Outerwears Pre-filters are designed to keep out particles of dirt, sand, and street debris as small as .005in. Our Pre-filters are run dry meaning no oils or additives are required so there is no dirt sticking to the oil creating premature loss of airflow and horsepower. Outerwears water repellent Pre-filters add another level of protection. The woven mesh is specially treated at the mill using a proprietary process to repel water. This hydrophobic water repellent process is not sprayed on; consequently there is no airflow loss. Our Pre-filters are used and recommended by top professional engine builders and race teams worldwide.