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To make generalizations one of the things common among commercially-available cold air intakes is that they replace the stock air filter box and short hose to the throttle body with a long tube placing a reusable filter near the front of the vehicle and somehow duct the stagnant air from the nose to this filter.
The "wave theory" brought up by whatthe could more than explain how such a setup would improve fuel economy. By using a longer intake duct you are actually tuning the intake to improve volumetric efficiency at a lower engine speed. Engines do not pull air at steady flow but rather in pulses as each cylinder fills only once every two revolutions and no two cylinders fill in unison. CAIs often produce a sound because they eliminate a resonant chamber (the air box) and replace it with an organ pipe (the tube) thereby allowing that unsteady flow pulse to resonate and produce sound. Since sound is an alternation of positive and negative sound pressures you can acoustically tune your engine to shift the torque curve to a lower engine speed. When it is in tune the positive air pressure pulse will continue past the throttle and hit the open intake valve improving port velocity (increasing swirl and chamber turbulence, improving fuel burn and reducing knock tendency). The acoustic pressure pulse induced by the unsteady intake flow will likely exceed any "ram" pressure seen at normal highway speeds.
If the torque curve were improved VERY low in the engine speed range, below 2000 rpm, this could very conceivably improve fuel economy by allowing more high-load operation without lugging or allowing earlier upshifts. Shifting torque to a lower speed could shift a BSFC iso-efficiency island lower as well. I envision improving ultra-low speed torque (and efficiency) improving hypermiling by allowing a shorter-duration higher-load pulse followed by an EOC, effectively decreasing the percentage of the time the engine is on and under load.
As much as hypermiling depends on DIY and testing to compare I'm confused by the overt hostility to a mod just because it is perceived as a "performance" mod for racer-types. I tend to think most people's perceptions are wrong and would rather see data. If someone could do a good a-b-a on any mod and prove a FE improvement I'm all for it but debating the merits in the absence of data using only preconceived notions or dismissing an improvement because it's negligible (it's *still* an improvement) seems like a waste.
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