Acceleration stronger with less than 100% throttle?

[ruxprncd]

In a separate thread, Copper pointed out that getting a good 1/4 mile time requires not flooring the throttle until the car is well under way; otherwise (for whatever reason?) the launch is sluggish.

Even in regular driving, when already at speed, my car feels faster when I use about 3/4 throttle instead of flooring it.

Has anybody else noticed this? :vhmmm:

I had a 6-speed B5 S4 with the same 'issue'.

[got boost]

Ive noticed this in both my RS6 and B5 S4. I believe there are a few threads on this issue. Something with the fueling I think, runs very rich at full throttle for engine safety, and runs more effecient at 3/4 throttle.

Someone correct me if I'm wrong.

[jonas21]

Can second this behaviour.

[EKaru]

I third this behavior with my now sold B6 S4....

[#oftheBeast]

Not tried this but will be doing so later. It does bog down if you give it all from a stand-still; sometimes I wish it was a manual box so I could the revs up before setting off.

[BLITZEN]

Sometimes not even 3/4! The other night (~30*F) I was on it only at about 1/2 throttle and BAM - I was pinned in my seat! I was surprised 'cause I wasn't expecting that hard a pull.

[Hy Octane]

Yes. You will also notice that the revs will actually go higher a tad past redline before shifting when you use 1/2 or 3/4 throttle as compared to full which will shift a little before redline.

[RS6-4dr911]

High intake velocity is a friend of torque. With partial throttle, there is less area available the air being demanded to flow through, therefore it has to flow faster to get all the air through in time (Q=A x V, Q = total volume per time, A= area, V=velocity).

Porsche's vario-ram (IIRC) essentially capitalizes on this. At lower rpms only a portion of the intake is available, at higher rpms, a second plate opens up, all at full throttle.:mech:

[tvrfan]

i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???

[LIRS6]

Same results for me as well, on a consistent basis

[SoCal]

My RS6 was the same. Full throttle never gave fastest starts.

[tvrfan]

i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???

[Aronis]

High intake velocity is a friend of torque. With partial throttle, there is less area available the air being demanded to flow through, therefore it has to flow faster to get all the air through in time (Q=A x V, Q = total volume per time, A= area, V=velocity).

Porsche's vario-ram (IIRC) essentially capitalizes on this. At lower rpms only a portion of the intake is available, at higher rpms, a second plate opens up, all at full throttle.:mech:

I am having some trouble with this explanation.

At lower RPM's the engine requires less air per unit of time and less fuel per unit of time. So to keep the balance between intake Air and intake Fuel, that variable IIRC thing makes some sense.

I really doubt that the linear velocity of the intake air has ANY effect on the development of Torque by the internal combustion engine...perhaps it does with a Ram Jet engine....

With a NA engine, the intake air is being PULLED in, so any resistance to inflow is going to limit the amount of air the engine can take in per unit of time, a turbo engine is pulling this air in with even more gusto. Matching the intake resistance, thus the intake air flow rate to the fuel flow rate is important to maintain the target air/fuel mixture.

I am intregued by the the explanation about running Rich to protect the engine and would like to hear more on that issue.

Mike

[SpinEcho]

I agree with Mike that the offered explanation doesn't cut it. As well, I'll be the odd one out here and say that I have never noticed this phenom with my Beast or any other car I've ever had. Anyone else?

[DuckWingDuck]

To be honest I've never tried it out though I'll be sure to when I get home tonight!

[ruxprncd]

With my B5 S4, I used a well-known mod (from audiworld's forum) to add a rubber cap to the throttle stop to reduce the effect of this issue.

I looked at the throttle stop on the RS6 accelerator pedal today. You'd think it would be just a piece of rubber...but it's a little assembly with a spring, plastic casing, and rubber end. The spring seems to require quite a bit of pressure to go down the last ~10mm or so. I don't know if this gets compressed when I go WOT, but suspect it might. Maybe this gizmo is part of the story here???

For what it's worth, this issue is pretty distinct on my car, as it was also on my B5 S4. When time allows I'll try to gather some empirical data with a VAG-COM.

Unfortunately (he he) I'll be busy driving the beast at Daytona this weekend, but hopefully I can get to this in the next coupla weeks.

[Aronis]

the little switch is possibly the kickdown switch that triggers a downshift as typical on many GM cars.

I've never done a comparison full vs partial but will weather permitting.

Mike

[RS6-4dr911]

Sorry if you two (Mike and Spin Echo) don't like physics or fluid dynamics but that's a good portion of the story. Do some research on Porsche's vario-ram system (IIRC is short hand for "if I recall correctly" not the name of the system) That system is a little more complex than this issue alone but it's dealing with the same dynamics.

While I am not sure what's going on with the air/fuel ratio throughout the rpm range, or throughout the throttle movement range, (perhaps someone who has instrumented these things can chime in) I doubt if that would account for such a dramatic difference. While not certain, I would guess they would calibrate the mass air flow sensor (MAF) to keep the mixture within a fairly narrow range for both performance and efficiency reasons. While richening under full load makes some sense to prevent high temps resulting from being too lean, too much fuel washes the oil off the cylinder walls creating other predictable problems.

It's not totally about the ram air effect that you allude to, although it's involved, it's about kinetic energy, turbulence and mixing, all combining to make a more efficient, ready to combust mixture. Combustion dynamics is a pretty complex area of study, well beyond most engineer's casual knowledge (and admittedly beyond mine as well).

As for this effect on normally aspirated v. turbo engines, it still holds, the only difference is one of magnitude, since in either case you have a pressure differential across the throttle plate, higher upstream than downstream. Remember, there is no such thing as suction, just a lower pressure in one place than another, meaning air is getting pushed into the engine in either case.

There are numerous examples of this increased velocity being beneficial to torque. Lots of devices, cam profiles, manifolds, etc. that aim for this effect. Ask any hot rodder what happened to his low end torque when he put that high lift cam in his engine (increasing the flow area across the valve seat, thus lowering the velocity)? Same thing that happened when I switched out a small runner intake on my high-school era Camaro for one with bigger runners and a bigger carb to boot - went from smoking the tires at will to bogging off the line. Yes, it can breathe easier (and certainly high rpm HP will increase), but it's not better at low rpm's. Why? Decreased flow velocity.:mech:

[dab]

I have noticed this as well with my rs/6. Mentioned it to the shop that did my apr chipset and he said it is consisten with audis. He mentioned that some people had a stop nob so to speak installed under the accelerator.

[tvrfan]

i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???

please help ! ! !

[tvrfan]

i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???

please help ! ! !

anyone :confused:

[Aronis]

This could get REALLY interesting....this is long so skip to the last five lines if you're impatient.

I have a pretty good background in both Physics and Fluid Dynamics (BS biomedical engineering) and certainly appreciate that many complex devices function in a way that we THINK we understand but really don't, be that an Engine or a Human Being.

What I was saying is that I don't believe the actual linear velocity of the intake air at the throttle body has any effect on the Torque developed by the engine.

The Throttle Body diameter and geometry and position of the Throttle Plate will effect two things for sure. Total amount of air passing per unit time and the amount of turbulence in that air. What the engine needs is a certain MASS of air, i.e. number of moles of a given gas (oxygen for combustion), and does not really depend on how fast the molecules are rushing into the cylinder.

I agree that the same MASS of air passing through two different sized openings will move at different linear velocities if indeed the same MASS of air moves in the same amount of time. But for a given fixed pressure and temperature of that moving air,you may end up with some faster moving molecules through that gate, but less of them per unit time, thus less MASS of air moving.

Fluid dynamics is a complex science. While a gas such as common Air is considered a fluid, just as much as gasoline or water are fluids, the fluid dynamics of a gas is different from that of a liquid. One particular difference is the compressibility of a gas vs a liquid. Gasoline is not that compressible compaired to air. So when the pressure produced by the fuel pump behind the fuel injector is allowed to produce flow of fuel through the injector of a given geometry and open for a specific pulse width, a very predictable amount of fuel is delivered.

Now the issue of suction vs pushing is interesting. The movement of air into a normally aspirated internal combusion engine is accomplished by a pressure gradient producted by the movement of the piston during the intake phase of the 4 strokes.

This pressure gradiant results in the movement of air from a higher pressure area to a lower pressure area which is the definition of SUCTION. Air is also PUSHED into an engine by a Turbo or Supercharger or by the "Ram Air" effect.

The linear velocity of that air now needs to be considered. What possible effect could the linear velocity of the air have on the development of torque as a function of that linear velocity alone???

The Throttle body is connected to the MAF then to the intake manifold where the fuel injectors are found. The fuel injectors inject gasoline right into the intake valve. (newer direct injections engines have either an injector in the intake manifold AND an injector in the cylinder head (Lexus) or just in the cylinder head (Audi RS4, S6, S8).)

The MAF sensor determines what MASS of Air is moving into the engine. The ECU needs to know the mass (ie. the number of molecules of AIR) to match the proper amount of fuel to mix. The resultant messurement at the Oxygen sensors helps fine tune the mixture further.

Torque is created by the combined internal explosions in each cylinder per unit time through a mechanical link of each piston to the crank staft, etc, and the explosive force of each internal combustion depends on the volume of the cylinder, compression ratio, starting pressure, temperature, position of the spark (better progression of ignition), completeness of air/fuel mixture, etc,etc. The linear velocity of the intake air not mentioned here.

Putting more air/fuel into a given sized cylinder at a given compression ratio etc, producted MORE torque. Thus add a turbo, add a supercharger, add both, etc. The other way to get more torque is to add more cylinders or use larger cylinders, but these both add weight to the engine and more mechanical complexity.

The throttle plate position is going to determine how much air can flow into the intake manifold in a given time at a given pressure/temperature/air density. Those variables are less of a concern to the ECU which relies on the MAF sensor to tell it the mass of air entering the intake manifold, then combines that information with other sensors (intake manifold pressure, oxygen sensor, temperature sensors etc) to determine how much fuel to mix with the intake air to optimize combustion. Again there is no Air Linear Velocity here.

Turbulence created by the partially closed throttle plate is not an issue as the MIXING of the air and fuel occurs at the intake valve, thus any prior turbulence in the intake manifold is not an issue by the time the air gets to that tiny intake valve opening.

How about resistence to air flow at the partially closed throttle body which causes increased intake air linear velocity???

If you consider the fluid (air) as an ideal fluid instead of a gas, and if the total potential across the system (pressure gradiant from outside air to inside of the "sucking" cylinder) is unchanged and then the resistance to flow is increased (the throttle body is partially closed) that the velocity of flow would actually be decreased (i.e. V=IR V= potential difference ie voltage, I = flow, ie current, R= resistance).

But since the Air is not an ideal fluid but a gas (not even an IDEAL gas) it is a compressible fluid, and the relationship of total mass flow to pressure to temperature to linear velocity of the air is MUCH MUCH more complex. And the simple Ohms law becomes much more complex for anything more complex than a DC circuit, and I'll argue that air flow into an engine is not a like a simple DC circuit, but rather complex with added consideration of the true pulsitile flow of air into and out of the engine, and the impedance, inductance and capacitance of each tube/air box/MAF/ etc. in the flow path.





Now the nuts and bolts - for the RS6...



Drive - By - Wire.


Period..


The actual position of the throttle plate is controlled by the ECU...and even though at many times the position of the

throttle place IS directly related to the position of the gas pedal, the ECU will change it to match the needs of the given

LOAD and many other variables at a given moment.

So where your FOOT is on the gas pedal vs where the Throttle body plate is position may not be what you think.


and....I don't think linear velocity of air has anything to do with torque. I may be wrong...but I doubt it..


Mike

[tvrfan]

@ Aronis,

i have the same on my A3 1.8L on half throttle it accelerates faster.

SO is that just on audi models or on other cars too ( BMW, Mercedes........)???

[mmaturo]

Add me to the list that I find this true...if i fully gun it from a stop the car bogs down...one (and primarily in my opinion) from the traction control kicking in (from starting to break away the tires) and two the engine as noted. My best launches are to ease into it (roll off) then go to 3/4 in the next seconds then all in. If i dive all the way in I find I have to let off to 3/4 then go all in again. When already moving such as an interstate acceleration I find I get the random all out occasionally that someone mentioned at half or more throttle...the car does have a mind of its own on what it thinks i want to do with the drive by wire (I sense it especially as it reads your recent behavior).

[Aronis]

@ Aronis,

i have the same on my A3 1.8L on half throttle it accelerates faster.

SO is that just on audi models or on other cars too ( BMW, Mercedes........)???

Is it Automatic or Manual?

Traction control?

AWD or Front?


Mike

[RS6-4dr911]

Nothing like two geeks debating . . . .

[quote=Aronis;115865]This could get REALLY interesting....this is long so skip to the last five lines if you're impatient.

I have a pretty good background in both Physics and Fluid Dynamics (BS biomedical engineering) and certainly appreciate that many complex devices function in a way that we THINK we understand but really don't, be that an Engine or a Human Being. I ACKNOWLEDGE YOUR PRESENCE

What I was saying is that I don't believe the actual linear velocity of the intake air at the throttle body has any effect on the Torque developed by the engine. I DISAGREE

The Throttle Body diameter and geometry and position of the Throttle Plate will effect two things for sure. Total amount of air passing per unit time and the amount of turbulence in that air. KEY POINT What the engine needs is a certain MASS of air, i.e. number of moles of a given gas (oxygen for combustion), and does not really depend on how fast the molecules are rushing into the cylinder. DISAGREE AGAIN, see notes on camshaft profile effects.

I agree that the same MASS of air passing through two different sized openings will move at different linear velocities if indeed the same MASS of air moves in the same amount of time. But for a given fixed pressure and temperature of that moving air,you may end up with some faster moving molecules through that gate, but less of them per unit time, thus less MASS of air moving. AGREED

Fluid dynamics is a complex science. While a gas such as common Air is considered a fluid, just as much as gasoline or water are fluids, the fluid dynamics of a gas is different from that of a liquid. One particular difference is the compressibility of a gas vs a liquid. Gasoline is not that compressible compaired to air. So when the pressure produced by the fuel pump behind the fuel injector is allowed to produce flow of fuel through the injector of a given geometry and open for a specific pulse width, a very predictable amount of fuel is delivered. AGREED

Now the issue of suction vs pushing is interesting. The movement of air into a normally aspirated internal combusion engine is accomplished by a pressure gradient producted by the movement of the piston during the intake phase of the 4 strokes.

This pressure gradiant results in the movement of air from a higher pressure area to a lower pressure area which is the definition of SUCTION. Air is also PUSHED into an engine by a Turbo or Supercharger or by the "Ram Air" effect. SEMANTICS, you're making a distinction based on how the higher pressure is created. I understand the definition of suction, was simply pointing out the more technically correct definition (e.g., strictly speaking - there is no such thing as cold, only the absence of heat). So it doesn't matter why the higher pressure exists on one side of the throttle, it's just a question of magnitude. The turbo/supercharger is creating roughly 12psi of pressure on the upstream side, the engine is still sucking, to use your term, creating a loss of pressure (maybe 8-10psi) on the downstream side for a total difference of roughly 20psi across the plate. With a normally aspirated version, there is only the 8-10psi differential.

The linear velocity of that air now needs to be considered. What possible effect could the linear velocity of the air have on the development of torque as a function of that linear velocity alone???

The Throttle body is connected to the MAF then to the intake manifold where the fuel injectors are found. The fuel injectors inject gasoline right into the intake valve. (newer direct injections engines have either an injector in the intake manifold AND an injector in the cylinder head (Lexus) or just in the cylinder head (Audi RS4, S6, S8).) AGREED

The MAF sensor determines what MASS of Air is moving into the engine. The ECU needs to know the mass (ie. the number of molecules of AIR) to match the proper amount of fuel to mix. The resultant messurement at the Oxygen sensors helps fine tune the mixture further. AGREED

Torque is created by the combined internal explosions in each cylinder per unit time through a mechanical link of each piston to the crank staft, etc, and the explosive force (KEY PHRASE, remember this one) of each internal combustion depends on the volume of the cylinder, compression ratio, starting pressure, temperature, position of the spark (better progression of ignition)BINGO, completeness of air/fuel mixture, etc,etc. The linear velocity of the intake air not mentioned here. Not directly, but it's effect is there. The more turbulent air HAS an effect on combustion. You're idealizing a bit much here. The speed of the flame front has a big impact on torque. The faster it happens, the more pressure is built up in a relatively fixed volume cylinder. That's why twin plugs have an effect. If it travels more slowly, the piston has already started it's downward path, lessening the effect of the expanding gasses.

Putting more air/fuel into a given sized cylinder at a given compression ratio etc, producted MORE torque. Thus add a turbo, add a supercharger, add both, etc. The other way to get more torque is to add more cylinders or use larger cylinders, but these both add weight to the engine and more mechanical complexity. These are big picture generalities, irrelevant to our fun little debate.

The throttle plate position is going to determine how much air can flow into the intake manifold in a given time at a given pressure/temperature/air density. Those variables are less of a concern to the ECU which relies on the MAF sensor to tell it the mass of air entering the intake manifold, then combines that information with other sensors (intake manifold pressure, oxygen sensor, temperature sensors etc) to determine how much fuel to mix with the intake air to optimize combustion. Again there is no Air Linear Velocity here. Again you're oversimplifying. MAF's are not perfect and are taking an average airflow reading. If my understanding is correct, they measure the ability of a wire to retain heat. The more airflow, the less heat retained. In your ideal model, there is perfectly laminar flow with perfectly even pressure distribution. I think we have both acknolwedged that there is turbulence in the airflow. While I am not positive of this, it is certainly not wild to theorize that turbulence could "fool" the MAF briefly into an incorrect assumption.

Turbulence created by the partially closed throttle plate is not an issue as the MIXING of the air and fuel occurs at the intake valve, thus any prior turbulence in the intake manifold is not an issue by the time the air gets to that tiny intake valve opening.

How about resistence to air flow at the partially closed throttle body which causes increased intake air linear velocity?????

If you consider the fluid (air) as an ideal fluid instead of a gas, and if the total potential across the system (pressure gradiant from outside air to inside of the "sucking" cylinder) is unchanged and then the resistance to flow is increased (the throttle body is partially closed) that the velocity of flow would actually be decreased (i.e. V=IR V= potential difference ie voltage, I = flow, ie current, R= resistance). I think you really meant to reference Bernoulli's equation, not Ohm's law. But anyway, ON AVERAGE, yes. But not at the throttle plate and possibly not at the MAF

But since the Air is not an ideal fluid but a gas (not even an IDEAL gas) it is a compressible fluid, and the relationship of total mass flow to pressure to temperature to linear velocity of the air is MUCH MUCH more complex. And the simple Ohms law becomes much more complex for anything more complex than a DC circuit, and I'll argue that air flow into an engine is not a like a simple DC circuit, but rather complex with added consideration of the true pulsitile flow of air into and out of the engine, and the impedance, inductance and capacitance of each tube/air box/MAF/ etc. in the flow path. (I'm appreciative of the many analogies of electricity to fluid flow, but I think your biomedical roots are showing here, we really are talking about fluids, not electricity)





Now the nuts and bolts - for the RS6...



Drive - By - Wire.


Period..


The actual position of the throttle plate is controlled by the ECU...and even though at many times the position of the

throttle place IS directly related to the position of the gas pedal, the ECU will change it to match the needs of the given

LOAD and many other variables at a given moment.

So where your FOOT is on the gas pedal vs where the Throttle body plate is position may not be what you think. Interesting twist indeed, forgot about that little detail. However, it seems highly illogical (ESPECIALLY given Audi's past experience with the mythical unintended acceleration issue) that they would program the actual throttle to provide more power at a decreased level of pedal depression, don't you think?


and....I don't think linear velocity of air has anything to do with torque. I may be wrong...but I doubt it.. While you've certainly shown your engineering mettle, and I'll grant you that electronic controls may have a lot more to do with it than either of us know, to deny that the velocity of intake air has an impact on the performance of an engine flies directly in the face of the real world. Refer to my examples of camshaft profile, intake runner cross sections, carburetor sizing, etc. They DO impact performance directly. If you take all of the points you made, especially one of the later paragraphs about looking at the total system, if you were correct, these components would have zero effect on an engine's performance, but in fact they most definitely do. I'll also concede that I have explained this effect to the best of my ability, and any better explanation and/or proof will have to be left up to someone brighter and more educated than myself.

It's been fun.

[Aronis]

Torque is created by the combined internal explosions in each cylinder per unit time through a mechanical link of each piston to the crank staft, etc, and the explosive force (KEY PHRASE, remember this one) of each internal combustion depends on the volume of the cylinder, compression ratio, starting pressure, temperature, position of the spark (better progression of ignition)BINGO, completeness of air/fuel mixture, etc,etc. The linear velocity of the intake air not mentioned here. Not directly, but it's effect is there. The more turbulent air HAS an effect on combustion. You're idealizing a bit much here. The speed of the flame front has a big impact on torque. The faster it happens, the more pressure is built up in a relatively fixed volume cylinder. That's why twin plugs have an effect. If it travels more slowly, the piston has already started it's downward path, lessening the effect of the expanding gasses.

It's been fun.

The linear velocity AT the throttle body is not necessarily the same at the Valve opening at the head.

This is Pulsitile flow, and the flow of the air actually STOPS until the valve opens again, so what ever velocity the molecules had at the THROTTLE BODY location is not NOW present.

I'll go through your entire post later, I have to run....

I think its GREAT discussing things we basically know little about....LOL....better than politics,

Mike

[RS6-4dr911]

The linear velocity AT the throttle body is not necessarily the same at the Valve opening at the head.

This is Pulsitile flow, and the flow of the air actually STOPS until the valve opens again, so what ever velocity the molecules had at the THROTTLE BODY location is not NOW present.

I'll go through your entire post later, I have to run....

I think its GREAT discussing things we basically know little about....LOL....better than politics,

Mike

If you think I'm gonna let you have time to research some actual facts to continue this debate you're nuts. I did, however. My RS6 training manual indicates that the MAF's are well upstream of the throttle body. So, any comments I previously made about the throttle plate affecting the MAF's must, sadly, be retracted.

I'll grant you that the activity at the throttle body may begin to diverge from that at the valve seat, although it's position just ahead of the manifold means it's impact is greater than when I thought it was farther upstream of the valve seats. So now we're arguing over whether the linear velocity of airflow through the "system" has any impact. On small bore throttle bodies or carburetors, the higher velocity provides a stronger signal to the fuel metering circuits improving throttle response. Regardless of the size of the given intake runners, a change in throttle body bore size impacts performance, on a carburetor. Can't say I am convinced it would do so on an EFI throttle body.

You're right on the pulsitile flow (I admit, I learned a new word today) however, think of it this way - since momentum is m x v, and given we have the same m for a given rpm, the higher the velocity, the higher the momentum. If we have higher momentum, then when the valve slams shut "stopping" the flow (this is where our ideal gas becomes non-ideal, and the flow doesn't really stop, it just re-directs momentarily) the momentum starts to create a packing effect of the intake airstream, compressing it. When the valve opens up, a dozen milliseconds later, the high-pressure packed airstream wants very badly to enter the combustion chamber. Here's the ram air effect you doubted existed. I also wouldn't be so sure that the "pulsitile" flow doesn't find it's way back to the throttle body. I remember seeing a dyno test of a high performance 911 engine and at certain steady rpm's you could see a standing wave of fuel vapor just above the air horns (several inches UPSTREAM of the throttle plates) resulting from this very effect (of the valves slamming shut).

Summary, I'm ready to concede that I don't know, for SURE, why there is better performance at part throttle, but I'll fight to the rhetorical death over airstream velocity in the combustion chamber making a difference.

Your turn.:argue: (it's still fun, lol. And if we keep it at it in this way, we may very well solve this mystery!!)

[Antagonist]

Well if there is one thing I've learned here it's that I really need to go back to school. GREAT debate! I think you two are impressive to say the least. Thank you for making me feel like a complete idiot! :trash:

I have noticed the improvement in acceleration from a digg at 1/2-3/4 throttle. I raced a BMW today from a stand still and she really bogged for a couple of seconds. Every now and then I'll unintentionally rocket away from a light at what seems like 1/4 throttle. Always leaves me scratching my head....kinda like this debate.

[MrBucket]

Are all the cars you guys are talking about automatics? I dont notice this in any of the manual transmission Audis Ive driven. They also dont have that extra bit of effort required to press the gas pedal in the last bit, the action is smooth for the entire travel of it.

[ruxprncd]

MrBucket, my B5 S4 6-speed stick had this issue, and quite a few other B5 S4 owners (mostly 6-speed) on audiworld's forum reported the same (back in ~2002ish).

[tvrfan]

Is it Automatic or Manual?

Traction control?

AWD or Front?


Mike

@ aronis

its manual and front. no traction control

few questions tough:

1::: is that problem on all cars (bmw mercedes.......) or just on audi ???

2::: that means the car acceleration faster with half throttle from 100-200km/h then with full throttle???

[ZCD2.7T]

@ aronis

its manual and front. no traction control

few questions tough:

1::: is that problem on all cars (bmw mercedes.......) or just on audi ???

2::: that means the car acceleration faster with half throttle from 100-200km/h then with full throttle???

1) This behavior COULD show itself on any car with a DBW (drive by wire) throttle, which means most cars sold today.

2) Not exactly, but it depends:

With DBW throttles, unless you can see the programming curve of the actual throttle butterfly, there's no way to predict how the engine will respond. I remember a thread on AW where someone actually looked at the programming of the "Sport" button on the RS4. In that case, the throttle mapping was MUCH different when "Sport" was engaged. Bascially, the first 1/4 of the pedal travel delivered about 70 % of *actual* full throttle, and the last 3/4 only the remaining 30%. Some RS4 drivers like that feeling, some don't.

Personally, I can also add my family's 3 total 2.7Ts to the list of cars that feel quicker at 3/4 throttle than at full throttle, in some instances.

[DuckWingDuck]

tvrfan -

1. LOL, I don't think there are many folks here who have Mercs or BMWs as you've asked that question quite a number of times. Though, I guess I can ask my friend what it's like in his E55.


2. I think this is talking about acceleration, at any point. Though we're primarily addressing off the line.

[tvrfan]

its confusing me, a car should accelerate faster on full throttle as not 3/4 or 1/2 throttle!

[DuckWingDuck]

its confusing me, a car should accelerate faster on full throttle as not 3/4 or 1/2 throttle!

LOL, that's why you gotta read the physicist/mechanical engineers go at it up there for information!

[MikeL01]

Forum members capable of cognoscente thought. Very Cool. No mention of peak particle velocity? From a Civil Engineers point of view…3/4 throttle works best for me. :cheers:

[peiserg]

fwiw i spoke with the master tech rs6 guru here in phx. he did my OCT work and used to work with audi, now out on his own. he agreed that the less than full throttle is typically a bit better, but said it had more to do with regulation of torque at the torque converter and tranny vs. all th ephysics you guys are discussing.

[BLITZEN]

The flux capacitor takes you back you in time a fraction of a second for each second that the throttle is full. :jlol:

[RS6-4dr911]

Hey now peiserg, uhhh-uhhh, no sir, noooooooo, you're not gonna rain on our little parade by bringing someone knowledgeable on the topic into the fray. fuhget about it.:nono:


Honestly though, I'm not sure I'll buy that. I thought these were locking torque converters so they act pretty much like a clutch once locked up. Could once again be getting into the whole electronics blackhole/blackbox (see Blitzen for details) who knows what they're doing issue.:confused:

[emc2rs4]

Maybe that's the reason why I feel so slow to gain speed because I was 100% down on my foot ><~~ , it really feel slow, so maybe I will try 3/4 flooring next time if catch Lambo agagin ^^

http://www.rs6.com/forum/showthread.php?t=13702

[Antagonist]

I've been trying the 3/4 thing now and it does seem as if she doesn't bog as much. There is certainly no difference in acceleration. Hmmmmmm

[Aronis]

I'm killing time waiting to do an appendectomy so more fun....


If you think I'm gonna let you have time to research some actual facts to continue this debate you're nuts. I did, however. My RS6 training manual indicates that the MAF's are well upstream of the throttle body.

my point was that the MAF's are THERE after the Throttle plate and their function is to determine the MASS of Air so as to match the FUEL...the exact location is not the point.


I'll grant you that the activity at the throttle body may begin to diverge from that at the valve seat, although it's position just ahead of the manifold means it's impact is greater than when I thought it was farther upstream of the valve seats.

So now we're arguing over whether the linear velocity of airflow through the "system" has any impact.


The "Arguement" is simply that I believe the Linear Velocity of the Air at the Throttle body has NO Effect on the development of Torque, where the partially closed throttle plate supposedly increases the linear velocity of a gas vs the fully open throttle plate


On small bore throttle bodies or carburetors, the higher velocity provides a stronger signal to the fuel metering circuits improving throttle response.

I'll believe that, but with EFI the reason why we have a MAF sensor is to determine how much input air is coming in to match the fuel supply. So a faster or slower linear velocity is not an issue.

Regardless of the size of the given intake runners, a change in throttle body bore size impacts performance, on a carburetor. Can't say I am convinced it would do so on an EFI throttle body.

Yes, Yes, some other high performance engines have separate Throttle Bodies for each cylinder, first M3 (four cylinder), latest M3 (new v8 M3 also), all M5s that I can recall they all did), and this is to increase the MASS of air arriving at each separate cylinder, regardless of that Mass of Air's linear velocity LOL.

You're right on the pulsitile flow (I admit, I learned a new word today) however, think of it this way - since momentum is m x v, and given we have the same m for a given rpm, the higher the velocity, the higher the momentum.

What this now, for a higher engine RPM, the engine needs to breath in MORE air per unit time, ie. a Higher MASS of air regardless of it's velocity. This requires less restriction to inflow, i.e. a wider open throttle plate.

If we have higher momentum, then when the valve slams shut "stopping" the flow (this is where our ideal gas becomes non-ideal, and the flow doesn't really stop, it just re-directs momentarily) the momentum starts to create a packing effect of the intake airstream, compressing it. When the valve opens up, a dozen milliseconds later, the high-pressure packed airstream wants very badly to enter the combustion chamber. Here's the ram air effect you doubted existed. I also wouldn't be so sure that the "pulsitile" flow doesn't find it's way back to the throttle body.

I agree here since the partially closed Throttle Body may perhaps provide some BACK Pressure of that pulse wave but not sure.

I remember seeing a dyno test of a high performance 911 engine and at certain steady rpm's you could see a standing wave of fuel vapor just above the air horns (several inches UPSTREAM of the throttle plates) resulting from this very effect (of the valves slamming shut).

Summary, I'm ready to concede that I don't know, for SURE, why there is better performance at part throttle, but I'll fight to the rhetorical death over airstream velocity in the combustion chamber making a difference.

Perhaps in the Combustion Chamber...but remember that combustion starts near full compression and at that point the Linear Velocity of the air in the Cylinder is ZERO.


Your turn.:argue: (it's still fun, lol. And if we keep it at it in this way, we may very well solve this mystery!!)

yes I find this very fun!!!

My last point was that since the car has Drive By Wire, we the driver really don't know for sure what position the Throttle plate is in for any given engine load situation.

Asking the Throttle Plate to be wide open when the Engine is not YET approached a higher RPM would result in TOO MUCH AIR to match a given preprogrammed RPM VS Fuel injection pulse length and timing. If indeed the actual position of the throttle plate is being reduced (not matching your foot position) by the ECU, then that is because the ECU knows better than your foot.

Now it would be very interesting to have info on the given position of the throttle plate, vs your gas pedal position, vs load vs acceloration of the car...

Boy, I have some time to kill, on to the next thread....
:incar:
Mike

[Aronis]

Don't forget these acceloration "tests" have other data....starting from a stop, accelorating from a given speed, given gear, downshift one or two gears, lots of variables.....

Mike

[peiserg]

iirc he said something about the TC being regulated to prevent all the torque from hitting the tranny from a standstill. he said it was the equivalent of "slipping" a clutch, even though you don't really slip any clutch in an auto. he suggested i try a 5mph roll on...he was exactly right...i just had the oct chipwork done, and have tried some 5mph roll ons....hooo! :addict:

[RS6-4dr911]

Peiserg - still not sure I buy this logically. If I'm an engineer tasked with reducing transmission warranty claims on a hot rod, why would I protect it under full throttle (by slipping or delaying the lockup clutch) but overlook a scenario easily encountered whereby half throttle results in higher torque and abuse on the tranny? In my opinion, there are some pretty bright guys designing these things, can't believe they'd overlook that one.


As for you my worthy adversary . . .
I'm killing time waiting to do an appendectomy so more fun....



yes I find this very fun!!!

You're starting to wear me down on the whole velocity at the throttle versus the valve opening argument - perhaps they are too far apart to matter - I'm not conceding the point, mind you, merely opening the door for my escape. However, I now have issues with your comments on the throttle position v. pedal position. First of all, although I have never used one, shouldn't this type of info be available on a VAG-COM? There's a throttle position sensor (TPS) and even if there weren't a pedal position sensor or sensor reading, with some careful testing, someone could at least correlate the results to their approximate pedal position. And further, someone ought to be able to instrument some 0-60 and 1/4mi times using this phenomenon to advantage and see what pedal (and/or throttle) position really provides the best performance. Any volunteers?

Second of all, it seems totally bass-ackwards to program a drive-by-wire device to add more throttle(power) at less "demand". If you gave it half pedal, it seems it would move the throttle to provide half-power. If you give it full pedal, wouldn't it sense the demand for full power and move the throttle to the position that provides max power? Otherwise, how could even an enthusiast driver know how to press on the pedal for a given desire? Much less the average fool.

If it were programmed counterituitively (yet logical in that 1/2 pedal=1/2 power), then it seems that would mean 1/2 pedal=2/2 throttle=1/2power, and 2/2 pedal=1/2 throttle = 2/2 power, which, would mean that 1/2 throttle provides more power, which would mean higher airstream velocity=more power(because of more torque). But this goes against what we all seem to feel in the butt dyno. So either it's programmed highly illogically to give 2/2 throttle=2/2 power at 1/2 pedal, and 2/2 throttle does provide more torque/power, or it's programmed intuitively (1/2 pedal=1/2 throttle, but it's not intended that the ECU make major corrections to pedal v. throttle position), but 1/2 throttle provides more torque because of higher airstream velocity.

Instrumentation should prove or disprove this. If it were programmed intuitively, with the drive-by-wire only making minor adjustments, then it seems this would prove my point since 1/2 throttle would be resulting in more torque (based on what we all feel) and we seem to agree that the velocity would in fact be higher at half throttle for a given engine rpm.

As for the individual throttle plates on the M's, I think this may be in my general argument's favor as well. Everything I ever read said that this was to provide better throttle response, not better power, and it seems like that's what's really at issue in this phenomenon. Further, adding throttle plates seems (I'll admit I don't KNOW) like it would add to the total drag, not reduce it, so they wouldn't do it without a more than offsetting benefit. My quick analysis on why this might be is that it brings the location of the highest delta P (at the throttle plates) closer to the combustion chamber, so that when you crack the throttle, the ram effect of the air that's backing up on the upstream side of the plate can push right in to the cylinder without having to find it's way through the manifold and incurring the damping effects that would imply.


My last point was that since the car has Drive By Wire, we the driver really don't know for sure what position the Throttle plate is in for any given engine load situation.

Asking the Throttle Plate to be wide open when the Engine is not YET approached a higher RPM would result in TOO MUCH AIR to match a given preprogrammed RPM VS Fuel injection pulse length and timing. If indeed the actual position of the throttle plate is being reduced (not matching your foot position) by the ECU, then that is because the ECU knows better than your foot.Whoah cowboy, sounds like you might be agreeing with me. As long as the engine is gaining rpm's, it has not hit the limit of the amount of air that can flow through that particular configuration (of throttle position). So, the RPM will dictate how much air actually comes in, not the throttle or the MAF, and the MAF will match the right amount of fuel to it. And the MAF is well upstream of the throttle, so why does it matter what the throttle position is? As far as fuel added to the airstream, I agree not much if any. So what else is happening?

Now it would be very interesting to have info on the given position of the throttle plate, vs your gas pedal position, vs load vs acceloration of the car...This would be key info in our ongoing saga. Come on VAG-COMmer's . . anyone . . ??? How about an actual tuner out there than can really instrument this situation - anyone??:vhmmm:

Boy, I have some time to kill, on to the next thread....
:incar:
Mike

[ZCD2.7T]

1) This behavior COULD show itself on any car with a DBW (drive by wire) throttle, which means most cars sold today.

2) Not exactly, but it depends:

With DBW throttles, unless you can see the programming curve of the actual throttle butterfly, there's no way to predict how the engine will respond. I remember a thread on AW where someone actually looked at the programming of the "Sport" button on the RS4. In that case, the throttle mapping was MUCH different when "Sport" was engaged. Bascially, the first 1/4 of the pedal travel delivered about 70 % of *actual* full throttle, and the last 3/4 only the remaining 30%. Some RS4 drivers like that feeling, some don't.

Personally, I can also add my family's 3 total 2.7Ts to the list of cars that feel quicker at 3/4 throttle than at full throttle, in some instances.

Please see this link for an explanation (from Audi) of how "Sport" mode works in the RS 4.:

http://forums.audiworld.com/rs4b7/msgs/61053.phtml

I post this as an illustration of how a DBW throttle can be programmed to do whatever the engineers want.

The whole fluid dynamics discussion is WAY beyond me, and far beyond what I think is actually happening here, with apologies to those who have typed ALL THAT STUFF! :bigeyes:

[peiserg]

I do agree that it would be ridiculous to purposely have the car put more power to the ground at 3/4 throttle. Perhaps I'm not passing along the info correctly. But i'm pretty certain he said that with a full throttle stomp teh TC won't allow the tranny to take the full load instantaneously.

[SpinEcho]

Peiserg's post about transmission behaviour is very interesting. I've noticed my Beast has a rather unpredictable throttle response when in regular 'D' mode, something I always attributed to a combination of turbo lag and torque converter slip. I wondered if this might be responsible for the behaviour that started this whole post, as I have also noticed a sudden surge of acceleration at part throttle in certain cases. I can eliminate this by selecting 'S' mode (which I had always assumed locked up the torque converter more quickly). But I guess this explanation doesn't work, since people have also noticed this behaviour on manual-equipped cars...

[Z07]

High intake velocity is a friend of torque. With partial throttle, there is less area available the air being demanded to flow through, therefore it has to flow faster to get all the air through in time (Q=A x V, Q = total volume per time, A= area, V=velocity).

Porsche's vario-ram (IIRC) essentially capitalizes on this. At lower rpms only a portion of the intake is available, at higher rpms, a second plate opens up, all at full throttle.:mech:
This does actually make sense to me, albeit with my limited Bernoulli knowledge.

Total Pressure = Static Pressure + 0.5*Density*Velocity^2 + Density*g*z

We can forget the last term as we're dealing with air travelling at constant altitude/depth.

The static pressure is the ambient pressure, i.e. the pressure of the air outside. At speed, this air has a velocity relative to the car. When the air is brought to rest in the cylinder, the dynamic head (0.5*Density*Velocity^2), is realised as static pressure. A slightly higher static pressure means a slightly higher air density, which means a slightly higher mass of air will fit into the cylinder. The MAF(s) will detect this and inject marginally more fuel accordingly.

Note though that there is a limit to the applicability of this affect. At low rpm it will likely be pronounced but at higher rpm the demand for air will be too great for the smaller orifice to facilitate.

Layman's terms - A human sucking through a straw works well. A human sucking through a drain pipe doesn't work so well. A centrifugal pump sucking through a drainpipe works well.... through a straw, not so good.

That said and done, a good ECU would know this.;)

The same principles apply with cams. 304deg/11.8mm cams are great for delivering power at 10,000rpm, but are utterly useless at 2000rpm. 240deg/8mm cams are great at 2000rpm but are asmatic above 7000rpm.

The velocity of the air molecules also plays a part in combustion efficiency. When the fuel molecules enter the chamber, if they are hit by faster air molecules moving in a more turbulent motion, they are given more energy and, with more energy, both them and their properties are more evenly mixed throughout the cylinder. Turbulence is often deliberately introduced to flows to aid the mixing of fluid properties, e.g. golf balls. The dimples induce turbulent flow, which causes the fluid in the boundary layer to mix its velocity more uniformly over the depth of the boundary layer, hence preventing flow reversal at the skin, which leads to flow separation and a larger wake (which gives more drag). So with dimples, the balls fly further!

As regards AFR mapping, it varies depending on NA or FI application. However, generally the AFR is higher at low rpm and/or light throttle to yield good mpg (and help the turbos spin up AND pass emissions regs) and lower at high rpm and WOT to prevent detonation from occurring under the higher temperatures inside the combustion chamber. Boost obviously adds to the detonation problem with yet more heat and pressure.

EDIT: Just to add. The throttle response is usually better if you don't crack the throttle wide open straight away as well.

[Z07]

What the engine needs is a certain MASS of air
What it needs is a certain VOLUME of air. Hence why we trick it with turbochargers.:jlol:

[RS6-4dr911]

Ooooohhh, a 3 way and talking about balls - this is gettin' kinky.

Sounds like you agree with me Z07. Not sure I understand completely your analogy to straws v. drain pipes, (no personal experience with the latter) but your comments on combustion chamber turbulence match with what I've read and understand about the process.

I am not quite ready to agree with you on volume v. mass of air being what an engine needs - seems like it can't have one without the other. Certainly at a given rpm it will draw in a fixed volume every time, all else being equal. But I agree with Aronis, what it really wants to make power is a mass of air (hence oxygen) - mixed with a proportionate mass of fuel. That's why we pack in the mass with turbos, to get more power.

You also make a distinction of AFR with NA v. FI - but FI (fuel injected) is NA (normally aspirated) in most applications. I suspect you meant NA v. supercharged. Or did you mean carbureted v. FI? I think they're different in both cases nonetheless.

Carbureted is usually lower AFR to compensate for poor atomization and possible fuel/air separation as the mixture flows through the manifold. I suspect NA is higher AFR than supercharged to prevent detonation in the higher pressure of supercharged engines, regardless of fuel mixture mechanism.

Anybody that has hard facts or professional knowledge/experience to back up their arguments
, stay the hell out of this, ya hear me?:nono:

[Z07]

Ooooohhh, a 3 way and talking about balls - this is gettin' kinky.

Sounds like you agree with me Z07. Not sure I understand completely your analogy to straws v. drain pipes, (no personal experience with the latter) but your comments on combustion chamber turbulence match with what I've read and understand about the process.

I am not quite ready to agree with you on volume v. mass of air being what an engine needs - seems like it can't have one without the other. Certainly at a given rpm it will draw in a fixed volume every time, all else being equal. But I agree with Aronis, what it really wants to make power is a mass of air (hence oxygen) - mixed with a proportionate mass of fuel. That's why we pack in the mass with turbos, to get more power.
That's exactly my point. The piston draws in a given volume of air during the induction stroke (the swept volume of the cylinder). By altering the pressure of the air within that volume, you alter the mass. Fixed volume + higher pressure = higher mass = ability to burn more fuel. But looking at it in basic terms, the downward moving piston is drawing in a volume of air. It doesn't know what mass is. Faster moving air will end up at a higher static pressure when stagnated.

In hindsight, maybe we were saying the same thing. I was looking at the engine as an air pump in terms of its 'needs' (not what's actually good for it).



You also make a distinction of AFR with NA v. FI - but FI (fuel injected) is NA (normally aspirated) in most applications. I suspect you meant NA v. supercharged. Or did you mean carbureted v. FI? I think they're different in both cases nonetheless.
FI = Forced Induction.;) :hihi:

[Aronis]

What it needs is a certain VOLUME of air. Hence why we trick it with turbochargers.:jlol:

the volume is fixed by the cylinder size. The MASS of air is variable ie higher pressure =higher Density for a given fixed volume ie more MASS ie number of moles of air molecules.

[Z07]

the volume is fixed by the cylinder size. The MASS of air is variable ie higher pressure =higher Density for a given fixed volume ie more MASS ie number of moles of air molecules.

Yeah, I realised that we were saying the same thing a different way round. The piston draws in a volume of air dictated by the cylinder's swept capacity. However, the power produced is determined by the mass of air that fills this volume.

This is what happens when you start discussing 'the needs' of a non-emotional entity. You start confusing its needs and your needs.:hihi:

I'm not entirely sure about my velocity/pressure argument either. I need to talk to my old turbomachinery professor.

"Master. Why is it that car accelerate faster with only part throttle?"

I'm sure there's a psychological element to this too. At part throttle, you aren't expecting/wanting the car to accelerate that fast. At full throttle, you're usually expecting/hoping for a lightning storm and a chorus of angels. Hence marginal disappointment. Lazy-ass angels!

[RS6-4dr911]

Your last paragraph is exactly what I was wondering, that's why I'd love to see some actual performance data. If you look at the link that ZCD2.7T posted, the requested torque curve is hugely disproportionate on the early part (steep part) of the curve but it never slopes downward.

Somebody with a day to kill and lots of instruments really needs to get on this.:mech:

[JCT2033]

ok, only 1/2 to 3/4 throttle for me now.. ha