AEM Pre-Filters are manufactured from highly durable polyester. Uniform micron openings allow the filtering of debris .005 or larger. The AEM Pre-Filter is also factory treated using an embedded hydrophobic process. This offers consistent airflow, water repellency and maintains horsepower gains. Simply put, the AEM Pre-Filter keeps larger water and dirt particles out, but allows smaller air particles to pass through it (Please note that this item is water repellent, not water proof). The AEM Pre-Filter drastically improves filter life and the time you can go between cleanings, especially for people doing off road driving or driving in dusty areas. The part number for this Pre-Filter is 1-4001 and it fits over filters up to 6" Base, 5-1/4" Top and 9" Tall.