Focus ST 2013+ K&N E Series Round Air Filter

Most air filters on the market are a compromise. Many high-performance filters deliver better airflow, but sacrifice filtering ability and engine protection in the process. OEM filters, which are usually made of paper, filter adequately but at the expense of airflow. And airflow decreases as these air filters become dirty. Not so the K&N air filter. K&N air filters increase airflow without decreasing filtration ability, and the increased airflow remains consistent throughout the filter's service interval. The secret is in the K&N filter's construction.

K&N's initial goal was to develop a filter that would deliver increased airflow during dirty, dusty off-road racing conditions. Increased airflow improves the efficiency of the combustion process, increasing horsepower and torque. But just increasing airflow without ensuring adequate filtration under those conditions would result in a ruined engine. K&N's solution was a filter design consisting of 4 to 6 layers of cotton gauze sandwiched between two aluminum wire screens. The cotton is then treated with a specially formulated grade of oil that causes tackiness throughout the cotton's microscopic strands. The resulting K&N filter not only delivers increased airflow while trapping dirt particles that could harm your engine, that level of airflow is maintained even as the filter gets dirty. The layers of oiled cotton fibers in the K&N filter trap dirt particles that are smaller than the holes in the filter media. The thickness of the layers also allows the K&N filter to trap dirt and dust on many levels, trapping more dirt without decreasing airflow. Contrast this with a paper filter that only traps dirt on its surface; as the surface area becomes clogged, the airflow rate goes down.

K&N is the inventor and leading innovator of reusable cotton gauze filter technology for automotive applications. The first K&N filter was conceived for racing over 40 years ago and a performance enthusiast mindset continues today throughout all levels of the company. K&N is the sales and brand leader for performance filters, including an extensive line of factory replacement filters and performance air intake kits. The company actively promotes the environmental, economical and performance advantages of long-life reusable cotton gauze filters.