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Old 03-05-2016, 01:52 AM   #101
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Originally Posted by speedengineer View Post
Another way to think of is to consider the kinetic energy of a vehicle, which is 1/2*mass*velocity^2. At higher velocities, more energy is required for a 1mph increase in speed. Thus for a given power, more time is required for that larger increase in energy, thus acceleration must be lower.
******* weird. If I understand that right, the increase in kinetic energy is actually an exponential increase, so not even a flat torque curve would keep up?

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Old 03-05-2016, 02:00 AM   #102
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Originally Posted by guttedmiata View Post
It also had more than double the torque with the same basic hp.
By definition, if it had double the torque at a given RPM, it also had double the power at that RPM.
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Old 03-05-2016, 02:24 AM   #103
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Originally Posted by nitrodann View Post
If power is constant, and power is work/time.

Then as we increase RPM, the car travels further per unit of time.

If power is flat from 5000rpm>10,000rpm then torque is halved at 10,000rpm.

At 10,000rpm we have the same amount of work achieved per unit of time compared to 5,000rpm because horsepower is the same, but that same amount of work is stretched in half because it has to move the car double the distance.

Work achieved is how far the car is moved.

If the same amount of power has to move the car twice as far in the same unit of time, how can it possibly have enough energy left over to accelerate it just as forcefully as at 5,000 rpm when that same amount of power only had to do half of the work in a given period of time?
It's a flawed example. If you need X power to travel Y distance, you cannot travel 2Y distance with X power in the same period of time. What you're describing is a free energy machine. You said it yourself: Power is work/time, work is how far the car is moved. If you move the car twice as far in the same unit of time, then you have done twice the work. If power = work/time, and you double work, you double power.

If you re-gear the 10,000rpm example so that it only covers the same distance as the 5,000rpm example, it has 2x the gear reduction, which multiples the lower torque at the hub by 2x, resulting in the same torque output at the hub in both examples, and therefore the same acceleration force.
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Old 03-05-2016, 02:38 AM   #104
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Oops, I contradicted myself.

Quote:
Originally Posted by Savington View Post
Let's say you have a hypothetical car, and it's operating in the hypothetical universe we're talking about (no aero drag, no rolling resistance, no drivetrain losses, perfect efficiency). It is making a flat 10ft.lbs of torque at the flywheel, regardless of engine speed. (It's a very small engine, able to rev infinitely high.) At 5252rpm, you have 10hp available to you. At 52,520rpm, you have 100hp available to you. Will the car not accelerate harder with 100hp than it will with 10hp?

Multiply RPM by 10 again. 525,250rpm, 1,000hp. Still just 10ft.lbs of engine torque, and yet the car is now making a thousand horsepower. Can you honestly say that the car will not accelerate at a higher rate with a thousand horsepower than it will with just ten horsepower? Remember, engine torque has not changed at all.
It won't accelerate harder with higher power unless you maintain road speed by multiplying the gear ratios by 10 as well.

10hp, 10ft.lb, 5252rpm, 1:1 gear ratio, 10ft.lb at the hub.
100hp, 10ft.lb, 52,520rpm, 10:1 gear ratio, 100ft.lb at the hub. Same engine torque, more hub torque.
1,000hp, 10ft.lb, 525,200rpm, 100:1 gear ratio. 1,000ft.lb at the hub. Same engine torque (10ft.lb), but heavily gear reduced to create huge acceleration.
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Old 03-05-2016, 02:43 AM   #105
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So we agree that road torque not HP is what accelerates us?
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Old 03-05-2016, 02:47 AM   #106
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Originally Posted by nitrodann View Post
So we agree that road torque not HP is what accelerates us?
They rise and fall in a completely linear relationship. So yes, hub torque accelerates the car, but so does power. It's just a different way of thinking about it.

My point in the 40 posts-in-a-row above is that hub torque matters, but engine torque does not. You can have high acceleration rates without high engine torque, but you cannot have high acceleration rates without high hub torque or high power.
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Old 03-05-2016, 02:52 AM   #107
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Road torque is what is moving the vehicle however HP is what actually makes the car accelerate fast.

Think of torque like a clicker wrench. At that instant it's x. Think of horsepower of the entire motion of turning the nut and making the wrench "click".

Attached Thumbnails
Acceleration curve vs torque curve vs HP curve.-80-how_torque_is_created_a33d8f08f51c9018691998deb9241a5314efd807.gif  
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Old 03-05-2016, 03:18 AM   #108
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None of this answers the question.

In a single gear, with a flat powercurve from 4-8k, what rpm does the car accelerate hardest at?
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Old 03-05-2016, 03:27 AM   #109
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And by "powercurve" you're referring to the HP being flat or the torque being flat?
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Old 03-05-2016, 03:33 AM   #110
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HPower, and for christs sakes ignore variables like friction pls
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Old 03-05-2016, 06:08 AM   #111
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After some messing around this is what i got. Not sure how accurate the acceleration chart is though. Pretty sure its good but not 100%.
That said i'm very surprised.
Attached Thumbnails
Acceleration curve vs torque curve vs HP curve.-80-acceleration_520350d78f44817ee81e7a1b98a73789d496bb36.png  
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Old 03-05-2016, 06:35 AM   #112
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Thats exactly what I expected, real world graphs would be better but thats it.
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Old 03-05-2016, 08:47 AM   #113
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Quote:
Originally Posted by nitrodann View Post
HPower, and for christs sakes ignore variables like friction pls
HP can only be flat if rpm is steady at tq. you can't excel. with steady rpm. Excel will follow tq curve at output shaft. Hp doesn't exist without tq on rotational output engines.
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Old 03-05-2016, 09:28 AM   #114
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In a single gear with a completely flat power curve, the car will accelerate hardest at 0rpm.

In a single gear with a completely flat torque curve, the car will accelerate equally at all relevant rpms.

Sorry, I missed the last 10000 or so posts of this thread.
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Old 03-05-2016, 09:43 AM   #115
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Quote:
Originally Posted by fivehundredton View Post
HP can only be flat if rpm is steady at tq. you can't excel. with steady rpm. Excel will follow tq curve at output shaft. Hp doesn't exist without tq on rotational output engines.
HP can also be flat if TQ falls across the RPM band - just be careful of Torque approaching infinity as RPMs approach zero.



In this case, you'll have monster accelration (infinity) at zero RPM falling rapidly at first before trying to level off as to only approach zero torque at infinity rpm.

If acceleration is based on wheel torque, then notice how:

Quote:
Fooger: Acceleration will indeed dramatically reduce with engine speed *if power is constant*, because in order to keep power constant while engine speed increases we have to dramatically reduce engine torque.
Which is exactly what I said in the posting that I made that was somehow ripped apart.

Also:

Quote:
Fooger: We can also say that generally as engine speed increases, acceleration dramatically reduces *relative to power*
As far as suggesting that
Quote:
engine torque and acceleration have no correlation at all
, well that's either a drunken post or also a troll. If we double the engine torque, we double the acceleration. (You'll notice that I mentioned nothing about maintaining constant HP in this statment when I doubled engine torque, we are doubling acceleration by doubling torque across the entire RPM band - which also doubles HP)

Also, keep in mind that "Torque" and "Horsepower" are instantaneous measurements based on physics curves, and "Maximum Horsepower" and "Maximum Torque" are point measurements based on marketing hypes.
Attached Thumbnails
Acceleration curve vs torque curve vs HP curve.-theoretical%2520pwr%2520constant_zpsvclfvaoz.png  

Last edited by fooger03; 03-05-2016 at 10:10 AM.
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Old 03-05-2016, 11:13 AM   #116
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Originally Posted by Savington View Post
******* weird. If I understand that right, the increase in kinetic energy is actually an exponential increase, so not even a flat torque curve would keep up?

Yup it's goofy for sure. If you wanted to attempt to achieve constant acceleration, you would have to keep the ratio of P/v constant in the following equation:

a=P/(m*v) or regrouped as a=1/m*P/v

If P/V is constant, and mass is obviously constant, then acceleration is constant. So power has to increase linearly with vehicle speed.

Technically a constant torque vs RPM engine achieves this constant P/v as power in increasing linearly as RPM increases. BUT the very critical notation here is that this is only valid for one gear. As soon as you upshift, you've dropped back to low rpm and low power. Now you'll have constant acceleration again, but at a lower acceleration rate.


As Andrew was explaining earlier, here it is again:
Engine torque is a poor metric to use to determine vehicle acceleration as it alone is insufficient information to do anything with. You have to know and factor in all of the gearing in the trans, diff, tire diameter in order to have any idea of vehicle acceleration.
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Old 03-05-2016, 01:51 PM   #117
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Quote:
Originally Posted by nitrodann View Post
So we agree that road torque not HP is what accelerates us?
Torque at the wheels. Maximize torque to the wheels to maximize acceleration.

Quote:
Originally Posted by nitrodann View Post
None of this answers the question.

In a single gear, with a flat powercurve from 4-8k, what rpm does the car accelerate hardest at?
In this example, it would have maximum acceleration at 4K when it made peak power (which would be peak torque).

Some people posting physics need to read more.
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Old 03-05-2016, 01:53 PM   #118
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Quote:
Originally Posted by Savington View Post


What Jason (and I) are trying to say is that engine torque and acceleration have no correlation.. You can double or triple your engine torque, but if you only make that torque at 1/2 or 1/3 the engine speed, the power output remains the same, and acceleration will be the same. So yes, [i]engine[i] torque, as measured at the flywheel, is irrelevant.

If you change engine torque while maintaining the same power, acceleration will not change. If you change engine power while maintaining the same engine torque, acceleration will change. Thus, engine torque and acceleration have no correlation.



Sure, but only because wheel torque and engine power rise and fall in lock-step. You cannot increase wheel torque without increasing engine power at a given road speed.



This is completely false.

If engine power is constant, and the car is operating in a magical universe where there is no such thing as rolling resistance, drivetrain losses, or aerodynamic drag, then the car will accelerate at a constant rate.

The definition of power is a unit of energy consumed per time. Applying 200hp of energy to a body will cause that body to convert that energy into momentum. There is no engine speed at which the conversion of this energy into momentum will become less efficient, or happen at a slower rate, or cease to happen at all.

If that paragraph confused you, think about it this way. Let's say you have a hypothetical car, and it's operating in the hypothetical universe we're talking about (no aero drag, no rolling resistance, no drivetrain losses, perfect efficiency). It is making a flat 10ft.lbs of torque at the flywheel, regardless of engine speed. (It's a very small engine, able to rev infinitely high.) At 5252rpm, you have 10hp available to you. At 52,520rpm, you have 100hp available to you. Will the car not accelerate harder with 100hp than it will with 10hp?

Multiply RPM by 10 again. 525,250rpm, 1,000hp. Still just 10ft.lbs of engine torque, and yet the car is now making a thousand horsepower. Can you honestly say that the car will not accelerate at a higher rate with a thousand horsepower than it will with just ten horsepower? Remember, engine torque has not changed at all.



This is also completely false, for all the reasons explained above.
Holy crap there's some BS in here.
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Old 03-05-2016, 02:03 PM   #119
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Quote:
Originally Posted by nitrodann View Post
Hows this for an even easier example.

If I put smaller rolling diameter tyres on the car does it accelerate with more Gs or less G's in the same gear and at the same rpm and why?
It will accelerate harder. The reason why is that the force the tire exerts onto the road will be higher, due to the shorter leaver arm.
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Old 03-05-2016, 06:46 PM   #120
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Dont know why you are getting De-Kittied, you are right.

Point of this thread is to get a real world graph to prove it.

Dann
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