What are your thoughts on performance air filters for the BMW 318ti? I have seen ones that range from $50 to $300. Do they actually work (increase HP)? Also I read that you shouldn't drive in the rain if you have one. Please let me know any information you have on this subject. Thank you very much.

not driving in the rain is recommended for behind the bumper cold air intake systems. and even then hydrolock is unlikely.

intakes on our cars dont do much aside from sound. i replaced my stock airbox with a big cone filter, and it added a hissssssss noise at higher rpms. kinda cool. but unless you want the sound, and eye candy under the hood, intakes are useless on the 318's m42/m44.

sad, i know.

not necessarily. the M44 responds well to unobstructed cool air.. but that is the key, it needs to be COOL air, not hot air that was already circulating around in the engine compartment.

When Mr. Fogg devised his way to open up the stock airbox to more air (the M44's intake BEFORE the filter is extremely restricted) he noted an 8% increase in air flow to the engine as recorded from the computer's OBD port.

So the M44 DOES respond to a better intake, it just needs to be a GOOD intake

ah. very true. i have the m42. it did increase throttle response a little, but maybe i was imagining. the sound is there tho. a deeper engine noise with a whistle.

underhood air = decrease in power.

Fogg the airbox and call it a day.