Autobild Comparison - 550i vs. E550 vs. S6 vs. XF
07-28-10, 07:36 AM
#16
Am I the only one who finds something strikingly wrong with a V10 435 HP S6 doing 0 to 100 km/hr in 6 seconds?
It's lighter and has more peak horsepower than the 550i and gets to 100 km/hr a full second slower.
Now, obviously acceleration - particularly 0 to 60, is hardly just a function of peak horsepower and weight, but even still -
That S6 must have awful (for dig acceleration) gearing, or something.
I can't be the only one who noticed that, it's the first thing I noticed.
It's lighter and has more peak horsepower than the 550i and gets to 100 km/hr a full second slower.
Now, obviously acceleration - particularly 0 to 60, is hardly just a function of peak horsepower and weight, but even still -
That S6 must have awful (for dig acceleration) gearing, or something.
I can't be the only one who noticed that, it's the first thing I noticed.
Doesn't add up, or it's a pig.
07-28-10, 08:11 AM
#18
Brings all the data into question.
07-28-10, 04:27 PM
#20
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It's quite simple. For acceleration, power-to-weight is far less important than torque at the wheels. So, combine the BMW's highest flywheel torque figure here with the torque-multiplying benefits of eight closely stacked ratios and you have the answer.
When you look at the torque to weight ratios you get:
550i: 298 Nm/ton
E500: 289 Nm/ton
S6: 271 Nm/ton
XF: 271 Nm/ton
With the knowledge that the Audi's AWD mechanically saps energy by driving all four wheels, the acceleration differences between the cars are rational.
-- To go even into more detail --
Power is merely the product of engine torque x rpm. It's a calculation of the rate of work done. So a quoted power of 320 kW / 540 hp is purely a product of the engine torque (a Force) at those rpm. It's why we refer to it as maximum power - because it's purely a peak in a curve that extends throughout the rev range. So what bearing does a ratio of 320 kW of max power @ 6800 rpm to weight have on acceleration? Very little actually because an engine makes different power at different points in the rev range. It's obvious that during acceleration an engine has to get from X in the rev range to Y. It's how forcefully it does this that determines the rate of acceleration and not an arbitrary and brief instant in time at maximum effective rpm.
"In classical mechanics, for a body with constant mass, the acceleration of the body is proportional to the resultant (total) force acting on it (Newton's second law):
F = ma and hence a = f/m
where F is the resultant force acting on the body, m is the mass of the body, and a is its acceleration."
It is torque that is the rotational force.
Where max power is more relevant is when evaluating the top speed potential of a car in a particular gear - here power is clearly more relevant because the rate at which one can deliver the force to the wheels determines how fast the wheels can turn and hence how fast a car can go. Here, it's all about power-to-weight ratios - easy-peezy.
The correlation you have experienced between power-to-weight and acceleration is nothing more than that: a correlation as a result of power being a product of torque. It's mathematically only logical that a car with superior torque at X rpm will make superior power at X rpm. What you've been seeing is much more the effect of torque-to-weight ratio.
And you know what's so cool - here's a perfect example of exactly that. Due to turbocharging the BMW makes far superior torque through a broader portion of the rev range but makes less power ultimately because of the drop-off in BMEP as the pressure of air into the cylinders tails off as a result of a drop in the turbos' efficiency for volumetric flow at high rpm. We see this all the time: turbo's that boost early on in the rev range often "throttle" an engine at high rpm.
Therefore: the BMW engine makes more torque through a broader rev range and this is precisely why a deficit in power at maximum effective rpm has no bearing on the accelerative outcome.
07-28-10, 04:31 PM
#21
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And an even better explanation:
Power to weight ratio is not always the case. In truth, when it comes to accelerative performance, it is hardly the case.
The magic ingredient in overall vehicle performance isn't torque nor is it power - it's the relationship between the two. Torque a beautiful thing - it's the net effect of the twist force coming out of the engine multiplied by the gear ratio that gives you the sensation of accelerative force. It's why you feel more acceleration and the car accelerates faster in third gear than you'd experience in 5th gear. And it's completely obvious as to why this is so.
Think about it. How much power does an engine make in third gear at maximum effective rpm? Now, how much power does it make at the same point in fifth gear? Exactly the same amount! And yet, we all know that a car accelerates faster in 3rd gear than 5th gear. But why? The power-to-weight ratio hasn't changed... so how can this be a factor in acceleration? It isn't. Look at what has changed: it's the amount of torque reaching the rear wheels. Let's take a look at some theoretical numbers:
An engine makes 200 Nm of torque; gear is 3rd, typical ratio, 1.5:1; thus 200 Nm x 1.5 = 300 Nm before final drive (diff).
Same engine makes 200 Nm of torque. Gear is 5th, typical ratio, 0.85:1; thus 200 Nm x 0.85 = 170 Nm pre-final drive.
What changed? The amount of torque reaching the wheels. What was the result? The car accelerates slower in 5th than it does in 3rd. What didn't change? Power-to-weight - it remained constant.
So, taking this a step further, it's easy to see why a 550i with less power accelerates faster than an E39 M5 and an S6 which both have better power to weight ratios. The difference is clear - the 550i with 600 Nm of torque over a broader rev range multiplied by the closer-stacked gear ratios of the 8-speed gearbox result in more forceful acceleration because it has more torque to overcome its heavier weight.
550i - 600Nm over 1750 rpm - 4500 rpm
S6 - 540Nm over 3000rpm - 4000 rpm
550i Ratios - 1st: 4.71, 2nd: 3.14, 3rd: 2.11, 4th: 1.67, 5th: 1.29, 6th: 1.00, 7th: 0.84, 8th: 0.67
S6 Ratios - 1st: 4.171, 2nd: 2.340, 3rd: 1.521, 4th: 1.143, 5th: 0.867, 6th: 0.691
Wow, just look at how the closely stacked ratios coupled with engine torque give the 550i a huge torque advantage in every one of the first 6 gears. [Those are the ones you use for accelerating - not 7th and 8th.]
But here's the most telling number: in 6th gear at max torque - 600 Nm x 1 - the BMW transmits more far more torque to the final drive (600 Nm) than the S6 - 540 Nm x 0.867 - does in 5th gear (468 Nm).
Sure, if you want to see what the top speed potential of either vehicle is in a particular gear then power-to-resistance (weight, drag) is the relevant ratio to use. But when asking how quickly will it get to that speed it's torque that does the talking.
Power to weight ratio is not always the case. In truth, when it comes to accelerative performance, it is hardly the case.
The magic ingredient in overall vehicle performance isn't torque nor is it power - it's the relationship between the two. Torque a beautiful thing - it's the net effect of the twist force coming out of the engine multiplied by the gear ratio that gives you the sensation of accelerative force. It's why you feel more acceleration and the car accelerates faster in third gear than you'd experience in 5th gear. And it's completely obvious as to why this is so.
Think about it. How much power does an engine make in third gear at maximum effective rpm? Now, how much power does it make at the same point in fifth gear? Exactly the same amount! And yet, we all know that a car accelerates faster in 3rd gear than 5th gear. But why? The power-to-weight ratio hasn't changed... so how can this be a factor in acceleration? It isn't. Look at what has changed: it's the amount of torque reaching the rear wheels. Let's take a look at some theoretical numbers:
An engine makes 200 Nm of torque; gear is 3rd, typical ratio, 1.5:1; thus 200 Nm x 1.5 = 300 Nm before final drive (diff).
Same engine makes 200 Nm of torque. Gear is 5th, typical ratio, 0.85:1; thus 200 Nm x 0.85 = 170 Nm pre-final drive.
What changed? The amount of torque reaching the wheels. What was the result? The car accelerates slower in 5th than it does in 3rd. What didn't change? Power-to-weight - it remained constant.
So, taking this a step further, it's easy to see why a 550i with less power accelerates faster than an E39 M5 and an S6 which both have better power to weight ratios. The difference is clear - the 550i with 600 Nm of torque over a broader rev range multiplied by the closer-stacked gear ratios of the 8-speed gearbox result in more forceful acceleration because it has more torque to overcome its heavier weight.
550i - 600Nm over 1750 rpm - 4500 rpm
S6 - 540Nm over 3000rpm - 4000 rpm
550i Ratios - 1st: 4.71, 2nd: 3.14, 3rd: 2.11, 4th: 1.67, 5th: 1.29, 6th: 1.00, 7th: 0.84, 8th: 0.67
S6 Ratios - 1st: 4.171, 2nd: 2.340, 3rd: 1.521, 4th: 1.143, 5th: 0.867, 6th: 0.691
Wow, just look at how the closely stacked ratios coupled with engine torque give the 550i a huge torque advantage in every one of the first 6 gears. [Those are the ones you use for accelerating - not 7th and 8th.]
But here's the most telling number: in 6th gear at max torque - 600 Nm x 1 - the BMW transmits more far more torque to the final drive (600 Nm) than the S6 - 540 Nm x 0.867 - does in 5th gear (468 Nm).
Sure, if you want to see what the top speed potential of either vehicle is in a particular gear then power-to-resistance (weight, drag) is the relevant ratio to use. But when asking how quickly will it get to that speed it's torque that does the talking.
07-29-10, 12:33 AM
#22
Lexus Champion
07-29-10, 08:16 AM
#23
Going to copy and paste, it's other's info, but explains it well:
It's quite simple. For acceleration, power-to-weight is far less important than torque at the wheels. So, combine the BMW's highest flywheel torque figure here with the torque-multiplying benefits of eight closely stacked ratios and you have the answer.
When you look at the torque to weight ratios you get:
550i: 298 Nm/ton
E500: 289 Nm/ton
S6: 271 Nm/ton
XF: 271 Nm/ton
With the knowledge that the Audi's AWD mechanically saps energy by driving all four wheels, the acceleration differences between the cars are rational.
-- To go even into more detail --
Power is merely the product of engine torque x rpm. It's a calculation of the rate of work done. So a quoted power of 320 kW / 540 hp is purely a product of the engine torque (a Force) at those rpm. It's why we refer to it as maximum power - because it's purely a peak in a curve that extends throughout the rev range. So what bearing does a ratio of 320 kW of max power @ 6800 rpm to weight have on acceleration? Very little actually because an engine makes different power at different points in the rev range. It's obvious that during acceleration an engine has to get from X in the rev range to Y. It's how forcefully it does this that determines the rate of acceleration and not an arbitrary and brief instant in time at maximum effective rpm.
"In classical mechanics, for a body with constant mass, the acceleration of the body is proportional to the resultant (total) force acting on it (Newton's second law):
F = ma and hence a = f/m
where F is the resultant force acting on the body, m is the mass of the body, and a is its acceleration."
It is torque that is the rotational force.
Where max power is more relevant is when evaluating the top speed potential of a car in a particular gear - here power is clearly more relevant because the rate at which one can deliver the force to the wheels determines how fast the wheels can turn and hence how fast a car can go. Here, it's all about power-to-weight ratios - easy-peezy.
The correlation you have experienced between power-to-weight and acceleration is nothing more than that: a correlation as a result of power being a product of torque. It's mathematically only logical that a car with superior torque at X rpm will make superior power at X rpm. What you've been seeing is much more the effect of torque-to-weight ratio.
And you know what's so cool - here's a perfect example of exactly that. Due to turbocharging the BMW makes far superior torque through a broader portion of the rev range but makes less power ultimately because of the drop-off in BMEP as the pressure of air into the cylinders tails off as a result of a drop in the turbos' efficiency for volumetric flow at high rpm. We see this all the time: turbo's that boost early on in the rev range often "throttle" an engine at high rpm.
Therefore: the BMW engine makes more torque through a broader rev range and this is precisely why a deficit in power at maximum effective rpm has no bearing on the accelerative outcome.
It's quite simple. For acceleration, power-to-weight is far less important than torque at the wheels. So, combine the BMW's highest flywheel torque figure here with the torque-multiplying benefits of eight closely stacked ratios and you have the answer.
When you look at the torque to weight ratios you get:
550i: 298 Nm/ton
E500: 289 Nm/ton
S6: 271 Nm/ton
XF: 271 Nm/ton
With the knowledge that the Audi's AWD mechanically saps energy by driving all four wheels, the acceleration differences between the cars are rational.
-- To go even into more detail --
Power is merely the product of engine torque x rpm. It's a calculation of the rate of work done. So a quoted power of 320 kW / 540 hp is purely a product of the engine torque (a Force) at those rpm. It's why we refer to it as maximum power - because it's purely a peak in a curve that extends throughout the rev range. So what bearing does a ratio of 320 kW of max power @ 6800 rpm to weight have on acceleration? Very little actually because an engine makes different power at different points in the rev range. It's obvious that during acceleration an engine has to get from X in the rev range to Y. It's how forcefully it does this that determines the rate of acceleration and not an arbitrary and brief instant in time at maximum effective rpm.
"In classical mechanics, for a body with constant mass, the acceleration of the body is proportional to the resultant (total) force acting on it (Newton's second law):
F = ma and hence a = f/m
where F is the resultant force acting on the body, m is the mass of the body, and a is its acceleration."
It is torque that is the rotational force.
Where max power is more relevant is when evaluating the top speed potential of a car in a particular gear - here power is clearly more relevant because the rate at which one can deliver the force to the wheels determines how fast the wheels can turn and hence how fast a car can go. Here, it's all about power-to-weight ratios - easy-peezy.
The correlation you have experienced between power-to-weight and acceleration is nothing more than that: a correlation as a result of power being a product of torque. It's mathematically only logical that a car with superior torque at X rpm will make superior power at X rpm. What you've been seeing is much more the effect of torque-to-weight ratio.
And you know what's so cool - here's a perfect example of exactly that. Due to turbocharging the BMW makes far superior torque through a broader portion of the rev range but makes less power ultimately because of the drop-off in BMEP as the pressure of air into the cylinders tails off as a result of a drop in the turbos' efficiency for volumetric flow at high rpm. We see this all the time: turbo's that boost early on in the rev range often "throttle" an engine at high rpm.
Therefore: the BMW engine makes more torque through a broader rev range and this is precisely why a deficit in power at maximum effective rpm has no bearing on the accelerative outcome.
noob
07-29-10, 09:45 AM
#24
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...Chuckling...
Then I suggest you go out and drive all the cars, I truly enjoyed doing so the past couple of months. About the only one that gave me the same or better driving experience was the E63 AMG, and quite frankly didn't want to spend the difference.
Bought an F10 550i yesterday, wouldn't consider the 535i, very different from past 5 series, but in a good way.
Then I suggest you go out and drive all the cars, I truly enjoyed doing so the past couple of months. About the only one that gave me the same or better driving experience was the E63 AMG, and quite frankly didn't want to spend the difference.
Bought an F10 550i yesterday, wouldn't consider the 535i, very different from past 5 series, but in a good way.
07-29-10, 10:39 AM
#25
...Chuckling...
Then I suggest you go out and drive all the cars, I truly enjoyed doing so the past couple of months. About the only one that gave me the same or better driving experience was the E63 AMG, and quite frankly didn't want to spend the difference.
Bought an F10 550i yesterday, wouldn't consider the 535i, very different from past 5 series, but in a good way.
Then I suggest you go out and drive all the cars, I truly enjoyed doing so the past couple of months. About the only one that gave me the same or better driving experience was the E63 AMG, and quite frankly didn't want to spend the difference.
Bought an F10 550i yesterday, wouldn't consider the 535i, very different from past 5 series, but in a good way.
Where'd you come up with that? Physics class?
I was observing, as was carLx, that the OP's data for the 0-60 time for the Audi was incorrect.
07-29-10, 12:09 PM
#26
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You might want to read some reviews of the S6, one here that says:
Audi claims a 0-60-mph time in the low 5-second range, though in our testing we did no better than 5.7 seconds. That's slower than the supercharged V6-powered A6 3.0T, let alone its hard-core competitors from BMW, Jaguar and Mercedes-Benz.
07-29-10, 12:49 PM
#28
Lexus Fanatic
about the S6, i gonna go with parasitic drivetrain loss due to the AWD setup and torque band together add up to the slower 0-60 time... ppl wrongfully assume because it has more hp and AWD it should have a better off the line... this is not always the case due to my reasons above.
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