Injen RD1110P: Air Intake BMW 3-Series 2.5L / 2.8L 99-00, RD Series Polished Cold Air Intake System (RD1110P) by Injen. The RD Series is made for the enthusiast who wants improved mid-range power. This cold air system positions a high-flow air filter behind the bumper, where it can take in cooler, denser air. The cool air then flows to the engine through a fee-flowing aluminum alloy intake tube. Cooler air contains more oxygen for more complete combustion and more horsepower. Up to 9.6 HP Gain.