Cold Air Intakes do not work. Consumers spend hundreds of dollars for a nice sound. Making these yourself can be done for as low as $20.
For mine, I started with a cheap air filter off of eBay, and just put that on the MAF in place of the factory air box. This made it sound more like a Honda, and I didn’t like that, so I bought a piece of 2.5 inch turbo pipe that was pre bent at a 45 degree angle. Then I used screws and a hose clamp to put it all together. It sounds just like a $150 K&N Cold Air Intake for a fraction of the cost.
Something I wish I had done, was ugrading the whole intake to 3 inches. Meaning a 3 inch turbo pipe, air filter, and a Mass Airflow Sensor from a 7 series, or 540i. Click here to see a tutorial on how to wire it up.