This is where it all began. Over 40 years ago, the sandy, gritty soil of The Netherlands - and the resulting engine failures - proved that single-foam and paper filters gave riders air flow, but failed at protection. Twin Air's breakthrough, fused-dual-layer design (now widely copied) combined performance and protection. The fused layers of open-pore foam and fine inner foam trap even the smallest particles. The new, rounder design boosts airflow. So do the minimized seams made possible by a revolutionary micro-gluing process: Shorter seams = more foam = more flow. Replacement filter for use with Power Flow Kit. The Twin Air dual-stage off-road air filter traps even the smallest particles without restricting air flow to offer the best power output without harming the engine. With fused layers of patented open-pore foam and fine inner foam the Twin Air dual-stage off-road air filter protects from dust, dirt, and sand.