air filter

I popped off the air filter cover the other day for a look-see (curiosity mangled the cat...) and found this on the underside of the lid: There's no external line leading to it, no wires, no flashy-thingy; nuthin! What is it? Inlet noise reduction? Activated charcoal filter to help it breathe...

Any one change the stock filter to the K&N drop in? Thoughts?

To set up my question... I just replaced the stock air filter with a green drop in filter that I purchased on Amazon. The box seemed a bit old but the bag the filter was in was sealed. A sticker on the box with the model number had a date of 3/10/2017. Here's my concern...the filter seemed...

Hi. Looking for low-cost/simple horsepower gains. Looking at a drop-in K&N intake filter. 1. Is there a noticeable difference, or is it just mental? 2. What are the gains you are seeing/think you're seeing? 3. Does anything else need to be recalibrated? 4. Any other suggestions beside...