K & N Filter

Part#: re-0920 9" cone with a 3' flange.

With the way I understand it, when the efi sees the throttle position at wot, it goes into a loop mode that supplies fuel at a preprogrammed rate. This rate is based simply pre programmed. This is done because the engine output is changing so quickly that no "o2 sensor" possibly keep up. So a set mixture rate is applied based on the engine's capacity to move fuel and air through itself.
You have changed that capacity! Because that filter is less restrictive to the air flow, the engine is now capable of moving more air. But at WOT the EFI does not recognize this increase in air flow and inject more fuel, it simply runs it's predetermined program. Hence, you run lean and loose power.
At part throttle, the O2 sensor will adjust the mixture. This should show up as better fuel efficiency.

It is the increase in air flow the surface area of the conical filter provides that increases power. Of course cooler air will give more power, but the filter in the stock airbox will not flow nearly enough air for it to be beneficial. The exposed filter will give a 5 - 6 % horsepower gain on average vs. a 2 - 4 % gain the panel filter will give. This is dyno tested and proven. Placing the filter right where the airbox was is good because it will fed fresh air from the ducts that fed the stock airbox. You may loose the response at WOT but part throttle and mpg increase.

Of course there is an issue of a crud that it picks up, and water. In the rain, it may pick up a little water but that depends on how you positioned the filter . The usual bugs, small leaves, etc. are all there, but can be removed by shaking the thisng (a toothbrush works well).

Here is some more INFO:
MN12 Air Silencer Removal

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