[Delnari]

Intakes on turbo cars work better than those on normally aspirated cars. They only function better when the other components along the emissions path is upgraded as well. You want to get an intake that will work best with your other modifications, i.e. turbo upgrade or performance tune, downpipe, performance muffler, etc. The only formula you need to look for is the volume of air flow that can be drawn with least amount of heat. Sound effect is a by product of the non-baffled intake piping; OEM intakes are built to reduce excess noise with baffle chambers. One of the reasons why OEM is not as effective as aftermarket.

Look for an intake that will source it's air intake away for the engine as much as possible and has a large filter for air flow. The most expensive intakes on the market will use exotic materials such as real carbon fiber in trying to keep the heat soak at a minimum. Be careful on where the air inlet is position to avoid direct contact with water at the point of entry as these will cause engine problems and even hydrolock if enough water is drawn into the unit.

Late model BMW cars have the intake position up high in the engine bay with an air inlet channel directed from the front the front of the car for it's air flow. Due to this design you will not be getting the coolest air possible. A good alternative is to add a larger intercooler and/or methanol injection to BMW intake that will give the most dense (oxygen condensed) air for combustion.