. Your teacher is using one or the other depending on which is more convenient. But that doesn't change anything about the real world they are attempting to model.

    Math models the real world, not the other way around.

    - Skant
     
  7. ZeratulSG

    ZeratulSG

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    Okay, so why don't RL racers shift directly to the gear they want for the upcoming corner, rather than heel/toeing their way down through the gears? :odd:

    Okay, so then in simplified terms, what I said is true, maximum RPM below the redline per downshift (all other factors such as car balance aside) provided maximum engine braking. Realistically, you want to shoot for the maximum RPM per downshift that does not throw too much torque at the wheels.
     
  8. Skant

    Skant

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    146
    Okay. History lesson time... :)

    A long time ago, brakes really sucked. They couldn't cool down nor could they survive the heat levels generated from repeated hard usage. Even though it destabilized the car, it was best for racers to row through all the gears on the way down in order to get as much engine braking as possible. Stability was less important than brake failure.

    Besides... it made really cool noises and impressed fans.

    Then brake systems got better. Disc brakes were developed. Pad materials that could operate without fading at extreme temperatures were developed. Brake fluid that was much harder to boil was formulated. No other automotive system has improved as much as brakes have over the years.

    It became unnecessary to 'save the brakes'. Once the brakes could stand up to it without the help, racers stopped rowing down through the gears. It was better to keep the car stable and to preserve the driveline.

    Well, not all racers stopped doing it. This was a point of some disagreement. The primary argument seemed to be that the fastest guys were still rowing down through the gears, so it must still be effective for some reason. But this argument was deflated when said fastest guys started admitting that they were actually just blipping the throttle on the way down to keep the turbos spooled up and weren't actually shifting through all the gears anymore. They were happy to let their competitors think they were, though. It made them slower.

    So... for a while rowing down through the gears went out of style.

    Then came sequential gear boxes. Sequential gearboxes are... well... sequential. So the racers were once again rowing down through all the gears. Simply because there is no other way on a sequential box. However, you might notice that they aren't downshifting into high revs. They're just trying to reach the lower gear, that's all.


    If you have a sequential gearbox, you should just downshift in such a way as to keep the rpms somewhere below upsetting the car too much... you don't shoot for the max possible rpms there. If you have a nonsequential gearbox, you should just slow the car and then shift only once into the gear you will use to traverse the turn. In some cases, you may want to blip the throttle just to keep the turbos spooled, but lag isn't such an issue for modern turbos anymore.


    All that said... there are still some racers that hang on to the old ways. Rowing down through the gears sounds neat and is a crowd pleaser. And it feels racey. And they've seen other racers doing it.

    The same could be said about wiggling the car back and forth during the parade lap and laps under yellow. This has no useful effect on tire temperatures. The little wiggles at low speed just aren't enough to heat the tires at all. But most of the drivers do it anyway. Because that's the way they've seen it done for years and years. Even the announcers think it does something. And it does do something... sometmes it causes a crash when cars wiggle right into eachother.

    - Skant
     
  9. chameleon2

    chameleon2

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    I happen to think that going down through all the gears works better for me. I suppose it's just personal. I don't like to go from 5th down to second. I think it's harder to guess the revs. I suppose it's just practise though.
     
  10. Soyfu

    Soyfu

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    405
    Does that explain why a car can go 150 mph in 4th at 6000 RPM, but only go 135 mph in 5th at 4500 RPM (my dad's Porsche, 180ft/lbs, 180hp)?

    That's how the rice rockets get their high HP?

    (130ft/lbs x 7500 RPM) / 5252 = 185.6 hp

    That's the advantage of having PEAK TORQUE close to the redline. The ENGINE TORQUE has the advantage of multiplying by the high RPM to maximize WHEEL TORQUE. Correct?

    2000 Acura Integra
    2 Dr Type R Hatchback

    Code:
    Acceleration (0-60 mph): 6.7 sec.  Braking Distance (60-0 mph): 113 ft.
    Road Holding Index: .88 g          Base Number of Cylinders: 4
    Base Engine Size: 1.8 liters       Base Engine Type: Inline 4
    Horsepower: 195 hp                 Max Horsepower: 8000 rpm
    Torque: 130 ft-lbs.                Max Torque: 7500 rpm
    Drive Type: FWD                    Turning Circle: 34.8 ft.
    http://www.edmunds.com/used/2000/acura/integra/12716/specs.html
     
  11. Jmac279

    Jmac279

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    What I find really funny are the people who say, "My V8 is faster because it makes more torque."

    News flash - F1 engines only make about 250 lb-ft of torque ;)
     
  12. Scaff

    Scaff Staff Emeritus

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    As Skant has said, this is as much a personal choice for drivers, as it is need. Skip Barbers book 'Going Faster' covers this in some detail, with viewpoints from a number of different instructors from the school, for some people its about maintaining a good flow and they find that by changing down through the 'box; others will just skip gears to reach the target gear. Which one is faster depends on the person involved, I personally think that this one is a case of personal taste.

    Your post does imply that you would only heel and toe when shifting through the gears and not when missing gears, while most RL racers would always heel and toe on any kind of downshift. Not sure if thats what you mean or not?
     
  13. Jmac279

    Jmac279

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    OT: Have you had a chance to play around with my excel calculator, Scaff ?
     
  14. Skant

    Skant

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    146
    Yup. That and that 5th may have too little torque multiplication in general. It's not uncommon in modern sports cars to have their highest gears be for fuel economy on the street.

    There is another situation that can arise... increased peak torque _can_ produce higher top speed... _if_... the car is reaching its top speed at a lower rpm in a higher gear. This does imply that the car is improperly geared. The vehicles best theoretical top speed has actually fallen inbetween the gears and can not be achieved.

    Yes, that's essentually correct. The combo allows for aggressive gear ratios to multiply the heck out of the engine torque.

    Yeah. I'm a V8 guy myself, and I even think it's funny when my fellows are mystified by little engines that go fast anyway.

    The thing is... "My V8 is faster because it makes more torque" is actually sortof correct just like "Torque = Accel, Horsepower = Top speed" is sortof correct.

    V8 engines typically have _very_ flat torque curves. The LT4 V8 in my vette doesn't vary by more than 10 foot pounds from 3500-6200rpm. And it's perfectly flat through most of that range. Compared to a smaller engine with an identical peak horsepower... but with a peaky torque curve instead of a flat one... it will be faster.

    Or another way of looking at it would be to say that the peak horsepower of the two engines is identical, but the average horsepower... the area under the curve... is quite different.

    It's a case of lying with statistics. Big engines still inherently produce more power than small engines. That shouldn't be a surprise.

    But the difference is not because of a high value for the peak torque figure on a V8. That's the part that is misunderstood. The difference is the shape of the curve.. and that's not expressed in any of the marketting figures!

    - Skant
     
  15. Scaff

    Scaff Staff Emeritus

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    Not yet, but as I'm off work this week I will be.
     
  16. Jmac279

    Jmac279

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    Most big V8 dynos I've seen have had wide power curves (not flat torque curves) ...

    Searching for LT4 dynos on google, I found most of the lightly modded or stock engines hit peak torque around 300-320 lb-ft @ 4500 RPM and fell off to about 250-260 lb-ft @ 6200 RPM ...

    What mods do you have done to yours ?

    [​IMG]
     
  17. oldskul

    oldskul

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    Sorry about the very delayed reply Jmac279.
    i dont mess around the default ratios of every gear because i dont really know much about it, im a racer not an engineer. what i do though is run qualifying and if my best times are really no match for the other cars then i just increase the final ratios a bit and run qualifying again to see if the adjustments had some improvements. I am a professional karter and a rookie production class touring car driver. As a racer and not an engineer i am just left with my senses to be one with the car. No power graphs in the production class, only the "brr". You will know you are overreving when the rev limiter hits and you will feel the car braking. Anyway, my point is: eyes on the apex and a bit on the rev limiter, ears for the engine revs, talent for winning, and numbers for the engineers.

    Anyway, on my experience with GT4, the only time that i applied the gear ratio adjustments is on the Nissan 350Z Fairlady LM, when i was trying to beat the minolta and the BMW V12. Outside that, its a needless excercise.

    And one more thing, GT4 really helps in improving my real life racing. It works.

    regards to everyone here.
     
  18. gumpy

    gumpy

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    1,119
    Are you sure about that F1 figure. I am not disagreeing with your general point (I don't know enough to really form a view). But I am surprised that F1 engines make so little torque. Why is it that they have so little torque ?
     
  19. Skant

    Skant

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    146
    Ummm... the dyno image you have there has a rather flat torque curve. Flat... as in close to being a horizontal line. As in that engine has nearly the same umph regardless of the rpm it's running at.

    Sounds about right to me... these cars have a red line at 18,000RPM. If they were torque monsters as well, they'd have enough power to tow battleships. :)

    - Skant
     
  20. gumpy

    gumpy

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    1,119
    My question is more this:

    Has the engine been designed to deliberately have that amount of torque or is it just a by product of the general engine design ?

    If the latter then me being a layman has the question what is it that limits the torque in those engines.
     
  21. wfooshee

    wfooshee Premium

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    The modern F1 engine is all about revs. They have almost no stroke, and stroke is where you get a wide torque band. And yes, the figure of 250 lb-ft is probably good. I used "less than 300" in one my own posts above. But multiply that 200-250 lb-ft of torque by 18,000 RPM, and you get gonzo horsepower. You also get really high (numerically) gearing for gonzo acceleration. So by designing for extreme RPM, they limit stroke, and by limiting stroke, they end up limiting torque. A generalization, yes, but basically how it is.
     
  22. gumpy

    gumpy

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    1,119
    Cool. Thanks for the response.

    I understand now (well enough for a layman like me).

    Cheers.
     
  23. Omnis

    Omnis Staff Emeritus

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    http://www.v8914.com/Horsepower-v-torque.htm

    i don't know if anyone's busted out that link yet...but it does a pretty good job of explaining things.

    it doesn't matter how much your torque to HP or HP to torque ratio is, it's how your car and the gears accomodate it, and how they deliver that to the wheels.

    guess i said nothing that hasn't already been said. came a little too late on this one.

    peace.
     
  24. Jmac279

    Jmac279

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    Yes, it's true that F1 engines only make around 250 lb-ft of torque ... I remember watching a show on Speed where they were discussing F1 engines and they said the Honda F1 engine (I think it was either 2002 or 2003) hit peak torque of 280 lb-ft @ 14,000 RPM and peak power of 790 hp @ 18,000 RPM ...

    I don't see how the LT4 has a flat torque curve ... Torque is down 15% by 6000 RPM and down over 25% by redline ...
     
  25. Soyfu

    Soyfu

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    405
    I guess it's "flat" because it starts nice and high. It's certainly much more flat than most of the torque curves I see in GT.

    ...which are more like the green line, here:

    [​IMG]
     
  26. Skant

    Skant

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    146
    Show me a torque curve that's flatter on an engine with similar peak horsepower.

    Torque output dives on all engines as they get near their redline. That's what makes it the redline. As you say, it's dropped 15% at 6000... but that's only 400rpm away from the 6400rpm rev limit, and it's a pretty shallow dive. The torque output is near constant through the entire range before that... even down to 2200rpm where the dyno chart stops reading.

    The LT4 is known for having a very wide and flat torque curve. It produces signficant torque even near engine idle. At the drag strip, I launch at only 1500rpm, and it's a mission to keep it from lighting up the tires even with the rpms down that low.

    If that isn't a flat torque curve in your book, please enlighten me as to what is. :)

    - Skant
     
  27. ZeratulSG

    ZeratulSG

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    917
    :lol: Indeed it does :lol:

    Wow thanks for the "history lesson" :tup:

    But let me just ask this: under racing conditions the brakes are being used to their maximum potential, so would it not stand to reason that providing engine braking in addition to those effective modern brakes would shorten stopping distances even further, thus meaning a driver could brake later?

    Have you seen the Motegi battle vid with the NSX-R, Gallardo, etc.? The NSX-R driver seems to downshift twice per slow corner entry (presumably 3rd and 2nd gears), would you say this is a personal driving style thing for him? He certainly seems to be using it to slow the car. :odd:
     
  28. ZeratulSG

    ZeratulSG

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    917
    No, I meant with every downshift.
     
  29. Jmac279

    Jmac279

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    It is a lot flatter than a lot of engines out there, but it's still not flat. As shallow it may be, it does start rolling-off at around 4600 RPM and that, by definition, means it's not flat ...

    Don't mind me, I just take the dictionary's defintions way too literally :p

    Anyways, my point was that a lot of big V8s will hit peak torque and quickly drop off from there ... especially the older ones (Hemi being the only exception I can think of) ...
     
  30. Soyfu

    Soyfu

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    Very small cylinders, too. Three liter V10 engines as opposed to the typical 3L V6 in a Camry.

    Yeah, great link. Thanks. As I was reading that, it all seemed to make so much sense (probably because I've got the basic understanding from reading this thread).

    What I've come to understand:
    1. PEAK ENGINE TORQUE is meaningless in terms of choosing shift points but can be useful to understand how an engine behaves.
    2. HORSEPOWER is the product of ENGINE TORQUE and RPM.
    3. For maximum performance, always keep the engine as close to max HORSEPOWER as possible. Shift accordingly.
    4. WHEEL TORQUE is the result of HORSEPOWER and gearing.
    5. Maximum acceleration occurs at PEAK WHEEL TORQUE (which occurs at max HP).
    6. Top speed occurs at PEAK WHEEL TORQUE in top gear (w/ proper gearing).

    Now I understand the tagline for this performance gearing shop:
    Thanks: Skant, Jmac, wfooshee :tup: