How does the Vmax work, really?
07-04-2015, 08:53 AM
#1
Junior Member
Thread Starter
How does the Vmax work, really?
Hello. Recently bought an E55. Nice car, I think I'll keep it.
One thing puzzles me though - I can't wrap my head around what the top speed is.
I live in a nordic European country, near a very large, flat, long and sparsely trafficked piece of road, with great visibility across the fields in every direction.
The other day the car went to 258 kph (160 mph). I thought that was odd since the speed was supposedly limited to 250 kph or some such (testimonies vary). I figured the speedometer was optimistic, so I thought nothing more of it.
Then I bought a Torque (smartphone app) Bluetooth logger and plugged it into the ODB2. It can log both the car computer registered speed and the GPS-measured speed, both current and min/max. They match quite well, the difference is about 1%.
Next time I floored it, it read 262 kph (163 mph) GPS-measured and 255 kph (158 mph) computer-measured speed.
I got bolder and made a second run. 278 kph (173 mph) GPS, 255 kph (158 mph) computer. While I was busy looking at the road, I did flash a glance at the speedometer needle and it did hover a hairswidth before the 280 line.
Third time is a charm: 289 kph (180 mph) GPS, 255 kph (158 mph) computer.
The second two runs were definitely going faster than the first, I could hear that on the engine sound and wind roar.
So what is going on here? I have three different meters- two telling me the same thing (that my car obviosuly has a Vmax delete) and the third one, coming from the computer, telling me that I have the expected 255 kph limit?
My limited knowledge about engine management tells me that this is wrong. If the car has a Vmax delete, then the computer should supposedly not cut at 255, and since it supposedly controls the speedometer needle that should supposedly stop at 255 kph if it HAS Vmax still on.
Or is the computer just... aloof? "I refuse to read more than 255 and I have no control over the speedometer!"
Also, I'm not sure if the GPS measurement comes from the car GPS or my phone GPS, and I don't really know if those are any more precise than the car computer. What I could tell though is that the perceived speed matches the GPS better than the locked-at-255 computer reading.
Also, I used to drive a superbike and did 250+ almost daily and I'm pretty sure this was beyond 255 kph, although that is just a subjective feeling.
So - what's going on here?
One thing puzzles me though - I can't wrap my head around what the top speed is.
I live in a nordic European country, near a very large, flat, long and sparsely trafficked piece of road, with great visibility across the fields in every direction.
The other day the car went to 258 kph (160 mph). I thought that was odd since the speed was supposedly limited to 250 kph or some such (testimonies vary). I figured the speedometer was optimistic, so I thought nothing more of it.
Then I bought a Torque (smartphone app) Bluetooth logger and plugged it into the ODB2. It can log both the car computer registered speed and the GPS-measured speed, both current and min/max. They match quite well, the difference is about 1%.
Next time I floored it, it read 262 kph (163 mph) GPS-measured and 255 kph (158 mph) computer-measured speed.
I got bolder and made a second run. 278 kph (173 mph) GPS, 255 kph (158 mph) computer. While I was busy looking at the road, I did flash a glance at the speedometer needle and it did hover a hairswidth before the 280 line.
Third time is a charm: 289 kph (180 mph) GPS, 255 kph (158 mph) computer.
The second two runs were definitely going faster than the first, I could hear that on the engine sound and wind roar.
So what is going on here? I have three different meters- two telling me the same thing (that my car obviosuly has a Vmax delete) and the third one, coming from the computer, telling me that I have the expected 255 kph limit?
My limited knowledge about engine management tells me that this is wrong. If the car has a Vmax delete, then the computer should supposedly not cut at 255, and since it supposedly controls the speedometer needle that should supposedly stop at 255 kph if it HAS Vmax still on.
Or is the computer just... aloof? "I refuse to read more than 255 and I have no control over the speedometer!"
Also, I'm not sure if the GPS measurement comes from the car GPS or my phone GPS, and I don't really know if those are any more precise than the car computer. What I could tell though is that the perceived speed matches the GPS better than the locked-at-255 computer reading.
Also, I used to drive a superbike and did 250+ almost daily and I'm pretty sure this was beyond 255 kph, although that is just a subjective feeling.
So - what's going on here?
07-04-2015, 10:09 AM
#2
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2006 E55 AMG
I guess you mean that the OBD reported speed is limited to the factory programmed Vmax? It's an interesting observation, but if the cluster indicates properly, then I don't see the issue.
BTW the GPS speed in Torque comes from your phone. Easy test is to turn off GPS.
What are your mods, and what is your final Vmax?
BTW the GPS speed in Torque comes from your phone. Easy test is to turn off GPS.
What are your mods, and what is your final Vmax?
07-04-2015, 03:40 PM
#4
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'04 E55 (Gone but not forgotten), '13 C63 P31 (RIP), another '13 C63 PP
Yeah, I have a tune (de-limited) and headers on my car, and while my speedo (digital and the needle) stop at 160, I have taken the car to 175 as measured by GPS.
07-05-2015, 10:14 AM
#5
Junior Member
Thread Starter
So the speedometer of a de-limited car stops at 255/160 even though the car continues to its physical top speed?
Yet my speedometer and car happily continues to the physical top speed, even though the computer adamantly claims 255/160?
This is what confuses me.
From what I know of cars, the computer controls the ignition and injection and I guess the speedometer needle as well in modern everything-by-wire cars.
So either the computer in my car should acknowledge the 289/180 speed, or I should hit the limit at 255/160, or this car in particular is wonky.
Or the engine control doesn't work the way I believe it does. Or I'm just imagining things and the car did actually plateau at 255/160 and my GPS is wrong.
I am also open for the possibility that Mercedes filled the car with a smattering of different computers that only cummunicate superficially with each other, and that the 255/160 ODB2 reading is not used for anything.
Yet my speedometer and car happily continues to the physical top speed, even though the computer adamantly claims 255/160?
This is what confuses me.
From what I know of cars, the computer controls the ignition and injection and I guess the speedometer needle as well in modern everything-by-wire cars.So either the computer in my car should acknowledge the 289/180 speed, or I should hit the limit at 255/160, or this car in particular is wonky.
Or the engine control doesn't work the way I believe it does. Or I'm just imagining things and the car did actually plateau at 255/160 and my GPS is wrong.
I am also open for the possibility that Mercedes filled the car with a smattering of different computers that only cummunicate superficially with each other, and that the 255/160 ODB2 reading is not used for anything.
07-07-2015, 07:12 AM
#7
Junior Member
Thread Starter
Yeah, well - aerodynamically speaking they actually ought to reach some 310 kph (190 mph), but it seems the gearing is not really optimal for that.
The highest gear is a little too tall, meaning that peak power lies beyond the theoretical top speed. Meaning that at 290/180 the car does not actually deliver all of the 470 BHP it could.
Oddly enough, the highest gear is neither cruise-friendly tall, meaning that I think the engine revs a bit too much at highway speed to be really fuel-efficient. My car does 2000 rpm at 110 kph (68 mph) which is about 500 more than it would really need.
But I suppose that is because these cars were not meant for cruising. They are autobahn trains, expected to shuttle down the road at 200+ kph (125+ mph) daily, a speed bracket where more power is needed than the high idle you need in the non-teutonic world...
Also, the 5-speed auto box is required for the torque levels of these cars, and with a very tall fifth gear you get a rather wide spread on the first four. Perhaps they thought this was unacceptable? It would make the sprint to 100 kph (60 mph) less legendary.
The highest gear is a little too tall, meaning that peak power lies beyond the theoretical top speed. Meaning that at 290/180 the car does not actually deliver all of the 470 BHP it could.
Oddly enough, the highest gear is neither cruise-friendly tall, meaning that I think the engine revs a bit too much at highway speed to be really fuel-efficient. My car does 2000 rpm at 110 kph (68 mph) which is about 500 more than it would really need.
But I suppose that is because these cars were not meant for cruising. They are autobahn trains, expected to shuttle down the road at 200+ kph (125+ mph) daily, a speed bracket where more power is needed than the high idle you need in the non-teutonic world...
Also, the 5-speed auto box is required for the torque levels of these cars, and with a very tall fifth gear you get a rather wide spread on the first four. Perhaps they thought this was unacceptable? It would make the sprint to 100 kph (60 mph) less legendary.
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07-07-2015, 07:42 AM
#8
MBWorld Fanatic!
What year is the E55? It is possible that they had the Vmax altered beyond the 155mph limit. Mine was changed and is set at 185mph... It's as simple as going in with Star Diagnostics and altering a setting.
07-07-2015, 08:12 AM
#9
MBWorld Fanatic!
Yeah, well - aerodynamically speaking they actually ought to reach some 310 kph (190 mph), but it seems the gearing is not really optimal for that.
The highest gear is a little too tall, meaning that peak power lies beyond the theoretical top speed. Meaning that at 290/180 the car does not actually deliver all of the 470 BHP it could.
Oddly enough, the highest gear is neither cruise-friendly tall, meaning that I think the engine revs a bit too much at highway speed to be really fuel-efficient. My car does 2000 rpm at 110 kph (68 mph) which is about 500 more than it would really need.
But I suppose that is because these cars were not meant for cruising. They are autobahn trains, expected to shuttle down the road at 200+ kph (125+ mph) daily, a speed bracket where more power is needed than the high idle you need in the non-teutonic world...
Also, the 5-speed auto box is required for the torque levels of these cars, and with a very tall fifth gear you get a rather wide spread on the first four. Perhaps they thought this was unacceptable? It would make the sprint to 100 kph (60 mph) less legendary.
The highest gear is a little too tall, meaning that peak power lies beyond the theoretical top speed. Meaning that at 290/180 the car does not actually deliver all of the 470 BHP it could.
Oddly enough, the highest gear is neither cruise-friendly tall, meaning that I think the engine revs a bit too much at highway speed to be really fuel-efficient. My car does 2000 rpm at 110 kph (68 mph) which is about 500 more than it would really need.
But I suppose that is because these cars were not meant for cruising. They are autobahn trains, expected to shuttle down the road at 200+ kph (125+ mph) daily, a speed bracket where more power is needed than the high idle you need in the non-teutonic world...
Also, the 5-speed auto box is required for the torque levels of these cars, and with a very tall fifth gear you get a rather wide spread on the first four. Perhaps they thought this was unacceptable? It would make the sprint to 100 kph (60 mph) less legendary.
07-07-2015, 08:50 AM
#10
Junior Member
Thread Starter
Well, at 345 km/h and stock gearing/tire size the car would rev ca 6300 rpm which sounds about the right peak power for a tuned car. Or thereabouts.
The drag coefficient for these cars is good, 0.26-0.28. I have the estate version so I would figure more 0.28 than 0.26.
I can't find any numbers on the front area, but it is usually around 2.2 m² for cars this size. Let's go with that preliminary.
The power required to move a car is mainly P = Cw·A·p·v³/2, measured in watts on the driving wheels and where "p" is my futile attempt to write the greek letter rho (r) which denotes the air density. Usually about 1.25 kg/m³ at sea level, with a little variation depending on temperature and humidity. v is here the velocity in m/s.
This means that at 289 km/h (80.3 m/s) my car needs about 199 kW, or 271 horsepower (metric). Assuming that the auto tranny steals about 25% this equals an engine power of 339 BHP.
The rolling resistance of the tires add to this, but at high speeds the proportion of that becomes small, by the order of single percents. That would mean some 370 HP.
My car only revs about 5300 rpm at 289 km/h though, well below the peak power @ 6100 rpm or so. Assuming a reasonably flat torque curve my car would deliver some 410 HP at 5300 rpm.
Add to the fact that here in Sweden we get worse gasoline than on the continent - many are the tales of people going on car holidays through Germany and gasping at the sudden increase in engine power and smoother idling - for supposed "cold climate" reasons. Our gas has a greater content of lighter components, the gasoline in Germany is visibly "oilier" and smells less. So basically, we are cheated on the peak power.
Examining your claim of 200 mph (322 km/h) and assuming that it is a sedan we talk about (Cw = 0.26) this required engine power is 434 HP.
At 345 km/h the same calculation gives 534 HP. Very reasonable output with a bit of tuning.
So basically, I believe you.
The drag coefficient for these cars is good, 0.26-0.28. I have the estate version so I would figure more 0.28 than 0.26.
I can't find any numbers on the front area, but it is usually around 2.2 m² for cars this size. Let's go with that preliminary.
The power required to move a car is mainly P = Cw·A·p·v³/2, measured in watts on the driving wheels and where "p" is my futile attempt to write the greek letter rho (r) which denotes the air density. Usually about 1.25 kg/m³ at sea level, with a little variation depending on temperature and humidity. v is here the velocity in m/s.
This means that at 289 km/h (80.3 m/s) my car needs about 199 kW, or 271 horsepower (metric). Assuming that the auto tranny steals about 25% this equals an engine power of 339 BHP.
The rolling resistance of the tires add to this, but at high speeds the proportion of that becomes small, by the order of single percents. That would mean some 370 HP.
My car only revs about 5300 rpm at 289 km/h though, well below the peak power @ 6100 rpm or so. Assuming a reasonably flat torque curve my car would deliver some 410 HP at 5300 rpm.
Add to the fact that here in Sweden we get worse gasoline than on the continent - many are the tales of people going on car holidays through Germany and gasping at the sudden increase in engine power and smoother idling - for supposed "cold climate" reasons. Our gas has a greater content of lighter components, the gasoline in Germany is visibly "oilier" and smells less. So basically, we are cheated on the peak power.
Examining your claim of 200 mph (322 km/h) and assuming that it is a sedan we talk about (Cw = 0.26) this required engine power is 434 HP.
At 345 km/h the same calculation gives 534 HP. Very reasonable output with a bit of tuning.
So basically, I believe you.
07-07-2015, 01:03 PM
#11
Junior Member
Thread Starter
Oh, and that math expression can be simplified a lot if you bake together Cw·A·p/2 and ignore rolling resistance, gear ratios and such: v = k·P-³
Meaning that for a given car, the top speed is proportional to the cube root of the power. For km/h the constant k is usually around 40 for cars, 50 for motorcycles.
Like so: To reach 250 km/h with a regular car, you need about (250/40)³ = ca 250 BHP. If you want to reach 322 km/h (200 mph) with the same car, you need ca 520 BHP. 400 km/h? 1000 BHP. 400 km/h with a motorbike? Ca 500 BHP.
Give or take a margin. The E55 in my last post would have a k = 42.5. Good aerodynamics, that.
Another fun rule of thumb: the power-to-weight ratio (in kg) is VERY close to the 0-100 km/h time for most vehicles in the normal bracket (i.e. not absurdly light, heavy, weak or powerful).
So my 2100 kg car with 470 BHP has 2100/470 = 4.5 kg/BHP, which incidentally is precisely the 0-100 km/h time listed. If I would want reach the magic 3.0 s barrier I would need about 2100/3.0 = 700 BHP.
Meaning that for a given car, the top speed is proportional to the cube root of the power. For km/h the constant k is usually around 40 for cars, 50 for motorcycles.
Like so: To reach 250 km/h with a regular car, you need about (250/40)³ = ca 250 BHP. If you want to reach 322 km/h (200 mph) with the same car, you need ca 520 BHP. 400 km/h? 1000 BHP. 400 km/h with a motorbike? Ca 500 BHP.
Give or take a margin. The E55 in my last post would have a k = 42.5. Good aerodynamics, that.
Another fun rule of thumb: the power-to-weight ratio (in kg) is VERY close to the 0-100 km/h time for most vehicles in the normal bracket (i.e. not absurdly light, heavy, weak or powerful).
So my 2100 kg car with 470 BHP has 2100/470 = 4.5 kg/BHP, which incidentally is precisely the 0-100 km/h time listed. If I would want reach the magic 3.0 s barrier I would need about 2100/3.0 = 700 BHP.
07-08-2015, 11:59 AM
#13
MBWorld Fanatic!
Well, at 345 km/h and stock gearing/tire size the car would rev ca 6300 rpm which sounds about the right peak power for a tuned car. Or thereabouts.
The drag coefficient for these cars is good, 0.26-0.28. I have the estate version so I would figure more 0.28 than 0.26.
I can't find any numbers on the front area, but it is usually around 2.2 m² for cars this size. Let's go with that preliminary.
The power required to move a car is mainly P = Cw·A·p·v³/2, measured in watts on the driving wheels and where "p" is my futile attempt to write the greek letter rho (r) which denotes the air density. Usually about 1.25 kg/m³ at sea level, with a little variation depending on temperature and humidity. v is here the velocity in m/s.
This means that at 289 km/h (80.3 m/s) my car needs about 199 kW, or 271 horsepower (metric). Assuming that the auto tranny steals about 25% this equals an engine power of 339 BHP.
The rolling resistance of the tires add to this, but at high speeds the proportion of that becomes small, by the order of single percents. That would mean some 370 HP.
My car only revs about 5300 rpm at 289 km/h though, well below the peak power @ 6100 rpm or so. Assuming a reasonably flat torque curve my car would deliver some 410 HP at 5300 rpm.
Add to the fact that here in Sweden we get worse gasoline than on the continent - many are the tales of people going on car holidays through Germany and gasping at the sudden increase in engine power and smoother idling - for supposed "cold climate" reasons. Our gas has a greater content of lighter components, the gasoline in Germany is visibly "oilier" and smells less. So basically, we are cheated on the peak power.
Examining your claim of 200 mph (322 km/h) and assuming that it is a sedan we talk about (Cw = 0.26) this required engine power is 434 HP.
At 345 km/h the same calculation gives 534 HP. Very reasonable output with a bit of tuning.
So basically, I believe you.
The drag coefficient for these cars is good, 0.26-0.28. I have the estate version so I would figure more 0.28 than 0.26.
I can't find any numbers on the front area, but it is usually around 2.2 m² for cars this size. Let's go with that preliminary.
The power required to move a car is mainly P = Cw·A·p·v³/2, measured in watts on the driving wheels and where "p" is my futile attempt to write the greek letter rho (r) which denotes the air density. Usually about 1.25 kg/m³ at sea level, with a little variation depending on temperature and humidity. v is here the velocity in m/s.
This means that at 289 km/h (80.3 m/s) my car needs about 199 kW, or 271 horsepower (metric). Assuming that the auto tranny steals about 25% this equals an engine power of 339 BHP.
The rolling resistance of the tires add to this, but at high speeds the proportion of that becomes small, by the order of single percents. That would mean some 370 HP.
My car only revs about 5300 rpm at 289 km/h though, well below the peak power @ 6100 rpm or so. Assuming a reasonably flat torque curve my car would deliver some 410 HP at 5300 rpm.
Add to the fact that here in Sweden we get worse gasoline than on the continent - many are the tales of people going on car holidays through Germany and gasping at the sudden increase in engine power and smoother idling - for supposed "cold climate" reasons. Our gas has a greater content of lighter components, the gasoline in Germany is visibly "oilier" and smells less. So basically, we are cheated on the peak power.
Examining your claim of 200 mph (322 km/h) and assuming that it is a sedan we talk about (Cw = 0.26) this required engine power is 434 HP.
At 345 km/h the same calculation gives 534 HP. Very reasonable output with a bit of tuning.
So basically, I believe you.
I like when people use math to explain things. It makes estimating the potential of something much easier and more precise.Do an ECU flash and get rid of the limiter, then you'll be able to go 200 mph easy.
