You are missing the point, power doesn't include those parameters in the literal sense.
The point is if you want to find acceleration using torque (in the simplest terms), you need to calculate what happens from the engine to the tire. You have to taken into account engine speed, gearing, and the tire size to figure out the force actually acting on the road.
On the other hand if you know the power at the engine it will be the same at the road because it is not affected by gearing, tire size, etc (ignoring losses). In this sense power combines all these parameter into a single value.
Therefore, if you have two identical vehicles but A) has 200 HP and B) has 250 HP. Then it is a fact that that B will always be capable of more acceleration at any given road speed.
However, this is not true for torque in that if you have two identical but A) has 200 lb-ft of torque and B) has 250 lb-ft of torque. Then either A or B could be the faster vehicle depending on which engine speed that this torque is produced at. Because the higher the engine speed the more gearing can be used, and thus more torque multiplication can be applied at the wheels.
"In theory, theory and practice are the same. In practice, they are not."
you are missing this: power= torque*rpm
as rpm are affected by, for example, the fact that the car is uphill or downhill going, power isn't what you are commanding with the accelerator, since it's influenced by all the internal (to the car) factors and even from external ones. torque is only affected by what is happening inside the car, so engine and transmission, but wich gear you are using is irrelevant, beside possible different losses between each gear.
and assuming there are two equal cars with just different power, the more powerful won't be always the best performer since they would have different power at different engine speeds, so the more powerful car could have less power than the other in a certain range of rpm, and more just at another range.
can't understand the last part of the post.
assuming the two cars have just different torque's levels, the idea is the same, the number released by automakers or at dyno is the bigger value during the test, but it doesn't mean that the car with more torque, or power, has a bigger value for all the rpm.
cant even understand what you mean with "The point is if you want to find acceleration using torque (in the simplest terms), you need to calculate what happens from the engine to the tire"
Novitec tuned a Fiat 600 up to 190 hp...
Last edited by LeonOfTheDead; 09-24-2008 at 02:46 PM.
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First, this isn’t some end all definition of power. There are other ways to define and/or measure power.Originally Posted by LeonOfTheDead
Secondly, the equation you stated is most often applied at the engine. Since it is force at the wheel that actually moves the vehicle a better equation to use is: Power = Force * Velocity.
Since at any engine speed power at the engine will generate the same power at the wheels (ignoring losses) this means that by knowing power alone one can calculate the force acting on the vehicle at any given road speed.
I am not sure I understand what you are saying. I read that to say that losses between the engine and wheels only affect power. But that would be wrong as any frictional or inertial losses, in the transmission for example, would be equal in proportion for both power and torque.Originally Posted by LeonOfTheDead
I wrote that the car with more power would be capable of more acceleration at any given road speed. This is true. I am saying that if you pick some road speed, the gearing can be chosen such that both cars make peak power at the road speed. Then the car with more power will always out accelerate the car with less power at the road speed.Originally Posted by LeonOfTheDead
The gearing could be chosen such that the car with less power out accelerates the car with more power initially. However, that implies the car with more power as the higher top speed. Again, you couldn’t reach this conclusion based on torque alone.
You are talking about using the numbers to compare performance between different cars and more often then not this will lead to erroneous conclusion. This is because there are many other factors beyond engine performance that affect vehicle performance. While this discussion is about power and torque only.Originally Posted by LeonOfTheDead
If you want to know how fast a vehicle is you want to know how much force can be generated at the tires. How much force is generate at the engine is irrelevant because you can use gears to multiply that force at the tire. However, the trade off to using gears to multiply force is that the speed (at the wheel) must be reduced accordingly.Originally Posted by LeonOfTheDead
By looking at torque the focus is only on force. By looking at power the focus is on both force and velocity.
"In theory, theory and practice are the same. In practice, they are not."
we are talking about about internal combustion engines, so the equation is right, and, AFAIK, the only one, the others are just equivalent.
but if you consider the power generated at the wheel, it doesn't matter anymore if the vehicle is propelled by an engine, a turbine or what else.
beside, the equation power=force*velocity (linear) is the same as saying torque(so force*distance)*rpm (so liear velocity/radius)
since power and torque are connected, the same can be said about torque
true, losses of any kind affect both power and torque, what I was saying is that power, being affected by rpm, and being rpm affected by factors external to the car, isn't a good way to say what your engine is giving you and that's why when you use the acceleartor pedal you are commanding more or less torque to your engine not power.
for example, you are demanding 75% of throttle, and a level road. and you are traveling at 75 mph, for instance. the road is now uphill, but you are still using 75% of throttle. the car will slow down a little, and so the rpm. for this reason now your car is delivering less power, because you are running at lower rpm. but you are still using 75% of throttle, so torque, which isn't affected by the uphill road.
hope that is clear enough
ok, assuming you can choose the right gear, is quite sure that the car with more power will accelerate faster.
what I was referring instead was this.
in two identical car (so weight, tires, aero...), but with two different engines (A 200 hp, B 250 hp), we are traveling at 60 mph. let's also assume we have just one gear, just to make things simpler.
flooring the throttle in both car will result in different acceleration, true, but it could be that at lower rpm the less powerful engine will actually have mroe power than the B car, because usually the more power you have, the more rpm you need to obtain it, and usually this affect power rpm. that's why for example Honda's Type R engines are very powerful at low rpm, because they are design to develop mpore power but at higher rpm.
so the point is just that even if you have more power, you need to be sure you have more of it at the very rpm you are running at.
basically what you said.
as I said, I wasn't referring to the gear, but to the fact that a less powerful engine usually have a good quantity (referring to its capabilities) of power at all rpm. for example, F1 engines have 800 cv, but at 19.00 rpm, while under 7.000 rpm they simply turn off, because of bad combustion and other factors, while the already mentioned Honda Type R is developing 170/200 hp and can take and advantage.
if we consider the possibility to change gear, obviously, there is no story any road car can win against and F1 car.
my point is that is silly to compare car performance just with power and torque, even if we are just considering a race on a straight line.
eve in this case, and F1 car develop a very low torque, about 450 Nm, so as much as a BMW 23d engine IIRC. even using the BMW engine in the F1 car, there is no way the car will go faster. or take the Veyron's engine with its 1300 Nm torque, beside its weight, it isn't so much faster than any other supercar, which all have about half the torque (and power too).
from what you said in the last sentence, you are repeating the fact that Power=torque*rpm=force*linear velocity
Force at the tire is connected (beside friction between ground and tire) to how fast the tire can go. this is connected to the power (or torque) the engine can deliver to the tire, but also to which gear you are using, and how big is the tire, its radius.
I suppose we are saying almost the same things, but in different terms.
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Not you can’t. For example, lets say at 60 MPH a 3000lb car is making 300 HP you could estimate the instantaneous acceleration (~.62G). Now lets say at 60 MPH that same car is making 300 lb-ft of torque, you couldn’t perform the same calculation without knowing what gear the car is in, what that gear ratio is, and tire size.
No the torque curve changes with engine speed just like the power curve.
What I am saying is that gearing and tire radius don’t affect power delivered to the road. These only affect torque and that is why it is more convenient to use ‘power’.
"In theory, theory and practice are the same. In practice, they are not."
are you considering power at the wheel though?
a car with 300 hp can't perform the same acceleration ignoring what gear is in. I would like to know how that is possible, and still can't agree with you.
torque and power curves don't change with the engine speed but with the load you are requiring. the curve is made of the value of the torque (or power) at a certain rpm, and there is a single curve for any position of the throttle, if we are talking of the same graph.
IMO the power delivering is affected by tires radius and gearing. because since you have a rotational movement (the tire), which deliver a certain power, to convert in a linear move (the car) this involve the dimension of the tire. as far as regard the gar, it's because we are transferring the movement between gears with different dimensions, this implies differences between different gears. a bigger gear would require more power/torque to be moved and that would affect the delivery.
I would like you to explain why something is what it is, and not just saying that it is so. in this way you are not convincing me I could be wrong and you could be right. not that you have to do it (convincing me), but otherwise this discussion would be a little pointless.
KFL Racing Enterprises - Kicking your ass since 2008
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"I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams
Torque and power are dependent on engine speed not load, plus acceleration is dependent on the torque curve and how wide it its and the power curve.
Eg
A diesel may make maximum torque(200nm) from 1500rpm and max power (111kw) at 3300, the most effective area for acceleration is between 1500-3300rpm which equals a very narrow powerband. Just when things are starting to happen its time to change gear as the powerband area is narrow, a close ratio 6 or 7 speed gearbox helps this situation though.
My petrol engine for eg makes 160nm @ 5800rpm and 137kw @ 9000rpm, the powerband is a lot bigger and helps with acceleration, it has a wider powerband and can be used longer in each gear, equalling better acceleration.
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"I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams
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