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?
I had a 6-speed B5 S4 with the same 'issue'.
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.
Can second this behaviour.
I third this behavior with my now sold B6 S4....
07' Boxster S
10' S550 Sport
2011 Audi R8 V10 Spyder
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.
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.
If you never exceed the limits, how do you know where they are?
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.
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.
i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???
Same results for me as well, on a consistent basis
GBNF: Mugello, silver, carbon, RNS-E, H&R coilovers, Hotchkiss
'14 GL550
'79 911SC Targa
My RS6 was the same. Full throttle never gave fastest starts.
SoCal
Current: S6 (2007), A3 2.0T (2008), RX-7 (1995)
Previous Audi: RS6 (2003)
i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???
I am having some trouble with this explanation.Originally Posted by 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.
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
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?
2010 XFR
2004 RS6 (sold)
2008 Exige S 240
2009 V8 Vantage
2008 X5 4.8i
To be honest I've never tried it out though I'll be sure to when I get home tonight!
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.
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
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.
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