Another torque versus horsepower question for a track car
01-02-2008, 03:58 PM
#106
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A clear example of this would be a vehicle traveling at max torque rpms and the resultant vehicle speed. say its 50mph. the hp level at max torque for most cars is some fraction of max HP. if you can downshift to a lower gear, and be close to the max HP levels, you will have more torque to the rear wheels, even though the engine torque levels is less. why??
acceleration = power/(mass x velocity)
max torque might be something like 200hp and max HP would be at 300hp. However, the torque available to the rear wheels would be proportionally greater as well.
Is that more clear?
MK
Last edited by DaveM993; 01-02-2008 at 04:18 PM.
01-02-2008, 04:04 PM
#107
"Acceleration is NOT function of power. It is a function of force."
this is why the dimensional analysis is helpfull, from my previous post
a - acceleration derived by dividing v by t, a =v/t dimension LT^-2
f - force derived by multiplying m by a, F =ma, it's metric is pound, newton, dyne, ton, poundal etc., dimension MLT^-2
P -power - derived by multiplying distance times force and dividing the result by time, P = sF/t = w/t, it's metric is hp, watt, kW, BTU/hr, ft-lb/min, cal/sec etc. dimensionally ML^2T^-3
rearranging the f equation a = f/m dimensionally a is still LT^-2 so acceleration is really just a function of distnce and the reciprical of time x time
" Power is derived from force but this does not make acceleration a function of power directly. To look at the the equation ACCELERATION = POWER/(MASS * VELOCITY) and thus conclude ACCELERATION IS A FUNCTION OF POWER is an incorrect interpretation of this equation. This is NOT, in fact, what that equation is saying. I will get to that in a moment. But first some basic postulates:"
again it is best to look at the dimensions of power ML^2T^-3 power is a function of mass, distance x distance and the reciprical of time x time x time
"2. I am saying that the acceleration experienced by a car is a function of one thing only…Torque. Assuming, of course, that friction, air resistance, etc. are ignored. We are speaking only of engine performance as characterized by a “dyno” curve."
exactly correct
"3. Therefore if you placed an accelerometer into a car and measured its reading through a WOT acceleration and the engine performed exactly as it did on the dyno (ie. leave it in the same gear), then the profile of the acceleration readings experienced by the car would THEORECTICALLY be identical to the profile of the TORQUE curve from the dyno and not the HP curve. IE. the maximum value for the g-force (acceleration) would be at the rpm where the
MAX torque was read by the dyno and NOT at the RPM where the max HP was recorded (again…theoretically)."
exactly, it won't because of aero and mechanical resistance
once again look at post #98, for max acceleration you continue to rev past the peak torque in a given gear into the peak power range only to set up for the maximum torque available in the next gear
"All I know is that, in real life, on real life technical tracks (like Motorsport Ranch), I am ~2 seconds a lap faster when I use more of my beefy torque, and less of the wheezy 7,000 RPM redline. " to the extent that you go past the point where you pick up the next gears max torque range, that is reasonable observation
this is why the dimensional analysis is helpfull, from my previous post
a - acceleration derived by dividing v by t, a =v/t dimension LT^-2
f - force derived by multiplying m by a, F =ma, it's metric is pound, newton, dyne, ton, poundal etc., dimension MLT^-2
P -power - derived by multiplying distance times force and dividing the result by time, P = sF/t = w/t, it's metric is hp, watt, kW, BTU/hr, ft-lb/min, cal/sec etc. dimensionally ML^2T^-3
rearranging the f equation a = f/m dimensionally a is still LT^-2 so acceleration is really just a function of distnce and the reciprical of time x time
" Power is derived from force but this does not make acceleration a function of power directly. To look at the the equation ACCELERATION = POWER/(MASS * VELOCITY) and thus conclude ACCELERATION IS A FUNCTION OF POWER is an incorrect interpretation of this equation. This is NOT, in fact, what that equation is saying. I will get to that in a moment. But first some basic postulates:"
again it is best to look at the dimensions of power ML^2T^-3 power is a function of mass, distance x distance and the reciprical of time x time x time
"2. I am saying that the acceleration experienced by a car is a function of one thing only…Torque. Assuming, of course, that friction, air resistance, etc. are ignored. We are speaking only of engine performance as characterized by a “dyno” curve."
exactly correct
"3. Therefore if you placed an accelerometer into a car and measured its reading through a WOT acceleration and the engine performed exactly as it did on the dyno (ie. leave it in the same gear), then the profile of the acceleration readings experienced by the car would THEORECTICALLY be identical to the profile of the TORQUE curve from the dyno and not the HP curve. IE. the maximum value for the g-force (acceleration) would be at the rpm where the
MAX torque was read by the dyno and NOT at the RPM where the max HP was recorded (again…theoretically)."
exactly, it won't because of aero and mechanical resistance
once again look at post #98, for max acceleration you continue to rev past the peak torque in a given gear into the peak power range only to set up for the maximum torque available in the next gear
"All I know is that, in real life, on real life technical tracks (like Motorsport Ranch), I am ~2 seconds a lap faster when I use more of my beefy torque, and less of the wheezy 7,000 RPM redline. " to the extent that you go past the point where you pick up the next gears max torque range, that is reasonable observation
01-02-2008, 04:26 PM
#109
Rennlist Member
Great, I got you thinking!! good argument, but Ill have to shorten my response here because i have to run, but you are fighting basic facts.
First of all, the equation, Acceleration=Power/(mass x velocity) is an identity. So, dont fight it.
F=ma
P=Fv
Therefore when you combine the two identities:
P=M*A*V
Therefore:
Acceleration=Power/(mass x velocity)
Algebra says that yes, Acceleration is proportional to power at any given speed, and with a constant mass (in otherwords at the same MPH and in the same car) It also says that acceleration is inversely proportional to speed. Meaning, if speed goes up, and power is constant, (best case would be CVT) acceleration will go down.
#1 Yes, we are talking about max acceleration at any speed. remember,ive been very clear to point out that with gears its tough to do this, so I qualified it by saying, " as close to max HP as possible". This gives the max acceleration at any speed.
#2. Yes, torque is what provides the acceleration, but it is the torque as found, multiplied through the gear box, at ANY speed, that will provide this thrust force
#3 Your entire problem lies in you are boucing back and forth with rear wheel forces, (ie torque as measured at the rear wheels) and engine torque)
All you have to do is find any speed, where a two cars have the same HP applied to the rear wheels, and the rear wheel torque is not identical (same diameter tires
) and you can prove yourself right.
speed HAS to be kept constant , or you can compare anything. just like the mass has to be kept constant.
This is not as simple as just A=Fm. why, because we are dealing with a variable power (no CTV), and because of the gears, we are dealing with a non constant variation of acceleration. (jerk is one thing, but this is variable jerk as well. Jerk is rate of change of acceleration), because we are dealing with the torque curves over any given speed range.
So, the equation is correct. Yes, as you apply a constant Power and speed goes down, yes, acceleration will go toward infinity. The key is applying the power at 0 speed. Look at a constant force , like gravity "g". drop a rock and it instantly starts to accelerate at a rate of 32f/s/s. that is constant force. like a rocket in space. apply the force long enough and you get infinite speed too!
ever watch the jet vs the Formula 1 car race? kind of a good example of constant force vs constant HP. car darts out and kills the jet to 60mph, why, constant power of near 800hp. Jet is lagging, with tons of thrust force, but less than the car, since power=F*speed, as the jet accelerates the power is going up with speed. the car on the other hand, who's power is limited at 800, acceleration (by following the equation) will go down directly inversely proportional to speed. at 1/4 mile they meet, after that the 100,000+ hp takes over and grows with speed with the jet. the thrust of the jet is still constant (force constant) acceleration is still constant, while the cars torque at the rear wheel is falling at the same rate as its acceleration. (assuming it has infinite gears and they are very close together)
All this says is that power determines acceleration. Power is the rate of doing work, by definition. (work is Force times a distance). and Power determines the rear wheel forces at any speed that create the acceleration. It all starts a step further with potential energy (ie Gas in the tank), converted to power which is made up of Torque and Rpms at the engine.
So, to your very last point. yes, the car will follow the torque curve if left in a particular gear, but the reason for this, is that the power is variable as well.
if you had another gear to shift into that was alowing the engine to operate at a higher rpm, closer to max HP, the acceleration of your car (looked at as a system) would be greater. (at any given vehicle speed) EVEN THOUGH, the acceleration in a particular gear has its greatest acceleration at max torque. Its not true for the vehicle that can select a lower gear and have greater acceleration at that same vehicle speed.
MK
First of all, the equation, Acceleration=Power/(mass x velocity) is an identity. So, dont fight it.
F=ma
P=Fv
Therefore when you combine the two identities:
P=M*A*V
Therefore:
Acceleration=Power/(mass x velocity)
Algebra says that yes, Acceleration is proportional to power at any given speed, and with a constant mass (in otherwords at the same MPH and in the same car) It also says that acceleration is inversely proportional to speed. Meaning, if speed goes up, and power is constant, (best case would be CVT) acceleration will go down.
#1 Yes, we are talking about max acceleration at any speed. remember,ive been very clear to point out that with gears its tough to do this, so I qualified it by saying, " as close to max HP as possible". This gives the max acceleration at any speed.
#2. Yes, torque is what provides the acceleration, but it is the torque as found, multiplied through the gear box, at ANY speed, that will provide this thrust force
#3 Your entire problem lies in you are boucing back and forth with rear wheel forces, (ie torque as measured at the rear wheels) and engine torque)
All you have to do is find any speed, where a two cars have the same HP applied to the rear wheels, and the rear wheel torque is not identical (same diameter tires
) and you can prove yourself right. speed HAS to be kept constant , or you can compare anything. just like the mass has to be kept constant.
This is not as simple as just A=Fm. why, because we are dealing with a variable power (no CTV), and because of the gears, we are dealing with a non constant variation of acceleration. (jerk is one thing, but this is variable jerk as well. Jerk is rate of change of acceleration), because we are dealing with the torque curves over any given speed range.
So, the equation is correct. Yes, as you apply a constant Power and speed goes down, yes, acceleration will go toward infinity. The key is applying the power at 0 speed. Look at a constant force , like gravity "g". drop a rock and it instantly starts to accelerate at a rate of 32f/s/s. that is constant force. like a rocket in space. apply the force long enough and you get infinite speed too!
ever watch the jet vs the Formula 1 car race? kind of a good example of constant force vs constant HP. car darts out and kills the jet to 60mph, why, constant power of near 800hp. Jet is lagging, with tons of thrust force, but less than the car, since power=F*speed, as the jet accelerates the power is going up with speed. the car on the other hand, who's power is limited at 800, acceleration (by following the equation) will go down directly inversely proportional to speed. at 1/4 mile they meet, after that the 100,000+ hp takes over and grows with speed with the jet. the thrust of the jet is still constant (force constant) acceleration is still constant, while the cars torque at the rear wheel is falling at the same rate as its acceleration. (assuming it has infinite gears and they are very close together)
All this says is that power determines acceleration. Power is the rate of doing work, by definition. (work is Force times a distance). and Power determines the rear wheel forces at any speed that create the acceleration. It all starts a step further with potential energy (ie Gas in the tank), converted to power which is made up of Torque and Rpms at the engine.
So, to your very last point. yes, the car will follow the torque curve if left in a particular gear, but the reason for this, is that the power is variable as well.
if you had another gear to shift into that was alowing the engine to operate at a higher rpm, closer to max HP, the acceleration of your car (looked at as a system) would be greater. (at any given vehicle speed) EVEN THOUGH, the acceleration in a particular gear has its greatest acceleration at max torque. Its not true for the vehicle that can select a lower gear and have greater acceleration at that same vehicle speed.
MK
OK Gang (and Mark K.)…here is my shot at it….please have at it.
Acceleration is NOT function of power. It is a function of force. Power is derived from force but this does not make acceleration a function of power directly. To look at the the equation ACCELERATION = POWER/(MASS * VELOCITY) and thus conclude ACCELERATION IS A FUNCTION OF POWER is an incorrect interpretation of this equation. This is NOT, in fact, what that equation is saying. I will get to that in a moment. But first some basic postulates:
1. The acceleration we are talking about is the instantaneous G-force experienced by the car. We are not talking about a 0-60 time…this is an AVERAGE acceleration. Making the statement that max acceleration occurs AT MAX. HP infers we are talking about instantaneous acceleration experienced at some point on the RPM curve, I just want to make sure we are on the same page of how we define acceleration.
2. I am saying that the acceleration experienced by a car is a function of one thing only…Torque. Assuming, of course, that friction, air resistance, etc. are ignored. We are speaking only of engine performance as characterized by a “dyno” curve.
3. Therefore if you placed an accelerometer into a car and measured its reading through a WOT acceleration and the engine performed exactly as it did on the dyno (ie. leave it in the same gear), then the profile of the acceleration readings experienced by the car would THEORECTICALLY be identical to the profile of the TORQUE curve from the dyno and not the HP curve. IE. the maximum value for the g-force (acceleration) would be at the rpm where the MAX torque was read by the dyno and NOT at the RPM where the max HP was recorded (again…theoretically).
First, let’s look at the correct physics.
Acceleration = Force/Mass. (F = ma), it gets no simpler than that.
Where:
a = acceleration of the car
m = mass of the car
F = the force applied to the car in order to induce the acceleration.
There is only one force that a car experiences that is generated by the engine…torque. It does not experience HP, that is a derived quantity. In general terms Torque = Force X Distance (T = Fd). It is a unit of energy (I am using T for torque here). In this case and in very simple terms the “distance” is the radius point of the rotation at which the force is measured. This is important to understand because we are not talking about the speed or velocity of the car…but the rotational speed at the point we area measuring the force on the rotation shaft to come up with our torque number. They are certainly related by the gearing...but the whole issue of gears confuses this discussion because we can actually ignore the gears altogether. We are not talking about absolute acceleration, but the relative acceleration over the functional RPM range of the engine. Pick any gear you want….just don’t change it during the test. This acceleration is being induced at the point where the "rubber meets the road". That is where it is measured...everything from there back to the engine is just a bunch of gears.
So with the above we can simplify F=ma even further to a = T/(md). Assuming that the mass of the car is constant and that the gear is not changed during the measurement then acceleration is a function of Torque (T) ONLY. Its as simple as that.
So back to our ACCELERATION = POWER/(MASS*VELOCITY) or to shorten:
a = P/(mv)
To say that acceleration varies according to power is an incorrect representation of this equation. This equation, in fact, COMPLETELY supports the fact that acceleration varies as a function of torque only. How?
It’s the pesky little “velocity” term. In order for acceleration to vary ACCORDING to power, it requires that all variables on the right hand side of the equation be kept constant and that power is then varied and the resulting change in acceleration is then measured. ONLY ONE PROBLEM….you cannot keep the other terms constant in order for the equation to be meaningful. The “velocity” in the denominator is the same "velocity" that is in the “POWER” term (remember P=Force X Velocity)) The acceleration is independent of the velocity term since it cancels out and you are left with….wait for it…torque. You want the math…here it is:
Power = Torque/time = force X (distance/time)
V = distance/time
m=mass
acceleration=distance/(time squared)
Everything cancels out and you are left with a = F/m, assuming the distance is kept constant (no change in gears) and we have shown that the time changes (ie. RPM increase) cancel out, the only dependent variables are mass and force...and since the mass is constant...the only dependent variable is Force and thus Torque.
In order for the equation a=P/(mv) to work the “distance” and “time” of the power (P) term and the velocity (v) term must be measuring the same thing. Therefore acceleration is NOT, BY DEFINTION dependent on Power...only Torque.
It would take too long to do here but you can see that one can manipulate these equations to increase power and keep acceleration constant, one can decrease power and increase acceleration...there is no direct relationship to power.
The ONLY way the acceleration vs. power interpretation of the equation works is if the “velocity” term in the denominator is independent of power. However, one quick look (beyond the math) shows why this is absurd. Let us say that it is independent and we can therefore hold the Power and the mass terms constant. As we decrease “velocity” the acceleration tends towards infinity. This is clearly wrong.
So the g-force (acceleration) experience by a moving vehicle at a given point of the RPM curve is a function only of the torque generated by the engine for a given gear, and thus MAX g-force (acceleration) is experienced at MAX torque...not MAX HP.
(Did an edit for clarity)
Acceleration is NOT function of power. It is a function of force. Power is derived from force but this does not make acceleration a function of power directly. To look at the the equation ACCELERATION = POWER/(MASS * VELOCITY) and thus conclude ACCELERATION IS A FUNCTION OF POWER is an incorrect interpretation of this equation. This is NOT, in fact, what that equation is saying. I will get to that in a moment. But first some basic postulates:
1. The acceleration we are talking about is the instantaneous G-force experienced by the car. We are not talking about a 0-60 time…this is an AVERAGE acceleration. Making the statement that max acceleration occurs AT MAX. HP infers we are talking about instantaneous acceleration experienced at some point on the RPM curve, I just want to make sure we are on the same page of how we define acceleration.
2. I am saying that the acceleration experienced by a car is a function of one thing only…Torque. Assuming, of course, that friction, air resistance, etc. are ignored. We are speaking only of engine performance as characterized by a “dyno” curve.
3. Therefore if you placed an accelerometer into a car and measured its reading through a WOT acceleration and the engine performed exactly as it did on the dyno (ie. leave it in the same gear), then the profile of the acceleration readings experienced by the car would THEORECTICALLY be identical to the profile of the TORQUE curve from the dyno and not the HP curve. IE. the maximum value for the g-force (acceleration) would be at the rpm where the MAX torque was read by the dyno and NOT at the RPM where the max HP was recorded (again…theoretically).
First, let’s look at the correct physics.
Acceleration = Force/Mass. (F = ma), it gets no simpler than that.
Where:
a = acceleration of the car
m = mass of the car
F = the force applied to the car in order to induce the acceleration.
There is only one force that a car experiences that is generated by the engine…torque. It does not experience HP, that is a derived quantity. In general terms Torque = Force X Distance (T = Fd). It is a unit of energy (I am using T for torque here). In this case and in very simple terms the “distance” is the radius point of the rotation at which the force is measured. This is important to understand because we are not talking about the speed or velocity of the car…but the rotational speed at the point we area measuring the force on the rotation shaft to come up with our torque number. They are certainly related by the gearing...but the whole issue of gears confuses this discussion because we can actually ignore the gears altogether. We are not talking about absolute acceleration, but the relative acceleration over the functional RPM range of the engine. Pick any gear you want….just don’t change it during the test. This acceleration is being induced at the point where the "rubber meets the road". That is where it is measured...everything from there back to the engine is just a bunch of gears.
So with the above we can simplify F=ma even further to a = T/(md). Assuming that the mass of the car is constant and that the gear is not changed during the measurement then acceleration is a function of Torque (T) ONLY. Its as simple as that.
So back to our ACCELERATION = POWER/(MASS*VELOCITY) or to shorten:
a = P/(mv)
To say that acceleration varies according to power is an incorrect representation of this equation. This equation, in fact, COMPLETELY supports the fact that acceleration varies as a function of torque only. How?
It’s the pesky little “velocity” term. In order for acceleration to vary ACCORDING to power, it requires that all variables on the right hand side of the equation be kept constant and that power is then varied and the resulting change in acceleration is then measured. ONLY ONE PROBLEM….you cannot keep the other terms constant in order for the equation to be meaningful. The “velocity” in the denominator is the same "velocity" that is in the “POWER” term (remember P=Force X Velocity)) The acceleration is independent of the velocity term since it cancels out and you are left with….wait for it…torque. You want the math…here it is:
Power = Torque/time = force X (distance/time)
V = distance/time
m=mass
acceleration=distance/(time squared)
Everything cancels out and you are left with a = F/m, assuming the distance is kept constant (no change in gears) and we have shown that the time changes (ie. RPM increase) cancel out, the only dependent variables are mass and force...and since the mass is constant...the only dependent variable is Force and thus Torque.
In order for the equation a=P/(mv) to work the “distance” and “time” of the power (P) term and the velocity (v) term must be measuring the same thing. Therefore acceleration is NOT, BY DEFINTION dependent on Power...only Torque.
It would take too long to do here but you can see that one can manipulate these equations to increase power and keep acceleration constant, one can decrease power and increase acceleration...there is no direct relationship to power.
The ONLY way the acceleration vs. power interpretation of the equation works is if the “velocity” term in the denominator is independent of power. However, one quick look (beyond the math) shows why this is absurd. Let us say that it is independent and we can therefore hold the Power and the mass terms constant. As we decrease “velocity” the acceleration tends towards infinity. This is clearly wrong.
So the g-force (acceleration) experience by a moving vehicle at a given point of the RPM curve is a function only of the torque generated by the engine for a given gear, and thus MAX g-force (acceleration) is experienced at MAX torque...not MAX HP.
(Did an edit for clarity)
01-02-2008, 04:32 PM
#110
Rennlist Member
could be due to less wheel spin out of the turns and less shifting .
But, in "real life" you will find if you can apply more power over a lap, you will have faster lap times. get a data logger and you will see this extremely clearly
mk
But, in "real life" you will find if you can apply more power over a lap, you will have faster lap times. get a data logger and you will see this extremely clearly
mk
01-02-2008, 04:38 PM
#111
Rennlist Member
I do not believe wheelspin in a 3,175 pound 240hp car is a big problem. 
As I said previously, your claim IMO is all dependent upon the track, the engine characteristics (torque & hp curves) and a host of other things. It is not absolute as you suggest.
About the only thing that IS an absolute is a slight modification of your claim above: "But, in 'real life' you will find if you can apply more throttle over a lap, you will have faster lap times.
As I said previously, your claim IMO is all dependent upon the track, the engine characteristics (torque & hp curves) and a host of other things. It is not absolute as you suggest.
About the only thing that IS an absolute is a slight modification of your claim above: "But, in 'real life' you will find if you can apply more throttle over a lap, you will have faster lap times.
01-02-2008, 04:41 PM
#112
Rennlist Member
you cant compare accelerations at different speeds and that is where you are going with the argument below. NOW that is comparing apples and oranges!
who cares about what gears you are in , how many you have to deal with, etc. you pick the gear at any speed, which gets you as close to max HP as possible and it will yield the greatest acceleration possible for the system.
If you say, acceleration is dependant on gear and torque, arent you saying that it is dependant on Power? In order to use the gear and get the torque at any speed, you need engine RPMS.
Find any speed where maximizing HP, not engine torque doesnt yeild the greatest rear wheel torque acting on the pavement. (remember, this is for the system, of engine and gears available)
mk
who cares about what gears you are in , how many you have to deal with, etc. you pick the gear at any speed, which gets you as close to max HP as possible and it will yield the greatest acceleration possible for the system.
If you say, acceleration is dependant on gear and torque, arent you saying that it is dependant on Power?
In order to use the gear and get the torque at any speed, you need engine RPMS. Find any speed where maximizing HP, not engine torque doesnt yeild the greatest rear wheel torque acting on the pavement. (remember, this is for the system, of engine and gears available)
mk
Mark, you are mixing apples and oranges. Where to start. First, I thought we were talking about max acceleration along the RPM axis of a given dyno curve...not max acceleration that the vehicle is capable of. Even if we are talking about that...it will be at the point of Max. torque on the torque curve (in the lowest gear possible) not the max hp. as my other, rather long winded post, points out. As to your example...it is meaningless for this discussion as you ARE mixing apples and oranges. To suggest that experiencing more torque at the wheels because of a lower gear means it is not dependent is absurd. All that you are saying here is that Torque at the rear wheels is only dependent on two things, engine torque and gear. Thus it is not dependent on POWER....your actually making my point. If the gear is left constant, the ONLY variable is torque. As I said in the other post, the reading of an accelerometer in the car is dependent on only one thing, torque at the wheels in contact with the pavement (excluding friction, air resistance, etc.). It will thus ONLY vary as the torque varies and for any constant gear the acceleration value will be at its max value for that gear at the RPM where MAX torque is experienced.
01-02-2008, 04:42 PM
#113
Drifting
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First of all, the equation, Acceleration=Power/(mass x velocity) is an identity. So, dont fight it.
F=ma
P=Fv
Therefore when you combine the two identities:
P=M*A*V
Therefore:
Acceleration=Power/(mass x velocity)
Algebra says that yes, Acceleration is proportional to power at any given speed, and with a constant mass (in otherwords at the same MPH and in the same car) It also says that acceleration is inversely proportional to speed. Meaning, if speed goes up, and power is constant, (best case would be CVT) acceleration will go down.
MK
F=ma
P=Fv
Therefore when you combine the two identities:
P=M*A*V
Therefore:
Acceleration=Power/(mass x velocity)
Algebra says that yes, Acceleration is proportional to power at any given speed, and with a constant mass (in otherwords at the same MPH and in the same car) It also says that acceleration is inversely proportional to speed. Meaning, if speed goes up, and power is constant, (best case would be CVT) acceleration will go down.
MK
When you say speed goes up and power is constant...how is it constant? The only way for power to stay constant and speed to go up is if torque goes down!!! which of course means acceleration goes down.The speed term is NOT THE SPEED OF THE CAR...it can't be...if you mean for it to be that then the equation is absolutely meaningless. It is the speed of rotation...ie RPM. so if you are increasing the RPM and the power is staying constant then the torque is going down and so is the acceleration.
Its basic physics my friend!
Better yet. Get in a car with a flat torque curve and run it through the flat torque range of the RPM in one gear. You will not fell ANY change in acceleration. If your logic is correct then given that most HP curves climb, one would feel increasing acceleration to the point of max HP. We don't...because...acceleration is not dependent on HP.
01-02-2008, 04:48 PM
#114
Race Director
For me in practice the are both the same. Here is why. I went go down to 2nd the higher RPM and extra downshift tend to slow my entry speed a bit. I do however have lots of hp on tap to get out of the corner fast and I can feel the surge as I get back on the gas. One major downside however is I need to shift to 3rd about 2/3 of the way through the long increasing radius corner. If take the corner in 3rd at entry the car is at very bottom of the power band and does not accelerate well out of the corner until the revs build some. Sounds it should be slower, but going in a higher gear means I carry more entry speed and it is much easir to put all of my hp down to the ground (since I have less) and no mid corner shift. End result is the same sector times and same RPM peak down the next straight. What I have learned however is to take 2nd in traffic, but run 3rd with clear space as if I get slowed down on entry due to traffic I can't make up ground and NEED 2nd pull me out. If I have clear track 3rd is easier to manage for me and is easlier on the car.
This situation could easily describe why in the real world on track you maybe "faster" running less RPM and taking turns in a higher gear.
01-02-2008, 04:48 PM
#115
Rennlist Member
LOL, do you really believe that????
let me give you a great story, I couldnt have asked you to prompt me with a better segway.
I ran an enduro at the Laguna Grand finale SCCA race . the guys car i was driving , had lost all the gears but one. Let me tell you , i was floored the entire time! lap times were lower, why????? HP-seconds !!!!!!
If you have no wheel spin and are short shifting and getting faster lap times, two things can be happening to keep you from your ultimate lap time. you have to shift faster! or, you have an engine that has a HP fall off dramatically, where a short shift gives you better average hp or more HP-seconds.
there is no logical reason why you would be faster by short shifting. as i said, coming off a turn at the low end of max torque, when you could be exiting closer to max HP in a lower gear provides more rear wheel torque and greater acceleration. if you dont believe me, find one spot on your track where this would not be true and proves your point. after all, we all agree that its the thrust force at the rear wheels that creates the acceleration.
MK
let me give you a great story, I couldnt have asked you to prompt me with a better segway.
I ran an enduro at the Laguna Grand finale SCCA race . the guys car i was driving , had lost all the gears but one. Let me tell you , i was floored the entire time! lap times were lower, why????? HP-seconds !!!!!!
If you have no wheel spin and are short shifting and getting faster lap times, two things can be happening to keep you from your ultimate lap time. you have to shift faster! or, you have an engine that has a HP fall off dramatically, where a short shift gives you better average hp or more HP-seconds.
there is no logical reason why you would be faster by short shifting. as i said, coming off a turn at the low end of max torque, when you could be exiting closer to max HP in a lower gear provides more rear wheel torque and greater acceleration. if you dont believe me, find one spot on your track where this would not be true and proves your point. after all, we all agree that its the thrust force at the rear wheels that creates the acceleration.
MK
I do not believe wheelspin in a 3,175 pound 240hp car is a big problem.
As I said previously, your claim IMO is all dependent upon the track, the engine characteristics (torque & hp curves) and a host of other things. It is not absolute as you suggest.
About the only thing that IS an absolute is a slight modification of your claim above: "But, in 'real life' you will find if you can apply more throttle over a lap, you will have faster lap times.
As I said previously, your claim IMO is all dependent upon the track, the engine characteristics (torque & hp curves) and a host of other things. It is not absolute as you suggest.
About the only thing that IS an absolute is a slight modification of your claim above: "But, in 'real life' you will find if you can apply more throttle over a lap, you will have faster lap times.
01-02-2008, 04:50 PM
#116
Drifting
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Acceleration is independent of speed. Period. A body at a constant velocity with no force acting on it has an acceleration of zero. The acceleration of a body is only induced by a force acting upon it and the amount of that acceleration is only dependent on the body's mass and the size of the force. The speed of the body HAS NO IMPACT on its acceleration.
I think you are confusing air resistance, friction, etc. for the reason why acceleration falls off at greater speeds in a car. That has NO IMPACT on our discussion.
If that is one you can't accept...then I can't go any further.
I did my best guys.
Dave
01-02-2008, 05:14 PM
#117
Rennlist Member
Mark, Mark, Mark.....when did I ever say I was short shifting? I said that on many tracks TQ is more valuable than HP...meaning that (for example) instead of downshifting to 2nd gear for a particular corner & then being near redline for the short time before having to upshift again, some cars with wide TQ bands (like the stock 3.2 E36 USA M3) are better left in 3rd so they can torque their way out of the corner. that is not short shifting---it is keeping the engine at lower HP in some places to use abundant TQ rather than fleeting (and sometimes declining) HP.
By the way, in your example, I would suspect that your Laguna Seca SCCA car had a built motor that focused on high RPM horsepower.....right? Oh yeah....and you had lost all gears. Way to make a major u-turn from the core premise of this thread....
I will leave you with one of the 2 most fundamentally educational things I learned about driving from a friend of mine named Redman (I think Larry knows him, too): "Dave, horsepower is for laptimes, but torque wins races." (the other was watching a master like him left foot brake).
Cheers---I am out of this thread at this point.
By the way, in your example, I would suspect that your Laguna Seca SCCA car had a built motor that focused on high RPM horsepower.....right? Oh yeah....and you had lost all gears. Way to make a major u-turn from the core premise of this thread....
I will leave you with one of the 2 most fundamentally educational things I learned about driving from a friend of mine named Redman (I think Larry knows him, too): "Dave, horsepower is for laptimes, but torque wins races." (the other was watching a master like him left foot brake).
Cheers---I am out of this thread at this point.
01-02-2008, 05:22 PM
#118
Rennlist Member
Dave, first of all when i refer to speed, im refering to vehicle speed. maybe ive left it out a few times, but i thought it was obvious. If i refer to engine speed, Ill often refer to it as "RPM".
you are arguing with an identity. basic newtonian identities.
It is valid and true. if you stop and think about how is this correct, rather than trying to prove it wrong, you might begin to understand.
Yes, if velocity can go to zero, and power can be at some constant, yes, acceleration will go to infinity. Just as in my prior example, speed can go to infinity if force can be kept constant.
So, also yes, if power is constant, acceleration is inversely proportional to speed. just look at any G-meter as you accelerate or anyones computer simulation. This also means that rear wheel torque also goes down with speed if power is kept constant, and that is true as well.
You say these things are clearly "wrong" but you put no meat into your argument. just saying something is wrong or obsurd, does not make yourself right.
so, look at your statement about power being constant and acceleration going down. yes, if power is constant, torque goes down (to the rear wheels) and it goes down at a rate inversely proportional to speed.
Remember, we are not talking about a car in a single gear where HP will be climbing, and torque rising and falling. we are talking about the entire system.
you can choose the gears you are in. you need to choose them as to maximize HP , not engine torque at ANY speed (vehicle speed)
You get yourself in deeper with your last paragraph about constant or flat torque. Yes, i do race, and think i have been suceesful by a keen understanding of the physics discussed here. Its usually one of my advantages over my competition that think some of the things you are suggesting here. If my VET competitor would shift at redline instead of short shifting, our .001 second different qualifying times might look like seconds in his favor!
Now, back to a flat torque curve. As far as feeling, even with a flat torque curve, (and i didnt want to bring this up, but you brough up "feeling".) you will not "feel" constant G forces. this is because the force to keep a constant acceleration at any speed, lhas to be a "NET" force and more will be required to fight things like the aero forces. The only thing relatively constant is rolling friction. So, if you have a constant torque, yes your acceleration will be close to constant, but not really. Your last sentence has the issue as most have with this topic. You are talking about varied speed (vehicle speeds) HP is rising with vehicle speed, but we are talking about comparative acceleration at any SAME vehicle speed. We can figure out the acceration rates with power only , if we wanted to . Power is the rate of change of kinetic energy, and that by definition is what we are talking about. At any speed higher on the HP curve, we are having a greater rate of change of kinetic energy. It doesnt nesessarily mean we are accelerating at a greater rate. (It can, but it doesnt have to and this is a divergence from the main point) However, if we want to maximize acceleration at any vehicle speed, we want the greatest HP available, and this is the point I am trying to make.
The problem is that you are not looking at the system. you are only looking at one gear. Yes, in one gear, acceleration will follow the torque profile of the torque curve, (to a certain extent, ingnoring aero drag, etc). to maximize acceleration at any vehicle speed (say you want come out of a turn with greater acceleration) find a gear that puts the rpms closer to max HP, not max torque, and you will have more torque at the rear wheels through the gear box.
acceleration= power/(mass x velocity) what this means is acceleration is proportional to power at ANY VEHICLE SPEED, and inversely proportional to VEHICLE SPEED.
Now, still think Im "Flat out wrong"?
mk
you are arguing with an identity. basic newtonian identities.
It is valid and true. if you stop and think about how is this correct, rather than trying to prove it wrong, you might begin to understand.
Yes, if velocity can go to zero, and power can be at some constant, yes, acceleration will go to infinity. Just as in my prior example, speed can go to infinity if force can be kept constant.
So, also yes, if power is constant, acceleration is inversely proportional to speed. just look at any G-meter as you accelerate or anyones computer simulation. This also means that rear wheel torque also goes down with speed if power is kept constant, and that is true as well.
You say these things are clearly "wrong" but you put no meat into your argument. just saying something is wrong or obsurd, does not make yourself right.
so, look at your statement about power being constant and acceleration going down. yes, if power is constant, torque goes down (to the rear wheels) and it goes down at a rate inversely proportional to speed.
Remember, we are not talking about a car in a single gear where HP will be climbing, and torque rising and falling. we are talking about the entire system.
you can choose the gears you are in. you need to choose them as to maximize HP , not engine torque at ANY speed (vehicle speed)
You get yourself in deeper with your last paragraph about constant or flat torque. Yes, i do race, and think i have been suceesful by a keen understanding of the physics discussed here. Its usually one of my advantages over my competition that think some of the things you are suggesting here. If my VET competitor would shift at redline instead of short shifting, our .001 second different qualifying times might look like seconds in his favor!
Now, back to a flat torque curve. As far as feeling, even with a flat torque curve, (and i didnt want to bring this up, but you brough up "feeling".) you will not "feel" constant G forces. this is because the force to keep a constant acceleration at any speed, lhas to be a "NET" force and more will be required to fight things like the aero forces. The only thing relatively constant is rolling friction. So, if you have a constant torque, yes your acceleration will be close to constant, but not really. Your last sentence has the issue as most have with this topic. You are talking about varied speed (vehicle speeds) HP is rising with vehicle speed, but we are talking about comparative acceleration at any SAME vehicle speed. We can figure out the acceration rates with power only , if we wanted to . Power is the rate of change of kinetic energy, and that by definition is what we are talking about. At any speed higher on the HP curve, we are having a greater rate of change of kinetic energy. It doesnt nesessarily mean we are accelerating at a greater rate. (It can, but it doesnt have to and this is a divergence from the main point) However, if we want to maximize acceleration at any vehicle speed, we want the greatest HP available, and this is the point I am trying to make.
The problem is that you are not looking at the system. you are only looking at one gear. Yes, in one gear, acceleration will follow the torque profile of the torque curve, (to a certain extent, ingnoring aero drag, etc). to maximize acceleration at any vehicle speed (say you want come out of a turn with greater acceleration) find a gear that puts the rpms closer to max HP, not max torque, and you will have more torque at the rear wheels through the gear box.
acceleration= power/(mass x velocity) what this means is acceleration is proportional to power at ANY VEHICLE SPEED, and inversely proportional to VEHICLE SPEED.
Now, still think Im "Flat out wrong"?
mk
Sorry Mark, but you are flat out wrong. Look at the equation, again closely. By your logic, looking at it strictly as an identity as you are, it creates its own absurdities. Have velocity go to zero....acceleration goes to infinity...wrong. Also, by saying "Meaning, if speed goes up, and power is constant, (best case would be CVT) acceleration will go down." This is also clearly wrong...unless Newton got it wrong When you say speed goes up and power is constant...how is it constant? The only way for power to stay constant and speed to go up is if torque goes down!!! which of course means acceleration goes down.
The speed term is NOT THE SPEED OF THE CAR...it can't be...if you mean for it to be that then the equation is absolutely meaningless. It is the speed of rotation...ie RPM. so if you are increasing the RPM and the power is staying constant then the torque is going down and so is the acceleration.
Its basic physics my friend!
Better yet. Get in a car with a flat torque curve and run it through the flat torque range of the RPM in one gear. You will not fell ANY change in acceleration. If your logic is correct then given that most HP curves climb, one would feel increasing acceleration to the point of max HP. We don't...because...acceleration is not dependent on HP.
When you say speed goes up and power is constant...how is it constant? The only way for power to stay constant and speed to go up is if torque goes down!!! which of course means acceleration goes down.The speed term is NOT THE SPEED OF THE CAR...it can't be...if you mean for it to be that then the equation is absolutely meaningless. It is the speed of rotation...ie RPM. so if you are increasing the RPM and the power is staying constant then the torque is going down and so is the acceleration.
Its basic physics my friend!
Better yet. Get in a car with a flat torque curve and run it through the flat torque range of the RPM in one gear. You will not fell ANY change in acceleration. If your logic is correct then given that most HP curves climb, one would feel increasing acceleration to the point of max HP. We don't...because...acceleration is not dependent on HP.
01-02-2008, 05:35 PM
#119
Rennlist Member
You are not reading the post.
Accelertion is proportioal to power, at any given vehicle speed, and with any same car (mass equal).
If you have a certain rate of burn of gas, you have some max potential of power. if you could keep that power constant, like with a CVT for example, acceleration would go down with the vehicle's velocity.
Now, the last sentence will have to force an intervention here. so, the only reason acceleration goes down with speed, is due to wind resistance??
PLEASE, someone give me a hand here. Newton is rollin over.......
forget about wind resistance. you saw the thust forces on this thread! they all go down with vehicle velocity. add wind resistance and then you start talking about NET forces. that can be an entire 'nother discussion!!
why do the forces (rear wheel torque) go down with velocity??
because
Acceleration = power/ (mass x velocity)
The only way that the acceleration can be kept constant, is if you had access to a rocket engine. then, force (thrust) would be relatively constant and acceleration would be constant. BUT, guess what my friend, power would go up with the velocity. so, power would be some HUGE number as speed went up. you have any idea what the acceleration of the F1 cars are???
Here is a graph to make my point. near constant Power of an F1 , decaying torque, decaying acceleration with speeds up to 180mph.
mk
Accelertion is proportioal to power, at any given vehicle speed, and with any same car (mass equal).
If you have a certain rate of burn of gas, you have some max potential of power. if you could keep that power constant, like with a CVT for example, acceleration would go down with the vehicle's velocity.
Now, the last sentence will have to force an intervention here. so, the only reason acceleration goes down with speed, is due to wind resistance??
PLEASE, someone give me a hand here. Newton is rollin over.......
forget about wind resistance. you saw the thust forces on this thread! they all go down with vehicle velocity. add wind resistance and then you start talking about NET forces. that can be an entire 'nother discussion!!
why do the forces (rear wheel torque) go down with velocity??
because
Acceleration = power/ (mass x velocity)
The only way that the acceleration can be kept constant, is if you had access to a rocket engine. then, force (thrust) would be relatively constant and acceleration would be constant. BUT, guess what my friend, power would go up with the velocity. so, power would be some HUGE number as speed went up. you have any idea what the acceleration of the F1 cars are???
Here is a graph to make my point. near constant Power of an F1 , decaying torque, decaying acceleration with speeds up to 180mph.
mk
Wow...you may just wear me down with the absurdity. Back to Mr. Newton (not of the fig variety...)
Acceleration is independent of speed. Period. A body at a constant velocity with no force acting on it has an acceleration of zero. The acceleration of a body is only induced by a force acting upon it and the amount of that acceleration is only dependent on the body's mass and the size of the force. The speed of the body HAS NO IMPACT on its acceleration.
I think you are confusing air resistance, friction, etc. for the reason why acceleration falls off at greater speeds in a car. That has NO IMPACT on our discussion.
If that is one you can't accept...then I can't go any further.
I did my best guys.
Dave
Acceleration is independent of speed. Period. A body at a constant velocity with no force acting on it has an acceleration of zero. The acceleration of a body is only induced by a force acting upon it and the amount of that acceleration is only dependent on the body's mass and the size of the force. The speed of the body HAS NO IMPACT on its acceleration.
I think you are confusing air resistance, friction, etc. for the reason why acceleration falls off at greater speeds in a car. That has NO IMPACT on our discussion.
If that is one you can't accept...then I can't go any further.
I did my best guys.
Dave
01-02-2008, 05:47 PM
#120
Rennlist Member
Veloce Raptor,
why dont you as the owner of the STOCK 928 that lost its gears before i took over. the point was now, i was at a huge deficit for HP. in fact, i was at my max torque level most of the time, in one gear!! but, without the other gears, i was at a HP deficit against the other cars. subsequently , it was easy to drive, not having to shift, but a few seconds slower. (no 2nd, no 3rd, only 4th gear for 1hour of racing)
If you are going around a turn to be at max torque rather than closer to max HP, you might (and i emphasize "might") be at a disadvantage by "trying" to take advantage of greater engine torque at a cost of less rear wheel torque by using a lower gear and having greater HP to play with. However, i can say there are a few turns like this where wheel spin is great, or the time to make the quick shift is not long enough to make it worthwile, and that probably is your point.
Hp for lap times, and torque to win races i the lamest thing ive ever heard. (mutiple times) its HP-seconds which wins races. (or ft-lb-seconds at the rear tires ) . Ever watch a speed GT race??? ever wonder why the porsche beats the vipers and vets, even at the "hp" tracks??? the vet has 2x the engine torque. so, the reason is???? its lighter, and has an equivilant HP to weight ratio as well as close ratio gears to make an applied HP curve that is almost the same shape!
Mk
why dont you as the owner of the STOCK 928 that lost its gears before i took over. the point was now, i was at a huge deficit for HP. in fact, i was at my max torque level most of the time, in one gear!! but, without the other gears, i was at a HP deficit against the other cars. subsequently , it was easy to drive, not having to shift, but a few seconds slower. (no 2nd, no 3rd, only 4th gear for 1hour of racing)
If you are going around a turn to be at max torque rather than closer to max HP, you might (and i emphasize "might") be at a disadvantage by "trying" to take advantage of greater engine torque at a cost of less rear wheel torque by using a lower gear and having greater HP to play with. However, i can say there are a few turns like this where wheel spin is great, or the time to make the quick shift is not long enough to make it worthwile, and that probably is your point.
Hp for lap times, and torque to win races i the lamest thing ive ever heard. (mutiple times) its HP-seconds which wins races. (or ft-lb-seconds at the rear tires ) . Ever watch a speed GT race??? ever wonder why the porsche beats the vipers and vets, even at the "hp" tracks??? the vet has 2x the engine torque. so, the reason is???? its lighter, and has an equivilant HP to weight ratio as well as close ratio gears to make an applied HP curve that is almost the same shape!
Mk
Mark, Mark, Mark.....when did I ever say I was short shifting? I said that on many tracks TQ is more valuable than HP...meaning that (for example) instead of downshifting to 2nd gear for a particular corner & then being near redline for the short time before having to upshift again, some cars with wide TQ bands (like the stock 3.2 E36 USA M3) are better left in 3rd so they can torque their way out of the corner. that is not short shifting---it is keeping the engine at lower HP in some places to use abundant TQ rather than fleeting (and sometimes declining) HP.
By the way, in your example, I would suspect that your Laguna Seca SCCA car had a built motor that focused on high RPM horsepower.....right? Oh yeah....and you had lost all gears. Way to make a major u-turn from the core premise of this thread....
I will leave you with one of the 2 most fundamentally educational things I learned about driving from a friend of mine named Redman (I think Larry knows him, too): "Dave, horsepower is for laptimes, but torque wins races." (the other was watching a master like him left foot brake).
Cheers---I am out of this thread at this point.
By the way, in your example, I would suspect that your Laguna Seca SCCA car had a built motor that focused on high RPM horsepower.....right? Oh yeah....and you had lost all gears. Way to make a major u-turn from the core premise of this thread....
I will leave you with one of the 2 most fundamentally educational things I learned about driving from a friend of mine named Redman (I think Larry knows him, too): "Dave, horsepower is for laptimes, but torque wins races." (the other was watching a master like him left foot brake).
Cheers---I am out of this thread at this point.