Air filters and air intakes. The expensive aftermarket “performance” options usually make no difference at all, or are actually worse than OEM.

K&N For example. The Oiled-gauze air filters allow far more contaminates through, and flow little to no more air than the paper OE because they have less surface area. They also clog quickly and in no time at all actually FLOW LESS than paper filters. Not to mention the oil will coat and ruin MAF sensors. Stick with the OEM-style replacements, preferably from the dealer, or high quality like MANN or WIX.

“Cold Air Intake” systems are all the rage, and they look really cool and sound cool, but really don’t make any improvement. Again, the filters are usually crappy, and most factory intakes pull air from the fender where it is actually cold, and the aftermarket intakes pull in hot engine bay air. The ones that actually pull in cold air are so close to the ground they risk sucking in water from puddles and hydrolocking your engine.

Most OEM air filters and intake systems flow well enough for almost 2x stock horsepower, so until you add forced-induction, or upgrade to bigger forced induction, 99 times out of 100 you are wasting your money.