over revving?

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

  1. Jmac279

    Jmac279

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    Peak acceleration will always happen at peak ENGINE torque in 1st gear (assuming no traction issues) ...

    Wheel Torque (lb-ft) = Engine Torque (lb-ft) * Gear Ratio * Final Drive Ratio

    Top speed happens when one of two things is true ...

    1) Engine has reached the rev-limiter in the highest gear.
    2) Force - Resistance = 0

    Force (lbs) = Torque (lb-ft) * Gear Ratio * Final Drive Ratio *24 / Tire Diamater (in)
     
  2. Skant

    Skant

    Messages:
    146
    Nope. A proper brake system has more than enough stopping power to yield maximum braking on all four tires. Properly tuned, it can bring all four tires to their threshold where they're still rolling a little, and thus providing the maximum braking possible.

    Engine braking interferes with the braking system by applying additional braking force to just the drive wheels, which makes them tend to lock up before the others. It also interferes with the operation of ABS (for cars so equiped) since it's applying a braking force to the tire which is not under the control of the ABS system.

    No, I haven't seen this video. Where can I find it? Sounds neat. :)

    It may be his driving style. You know... not every racer uses what would be considered a proper technique. In fact, one of the first things I learned racing in RL was that you shouldn't copy what everyone else is doing. Just because other racers are doing it doesn't mean it's the right way for you. Most racers copy eachother rampantly... mistakes and all.

    There's been more than once where I noticed that everyone was taking a particular corner in a way that seemed wrong to me. And I wondered... they must know something I don't? I did it my way. I won.

    Of course, at times it turned out that they _did_ know something I didn't... ooops...

    Rowing down through all the gears is a heritage that racers have been copying from eachother from way back when it was the only way to keep your brakes alive.

    Do you know why it's rude to wear your hat indoors? Because in the mideval ages, people wore hats with big feather plumes... and the indoor lighting was open flames. It wasn't rude to wear a hat indoors; it was stupid and dangerous. But that was passed down through the ages and in the modern day ends up with a kid getting detention for not taking his baseball cap off in class... even though the original reason for it is long since gone.

    But on the other hand, maybe this guy does have a particular reason he does it. You know... I could tell you anything about the typical techniques racers use, and you will always find someone who just does it a bizarre way that probably works only for him alone. Racing is a complicated animal, and there's more than one way to tackle it.

    The NSX is a tail heavy car that can have a bad tendency to become unstable under braking as the heavy tail tries to flip around to the front. Maybe he's got his brake bias toward the front, and then uses downshifts to dynamically apply more brake bias to the rear as long as the car is stable?

    Or maybe he just likes the way downshifts sound. Or maybe he's just copying the way his dad did it. Or maybe (and most likely) that was the way he was taught to do it by an instructor who learned to do it in the old days.

    You just never know. And that's why it's not a good idea to always copy what you see other racers doing.


    All that being said... it should be noted that rowing down through the gears or not is not a very big deal. It's theoretically a little slower to do it that way if you don't have to. But in the real world, if you're good at it, and you've tuned the car around the way you drive and all that.. it probably doesn't make any real difference in your lap times.

    It must wear the drivetrain a little faster. And if you blow any of the downshifts, you can lose time, spin, and/or blow the engine. So it's a case of... it's not really a helpful thing to do. It's pretty harmless if you do it right. It can be catastrophic if you do it wrong.

    - Skant
     
  3. full_wick

    full_wick

    Messages:
    41
    Just thought I'd post something about WHY torque drops off at higher RPMs, as it's kind of interesting and almost on topic ;-) Now, this is how I understand things as an amateur mechanic so don't take this as gospel and feel free to correct me or add things I have missed.

    Firstly, the basics. Most automobiles (the Mazdas being the main exception) work on a four stroke engine. The intake valve opens and the piston descends in the cylinder towards the crankshaft and away from the head. This sucks air (in a fuel injected engine) or air/fuel mixture (in a carbuerated engine) into the cylinder. In a turbo or supercharged engine, a mechanical device helps get the air or air/fuel into the cylinder. In a fuel injected engine (all modern engines) the fuel injector injects fuel into the cylinder during this part of the cycle. This is called the intake stroke.

    Then the piston reverses and comes back up, but the valves are all closed now, so it compresses the mixture, and naturally enough this is called the compression stroke. The difference in volume in the cylinder between when the piston is at the bottom and when it is at the top is called the compression ratio.

    Now, when the piston is at the top of its stroke and the air/fuel mixture is fully compressed, that is what one would think is the best time to ignite it (it's not always, and this is part of the reason for torque dropping off at higher RPMs, but we'll get to that later.) In any case, sometime at or before the piston reaches what is known as top dead center, the spark plug fires, the gas ignites (hopefully not explodes, but ignites in a controlled burn) and it expands, pushing the piston through the power stroke. Note that this is the only part of the four part cycle that produces any torque!

    Finally, as the piston rotates up towards the head again (powered by inertia or the power stroke of other pistons) the exhaust valve opens letting out the burnt gasses in what is called the exhaust stroke.

    Okay, so why would torque drop off at higher RPMs? Firstly, the piston is going in and out. It has to stop completely and reverse many times per second, and the faster it has to do this, the more pressure it puts on the bearings, resulting in more friction and loss of power at higher RPMs. This is only a small part of the story though.

    Remember how I said that ignition doesn't always take place when the piston is at top dead center? See, the thing is, the gas isn't exploding (which is very, very fast but produces 'knocking' and is very, very bad for engines.) it's burning, which is slow. In fact, at higher RPMs, there isn't enough time from top dead center to the bottom of the power stroke to burn all the gas, so the timing must be advanced to fire before the piston is allt he way at top dead center. This means the burning gas is expanding and actually fighting the compression stroke, robbing the engine of power.

    Finally, at very high RPMs (the point at which the torque curve really starts to drop rapidly, such that overall power drops even though RPMS are still going up) we get what is known as valve float. Remember, there are two valves (okay, more than two in many engines, but they all serve the same two purposes) per cylinder and each one has to open and close once per rotation. They look something like little golf tees. The round part seals the valve seat while the stem is what gets pushed on to open them. At high RPMS, this happens many times per second. The valves are opened by the cam shaft and rocker arms, and closed by heavy springs. What happens at high RPMS is that they slam shut so fast they actually bounce, and at points when the cylinder should be sealed, it isn't, resulting in lack of compression and miss-mixture of fuel and air.

    As far as I understand it, these three things are the main causes of torque dropping off at higher RPMs.
     
  4. Jmac279

    Jmac279

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    In most fuel injection engines, the fuel is injected into the throttle body. Only in diesel engines and Audi's new FSI engine is the fuel injected directly into the cylinder ...

    Combusion creates pressure ... Pressue = Force/Area (Force = Pressure*Area)
    The pressure is applied to the piston ... The piston has an area of Pi*(Bore/2)^2
    Torque = Force*Radius ... The Radius in an engine is half the stroke ...

    Therefore, Torque = Pressure*Pi*(Bore/2)^2*Stroke/2

    Since Bore and Stroke are constant, one comes to the conclusion that Torque signifies the quality of combustion at a given engine speed (for the reasons specified in abvoe post and others.)
     
  5. full_wick

    full_wick

    Messages:
    41
    Hmm, not to dispute, because I'm not an expert, but I thought throttle body was the old way and most fuel injection systems nowadays were multi-port. (does a quick google) Here's something from http://auto.howstuffworks.com/fuel-injection1.htm

     
  6. Jmac279

    Jmac279

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    Location:
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  7. full_wick

    full_wick

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    Ah, that makes sense. Putting the fuel injector in the cylinder would subject it to a lot of abuse. What I quoted says 'at the intake valve' and I assumed inside, which from what you linked is possible but is very new. Multi port actually puts one fuel injector right outside the valve. Learn something new every day :)
     
  8. Uncle Harry

    Uncle Harry

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    1,414
    Just a small comment on Skants reply on engine braking and race cars.

    Engine braking in race cars is used to help slow the car down while conserving the brakes so they last longer in a long race. It is not used to make them stop faster.

    Any of you guys watch Bathurst.
    ABS is not allowed here.
    If the drivers only used the brakes for braking they would not last the race.
    Engine braking helps them conserve the brakes for the full race distance.
    These guys are professionals so they know how to do it right.
    And sometimes these guys do get it wrong and they do spin the cars.
     
  9. ZeratulSG

    ZeratulSG

    Messages:
    917
    Okay, so the only effective reason for "rowing" through the gears, as you call it, would be to conserve brakes then?

    Unfortunately I had it e-mailed to me, so I don't know where to find it on the web. A quick Google for "motegibattle.wmv" seems to find a few results, so try that.
     
  10. Sephiroth976

    Sephiroth976

    Messages:
    52
    Or if you have a sequential gear box, where you have to go down through the gears in order.

    Thing I was taught when driving, brake pads cost less than gear boxes; and with modern brakes being as they are, they'll do the job. There is a good thread about brakes (well done guys, very interesting read) in the forums.

    Though I'd imagine if you were driveing a classic car in the game, then using engine braking may be needed, though saying that, you can upgrade the brakes on all the cars (more or less) so they won't fade...
     
  11. Scaff

    Scaff Staff Emeritus

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    I would be surprised if any profesional driver used compresion (engine) braking, even on endurance races. While it does help conserve the pads, it does no good to a race tuned engine, they are not designed to slow the car, but to power it.

    With Compresion braking it is very difficult to judge how much additional deceleration you will get, as a result if you are already at the threshold of braking (or near it) you could well overload the tyres grip level and lose control, flat spots on the tyres will result which will hammer your lap times.

    Changing the pads on a endurance race spec car is relatively quick and easy, and a task regularly carried out in the pits during endurance racing. Its also far less time consuming to have to change the pads, than it is replace an engine component (or retire through engine failiure) or lose time over a number of laps because you've just flat spotted a tyre.

    I have watched numerous endurance races, and have session reviews of Le Mans dating back to the 1960's, also a documentry on the Morgan team at Le Mans and Bathurst. I can't remember ever hearing a driver talk of using compresion braking out of choice. Now if the brakes have failed for some reason, you may not have a choice, but thats a different thing.

    You also seem to forget that unless the car in question is four wheel drive (still not that common in circuit ranging and banned in many series - such as the BTCC) compression braking will only directly effect the driven wheels, shifting the brake bias in that direction. With the majority of race cars rear wheel drive, this would mean a major brake bias to the rear wheels, if the car is anything other than straight you may now be in a situation of just trying to control the car rather than brake and setup the car for the corner entrance. Even if the car is in a straight line, if the car is runnning a high compresion engine (which increases the effect of engine braking) it can be enough to lock the rear wheels and get the back of the car twitching. Neither of these scenarios is worth conserving a little bit of brake pad material. You would get more of a saving on the brakes through good brake control and technique than you ever will through engine braking.

    A quick quote from Danny Sulivan illustrates this point
    "To put it into perspective, at Laguna Seca, which is hard on brakes, Rick Mears and I were team mates at Penske and Rick finished the race with only 70 thousandths of an inch of brake pad material left. I only used 70 thousandths of the pad in winning the race. People brake differently but can still run the same lap time, especially in a race"


    The following is an extract from the Russ Bentley books "Speed Secrets - Profressional Race Driving Techniques"

    "Again, the reason for downshifting is not to slow the car. I can't emphasise this enough. That's what brakes are for. Too many drivers try to use the engine compresion braking effect to slow the car. All they really achieve is upsetting the balance of the car and hindering braking effectiveness (if the brakes are right at the limit before locking up and you then engine braking to the rear wheels, you will probably lock up the rear brakes), and more wear and tear on the engine. Brake first, then downshift."


    This is from Skip Barber's "Going Faster"

    "What downshifting is really for.
    We ask this basic question of every racing school class. The most frequent (and incorrect) answer is, "to help slow the car down." In a racecar with good, durable brakes (the majority of modern racecars), downshifting to help slow the car down is unnecessary. The brakes slow the car down. You downshift to get the car in the proper gear to exit the corner."


    Now Russ Bentley has raced Indy cars, World Sports cars (including endurance) and is now an race instructor, Skip Barber should need no introduction, but the book I refer to has been writted with the assistance of ten instructors from the Skip Barber Racing school.

    Brakes slow the car, not the engine; unless your brakes are shot in which case you do not have a lot of choice, but unless you're Moss or Fangio you're not going to win like this.
     
  12. qsiguy

    qsiguy

    Messages:
    17
    Wow, there is a lot of long winded (albiet, educational) explainations on this thread! Just wanted to throw in one example of a car I personnally noticed (in the game) that seemed to do better shifting earlier. The Opel Speedster Turbo. I modded it all up and I noticed that the boost guage would drop quickly after around 4K rpm and at redline there was no boost at all. Also, checked the HP/Torque curve on it and sure enough it's torque, and I think HP as well, dropped significantly after 4-5K. So when driving it I tried to keep it under boost all the time. When boost started dropping I'd shift. Seemed to do well with this strategy. Seen similar issues on other cars but this one was the most obvious I'd seen.
     
  13. sucahyo

    sucahyo

    Messages:
    2,171
    I use HP curve to decide what gear I use and when to shift.
    I usually make my gear so that the hp at rpm when I shift the gear is the same as the hp at rpm drop.
    In other word if HP curve have raise at A point, and down at B point, where A hp is the same as B hp. I will shift at A and make my rpm drop to B after shift by gear tuning. I usually make the A and B distance as close as possible if the car have peaky power curve (tranny trick :p).

    When the car do not have down HP curve I always shift at max rpm.
    I always use excel file if I am going all out in gear tuning :). Without it it's impossible for me, thank's noQuarter :tup:!!! I can't thank you enough :D.
     
  14. opendriver

    opendriver

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    3,704
    I mainly use what I like to call the "butter-zone". What that is is the point where the engine makes both the most torque and the most horsepower. However, this does not apply to some cars, mainly supercharged ones:dunce:
     
  15. karelpipa

    karelpipa

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    I would like to use HP and torque curves for shifting too, but for me they are just too badly done. So you cannot be ultimately sure if you are using the most power/torque. And thats bothers me all the time.

    If you fire up the first GT1, you will see a proper power/torque curve.

    Enthusia had 2 LED lights on the RPM gauge where the one showed you that this is the peak power and the other that that is the peak torque. That would be very good. Better be showing like min. 80% power / torque. But at least the peak.