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Actually that short delay is due to the design of the feedback loop system. I am sure the BMW engineers are aware of that. There is always a way to diminish this delay but never totally eliminate. DBW is after all a mechantronic system.

There is a commonly known "waterbed effect" in the controls engineering world. Here's the situation, I want to decrease error (by attaining a desired goal) and suppress noise (here noise does not necessarily implies fuzzy audio quality, but includes all unwanted frequencies). Or in layman terms, when I stomp on the accelerator, I want an immediate response. However it turns out that the more you suppress noise the larger error you will encounter. This simply translates if you want an immediate response, expect a huge error. This is similar to sitting on one side the waterbed and observing a hump on the other side.

I have TA a couple of courses in feedback control systems and taught this waterbed effect a number of times. Strange but true phenomenon in controls engineering. It's happening in the DBW system too.

On the other hand, since mechanical throttle linkages are rigid bodies, there will be more of the so called "feel" and "responsiveness". But more mechanical parts means higher maintainance in the long run and more fatigue on the linkages. Delphi Automotive already have developed the DBW brake systems. Believe me, I love mechanism design. But wires are the way to the future.
 
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