View Poll Results: HP -> Acceleration... not Torque!
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HP -> Acceleration... not Torque!
02-01-2005, 03:54 PM
#361
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Originally Posted by daabc' date='Feb 1 2005, 03:41 PM
That physics student is assuming that you started to accelerate at maximum powerband only.
That's why he's always assuming that you are at peak power at launch and dropping the clutch. It's kinda deceptive actually.
By doing that he basically eliminated some of the distance the engine has to travel from the start to reach it's maximum powerband, and also elminated the time necessary to accelerate through those RPM's.
If both cars from his example started to accelerate from a dead stop, at idle, and force to accelerate through the entire engine rpm range and not at peak powerband, the graph would be different.
PS.
His calculations are correct. I'm just saying it's deceptive because it doesn't applied to "normal" driving. It only applies to accelerating by dropping your clutch at maximum power band.
That's why he's always assuming that you are at peak power at launch and dropping the clutch. It's kinda deceptive actually.
By doing that he basically eliminated some of the distance the engine has to travel from the start to reach it's maximum powerband, and also elminated the time necessary to accelerate through those RPM's.
If both cars from his example started to accelerate from a dead stop, at idle, and force to accelerate through the entire engine rpm range and not at peak powerband, the graph would be different.
PS.
His calculations are correct. I'm just saying it's deceptive because it doesn't applied to "normal" driving. It only applies to accelerating by dropping your clutch at maximum power band.
A telling quote from that article is: "Well, consider we want to accelerate a car with mass m from 0 to speed s using an engine with constant power p." Emphasis is mine.
Unfortunately for him, that never happens in the real world. Going from 6000 to 9000rpm, an S2000 engine goes from less than 150rwhp to about 200rwhp. The very same S2000 accelerates harder at 6300rpm (torque peak of 130 lb-ft) than at 7300 (hp peak, but only 120 lb-ft of torque.)
02-01-2005, 05:38 PM
#363
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Originally Posted by neko_cat' date='Feb 1 2005, 02:55 AM
I know this is an old topic but I believe I have very good pictures that will make this VERY clear to everyone, so I'm going to reply and bump this one.
Do a multigear graph and things will get foggy again.
02-01-2005, 08:09 PM
#364
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I've done multigear graphs. In fact you can do multiple gears with shift time and everything. I ran one tonight. Maybe I'll put it up soon.
It will be the same story in any gear. Torque when normalize to the acceleration curve will match perfectly. If I scale acceleration for each gear used the shape will then match that gear and overlay under it.
-mikey
It will be the same story in any gear. Torque when normalize to the acceleration curve will match perfectly. If I scale acceleration for each gear used the shape will then match that gear and overlay under it.
-mikey
02-02-2005, 07:59 AM
#365
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neko cat - of course acceleration is going to match the torque curve.
If you look at the equation:
hp = tq * rpm / 5252
You can think of torque as the slope of the horsepower plot. The more torque you have the steeper the horspower slope. The steeper the slope the greater the acceleration. While the torque and accleration curves do match up, that doesn't mean that torque is the important factor for determining acceleration. Torque by itself does not tell a person anything about the acceleration characteristics. For example, if I tell you that my car weighs in at 4000 lbs and has an engine with 400 lbs-ft of torque can you estimate how quick this car is? No, because you don't have enough information.
Horsepower on the other hand does tell a person about the acceleration characteristics. The reason is that horsepower is a measure of work performed while torque is just a measure of force. You can apply a force and perform no work, for example standing on a stuck lug nut. If the person is 150 lbs and is standing on a 1 foot long breaker bar then that person is applying 150 lbs-ft of torque. If the lug nut is not moving then that person is applying zero horsepower.
An easy example that I brought up on the previous page is to look at the correlation of hp/weight and tq/weight between 3 cars:
hp/weight:
S2000: 240/2800 = 0.085
Mustang GT: 260/3347 = 0.077
VW Jetta TDI: 100/3095 = 0.032
torque/weight:
S2000: 153/2800 = 0.054
Mustang GT: 302/3347 = 0.090
VW Jetta TDI: 170/3095 = 0.054
1/4 mile times:
S2000: ~14 seconds
Mustang GT: ~14 seconds
Jetta: ~17 seconds
As you can see there is a correlation between hp/weight and 1/4 mile times and not between tq/weight. This all comes down to horsepower is a measure of work and torque is a measure of instantanious force.
If you look at the equation:
hp = tq * rpm / 5252
You can think of torque as the slope of the horsepower plot. The more torque you have the steeper the horspower slope. The steeper the slope the greater the acceleration. While the torque and accleration curves do match up, that doesn't mean that torque is the important factor for determining acceleration. Torque by itself does not tell a person anything about the acceleration characteristics. For example, if I tell you that my car weighs in at 4000 lbs and has an engine with 400 lbs-ft of torque can you estimate how quick this car is? No, because you don't have enough information.
Horsepower on the other hand does tell a person about the acceleration characteristics. The reason is that horsepower is a measure of work performed while torque is just a measure of force. You can apply a force and perform no work, for example standing on a stuck lug nut. If the person is 150 lbs and is standing on a 1 foot long breaker bar then that person is applying 150 lbs-ft of torque. If the lug nut is not moving then that person is applying zero horsepower.
An easy example that I brought up on the previous page is to look at the correlation of hp/weight and tq/weight between 3 cars:
hp/weight:
S2000: 240/2800 = 0.085
Mustang GT: 260/3347 = 0.077
VW Jetta TDI: 100/3095 = 0.032
torque/weight:
S2000: 153/2800 = 0.054
Mustang GT: 302/3347 = 0.090
VW Jetta TDI: 170/3095 = 0.054
1/4 mile times:
S2000: ~14 seconds
Mustang GT: ~14 seconds
Jetta: ~17 seconds
As you can see there is a correlation between hp/weight and 1/4 mile times and not between tq/weight. This all comes down to horsepower is a measure of work and torque is a measure of instantanious force.
02-02-2005, 10:45 AM
#366
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AbusiveWombat: You know what you just stated right, because you just contradicted yourself.
"You can think of torque as the slope of the horsepower plot."
From a mathematical stand point, you basically told us that.........
HP= velocity function
Torque = Slope = acceleration function
Example:
Take the Derivative of a velocity function and you get it's slope, which is the acceleration function.
You just basically stated that Torque is the acceleration function, and HP is the velocity.
PS....
I don't think i ever stated my posititon on this
For me, what's important is how fast the engine can accelerate through the RPM range to reach it's maximum HP.
Example:
Work = Force X Distance
or for our purposes
HP = Acceleration X RPM
Yes, i know that HP, acceleration and RPM needs to be converted, but it's easier to see in this form for everyone.
Lets assume we have 2 cars that have exactly the same stats such as weight and such. Now have them both accelerate in 2nd gear only (it doesn't matter which) from a dead stop.
Car A
240 HP @ 9000 RPM
(our "high" revving example)
Car B
240 HP @ 3000 RPM
(our "high" torque, low RPM example)
Now, just using the above info, which of the above cars do you think can reach 240 HP the fastest?
"You can think of torque as the slope of the horsepower plot."
From a mathematical stand point, you basically told us that.........
HP= velocity function
Torque = Slope = acceleration function
Example:
Take the Derivative of a velocity function and you get it's slope, which is the acceleration function.
You just basically stated that Torque is the acceleration function, and HP is the velocity.
PS....
I don't think i ever stated my posititon on this
For me, what's important is how fast the engine can accelerate through the RPM range to reach it's maximum HP.
Example:
Work = Force X Distance
or for our purposes
HP = Acceleration X RPM
Yes, i know that HP, acceleration and RPM needs to be converted, but it's easier to see in this form for everyone.
Lets assume we have 2 cars that have exactly the same stats such as weight and such. Now have them both accelerate in 2nd gear only (it doesn't matter which) from a dead stop.
Car A
240 HP @ 9000 RPM
(our "high" revving example)
Car B
240 HP @ 3000 RPM
(our "high" torque, low RPM example)
Now, just using the above info, which of the above cars do you think can reach 240 HP the fastest?
02-02-2005, 11:48 AM
#367
Something that I've always followed is:
"Horsepower sells cars, torque wins the races."
To my understanding its the torque that does the intial pulling and once your into the mid to higher rpm band horsepower takes over and maintains the higher speed.
"Horsepower sells cars, torque wins the races."
To my understanding its the torque that does the intial pulling and once your into the mid to higher rpm band horsepower takes over and maintains the higher speed.
02-02-2005, 01:05 PM
#368
Registered User
Originally Posted by Mad_Paradox' date='Feb 2 2005, 02:48 PM
Something that I've always followed is:
"Horsepower sells cars, torque wins the races."
To my understanding its the torque that does the intial pulling and once your into the mid to higher rpm band horsepower takes over and maintains the higher speed.
"Horsepower sells cars, torque wins the races."
To my understanding its the torque that does the intial pulling and once your into the mid to higher rpm band horsepower takes over and maintains the higher speed.
Torque is what pushes you along, period. But horsepower is a really good measurement for how long you can maintain that torque, and therefore how low of a gearing you can run. (For example, acceleration in 1st is much higher than in 2nd.) So it's better to say - torque accelerates the car (from a physics standpoint) but it's the horsepower rating that tells you how well you can use it (from a benchmark racing standpoint.)Another way to look at it:
At 5000rpm, the S2000 in 2nd gear hits about 35 mph. This is approximately the same speed my truck hits in 1st gear at 5000rpm, since it has a 5500rpm redline. So the trucks's 1st and the S2000s 2nd are basically the same gear. The S2000 however has a lot of rev range left, so it's possible to create a 1st gear that's even lower than this, and therefore get usable, forceful acceleration. The truck has 220 lb-ft of torque compared to the S2000s 153, so if it only revved to 9000rpm it'd be a screamer!
Edit - corrected typo.
02-02-2005, 02:47 PM
#370
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actually i was talking about engine acceleration and speed only......
But yes, if both cars was geared correctly, which means both will be geared completely differnetly from each other, then both will have the same acceleration..........
but back to what i was talking about......
Like I said, both cars will have the same stats, which mean they have the same weight and gearing. Only difference is engine output.
Car A
240 HP @ 9000 rpm
Using my simplified formula.
HP = Acceleration X RPM
240 = A x 9000
A = acceleration = .0267
Car B
240 HP @ 3000 rpm
240 = A x 3000
A = acceleration = .08
Assuming both are using the SAME gears, basically:
The Engine in Car A gave it a slower acceleration but higher top speed.
The Engine in Car B gave it a faster acceleration but a lower top speed.
So knowing max HP doesn't tell you anything about acceleration really. Proper gearing only seek to eliminate some of the weaknesses in both engines by taking away some of it's strengths.
So for me,........knowing Max HP tells me nothing about how the car drives. Knowing HP @ RPM gives me a good idea. Knowing HP @ RPM, gearing, Torque @ RPM gives me a clearer picture.
But driving the car is how I know what the engineer did to the car.
But yes, if both cars was geared correctly, which means both will be geared completely differnetly from each other, then both will have the same acceleration..........
but back to what i was talking about......
Like I said, both cars will have the same stats, which mean they have the same weight and gearing. Only difference is engine output.
Car A
240 HP @ 9000 rpm
Using my simplified formula.
HP = Acceleration X RPM
240 = A x 9000
A = acceleration = .0267
Car B
240 HP @ 3000 rpm
240 = A x 3000
A = acceleration = .08
Assuming both are using the SAME gears, basically:
The Engine in Car A gave it a slower acceleration but higher top speed.
The Engine in Car B gave it a faster acceleration but a lower top speed.
So knowing max HP doesn't tell you anything about acceleration really. Proper gearing only seek to eliminate some of the weaknesses in both engines by taking away some of it's strengths.
So for me,........knowing Max HP tells me nothing about how the car drives. Knowing HP @ RPM gives me a good idea. Knowing HP @ RPM, gearing, Torque @ RPM gives me a clearer picture.
But driving the car is how I know what the engineer did to the car.