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Discussion Starter · #1 ·
Anyone notice a slight hesitation when it comes to stepping on the gas? As if
its not getting any gas or something.
Its usually that first 1/2" or so, that the car doesnt respond. Then it does.
Sometimes it does it, sometimes it doesnt. Sometimes its when your running
thru the gears and others when your driving it like your G-ma would. I've even
stalled it, just to see if it was user/ clutch error. After 3k miles I can say its definately\
not user error. I took it to the dealer and had a tech drive it. He noticed the same thing.
I left it for the day, got an oil change and car wash. He calls me and says, "I drove a brand
new one and it doing the same". I'm calling B.S.

THOUGHTS?
 

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I can't notice this delay any more... Maybe the ECU finally said "screw you, mr. leadfoot - do what you please" and let me drive like I want to
 

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Originally posted by zoltiz@Mar 10 2005, 08:22 PM
I can't notice this delay any more... Maybe the ECU finally said "screw you, mr. leadfoot - do what you please" and let me drive like I want to
Mine's still fighting back. Grr...
One day an ECU upgrade will be available and hopefully defeat this annoyance.
 

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You can bet that replacing the resistors in the gas pedal will cost more than replacing a throttle cable. I'm still trying to figure out the advantages here.
 

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Do you want the full technical reason why it should be better? I'd be happy to burn up a whole page describing the reasons why it should be the gnat's eyebrows.
 

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Former '05er
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Since you all asked, here it is.

WHY THE DRIVE BY WIRE THROTTLEBODY SHOULD BE SIGNIFICANTLY SUPERIOR TO A CABLE OPERATED THROTTLEBODY.

An engine depends on air entering and exiting the cylinder in a predictable fashion, with a goal of completely filling, or better yet, overfilling the cylinder with each breath it takes. Fundamental to making this work is the concept of mass flow. For a given flow of air, there will be a predictable mass containing oxygen molecules sufficient to completely oxidize the hydrocarbon fuel we are using to create cylinder pressure. This is fundamental to any hydrocarbon fueled engine.

The airflow has some interesting characteristics. Because it has mass, it has inertia. This means the air will resist changes in flow, either stopping or starting, and we can use this property to help us stuff the cylinder with as many oxygen molecules as physics will allow. One of the ways this is accomplished is through tuned lengths and tuned diameters of intake plumbing. For example, if we use a relatively long small diameter tube to connect to the cylinder, once the airflow is started, it will tend to continue to flow through the tube even after the throttleplate is closed because the air in the tube has mass and velocity, ergo inertia. We can use this inertia to fill the cylinder beyond it’s theoretically 100% full volume because the inertia in the column causes the pressure in the cylinder to slightly exceed atmospheric pressure (our standard for measuring what full means). When this happens, we say the cylinder has exceeded 100% volumetric efficiency because we have more air (and air means oxygen) molecules in the cylinder than we would have if the cylinder were allowed to fill unimpeded with “standard” air.

This concept leads to the often misunderstood concept of velocity. It is desirable to have high air velocity in the intake tube because of our inertia discussion. It is bad to have velocity exceed the speed of sound because undesirable things happen to airflow above Mach. So, the engine designer has a huge compromise to make. He (or she) has to determine how best to fill the cylinders with the most amount of air possible in the rpm range we have deemed important. So, our engineer will go through a lot of calculations to determine the optimum sizes for all the plumbing making up the intake to ensure we meet this goal (and about 50 other conflicting goals, including noise abatement). Key to making this all work is keeping the velocity as high as possible to ensure the inertia effect is working to our advantage in the rpm range we decided is most important.

The driver, or the engine’s operator, doesn’t understand the velocity problem, and shouldn’t need to be concerned about it at all. Unfortunately, with a mechanically controlled throttlebody, the operator needs to be keenly aware of this problem to operate the engine effectively. In the days of carburetors, this problem was huge because the carburetor depends on velocity to create pressure differentials across the fuel metering jets and force the fuel to atomize into the airstream. Electronic fuel injection has altered this issue, but not completely eliminated it.

What happens with the throttlebody when the operator opens it wide open very quickly? The velocity through the throttlebody drops like a rock because the throttleplate suddenly goes wide open whether it is a good thing or not. We lose all the advantage we had to designing the intake plumbing to perform at its optimum because we have no ability to maintain the desired velocity to create the inertia to fill the cylinder. The inertia we wanted is not there because velocity is not there.

How is drive-by-wire different? A computer controlled throttlebody is programmed to not open more than the engine is prepared to breathe, so velocity stays higher, the cylinders fill better, and the engine runs both more powerfully and more efficiently in the rpm range we determined to be important. Because it is impossible for the engine’s operator to open the throttle before it is appropriate, the engineer at the factory has even more freedom to size the intake plumbing to operate optimally across our all important rpm range because he or she knows the velocity won’t fall off when the operator says, “Give me full power.” The system is transformed from a “do exactly what I say” system to a “do exactly what I mean” system. The operator doesn’t need to know anything about the details of the engine’s operation, he or she just needs to tell the engine, “I want more power” and the computer algorithm takes care of the details in a way not previously possible with manual linkages.

Anyone who had had a really hotrod motorcycle with the famous “smoothbore” or “flat-slide” carburetors knows this problem, because engines equipped with those types of racing carburetors will complete stall if you open the throttle too much at too low an rpm. The airflow through the carburetors is too slow to create an adequate pressure differential to push fuel through the metering jets, so the engine stalls until the rider closes the throttle enough to bring velocity high enough to get fuel back into the engine.

Reliable fuel injection has eliminated the problem of getting fuel into the cylinder, but unfortunately, injectors don’t do the greatest job of atomizing the fuel, and regardless of atomization, if the velocity is too small, the engine starves for air, and power is way down. Having the computer controlled throttle allows the engineer to design the system with lengths and sizes of components not possible with a manually controlled throttle.

What does it means to us? We get an engine with MORE power, AND better efficiency than we could ever have without the computer controlled throttlebody. Sound pretty cool? It does to me, until I drive it, and my foot with its 38 years of experience using manually controlled throttles tries to do what it has always done to compensate for these weaknesses. All I manage to do is screw up the computer’s idea of how to control the throttle. I’m still trying to break the “bad” habits, and I find it gets better everyday, but it is really hard to teach an old dog new tricks.


OK. Not quite a whole page. Thank goodness!
 

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It sounds like this only applies at low RPM's when there is little or no inertia in the intake tubing. Which explains why the response is so much faster when we are already over 1500-2000 rpms. It's not necessarily a mechanical delay in the ECU, its just waiting for the right amount of air/fuel to enter the cylinders. Or rather, ramping up more slowly than we might be used to.

It also sounds like a shorter intake would allow faster response since pulling air through a longer tube would cause greater delay because it has to move so much more air to get the inertia it requires. This might also explain why the intake resonator is in place. It allows more air to enter the throttle body since it is effectively a resevoir of air in the intake tube that can be put into a slight vacuum during the initial ramping of the rpms.

Veddy intelesting...

Thanks Lance!
 
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