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The primary obstruction to intake isn't so much the direction of the intake, but the obstructions in the intake stream. That is why people like to upgrade to K&N filters; to allow the engine to breathe better.
You can also polish the intake manifold and do some other things like getting rid of the accordion hose in place of a smooth intake hose or pipe. You could improve on the intake a lot, with results that wouldn't make a whole lot of difference.
I think the design of the intake does two things:
One, it allows the intake box to not suck in everything that comes into view, therefore keeping debris and moisture out of the air filter.
Two, it allows the engine to operate under similar airflow conditions at idle and at speed. There are a hundred reasons why this might be necessary, or maybe it might not.
Let's say you put a huge air scoop on the front of the car. Sounds comical, but consider it just from design standpoint. The airflow would be normal sitting at idle or at very low speeds. However, wind speed (or "apparent" wind speed as the car travels faster) increases exponentially. Therefore, somewhere along the line, you would probably hit a point at which the intake airflow would cross from a vacuum to a positive reading, even at higher RPM.
Now, since the airflow is mapped into the engine control unit, I'm guessing that you would hit a point at which the airflow meter would not be able to interpret the airflow correctly, or the control unit would not be able to interpret the readings outside of the given parameters. Or the output parameters would be outside of what the fuel system would deliver, or what the oxygen sensor would interpret, or a host of other issues.
That's obviously an extreme example, but since the airflow system is designed to operate at a vacuum, it might not do it any good to forcibly introduce positive pressure (although you'd have to have a HUGE intake to force air in even at highway speeds). There is a point at which it would probably do the engine quite a bit of good, but you couldn't moderate the effects of the airflow as well as you can with turbocharging or an open intake system. They're controlled, and a simple ram air system is uncontrolled (due to wind, tailwind, etc). The airflow meter would accomodate some of that, as it already does based on barometric pressure.
The key to better performance would be to allow the engine to draw air in more easily, rather than to force air into it. Enter K&N filters, and some of the other things that you can do to allow easier induction.
Since it's hard to assume what the parameters of the control unit and airflow meter are, one can't really guess when it would be positive, and when it would be detrimental. But it is very apparent that turbocharged cars have different control units and airflow meters, probably because the parameters that are mapped internally, are very different. Even more than that, the car would have to literally EXPECT certain amounts of air at certain speeds, rather than expecting airflow based on air that the engine is taking in, or based on expected airflow as a result of turbocharging.
There are a lot of things that affect the operation and these are just a few. Just to give you a guide as to why intake systems are designed a certain way. I'm in no way an expert on ECU mapping, but some of the principles and parameters apply to all cars, and in all situations. That is why there are MAP sensors and airflow meters that ultimately control the way that the whole combustion process is controlled. The examples I've given are, again, extreme, but they give you an idea of why it might either a) not make a difference, or b) not make enough of a difference to do it. And why you might be better off exploring a more controlled, but open induction system if you're looking for little boost in performance.
More likely, though, is that the current design keeps the intake from getting fouled with debris.
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Chris Herbst, near Chicago.
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