One of the first and easiest ways to improve performance in any engine it to open up the intake. Zipper’s MaxFlow Air Cleaner Upgrade Kits include a specially constructed 100-percent washable and reusable filter element made from multiple layers of surgical-quality cotton gauze that has over 60% more filter area than the factory performance element. This greatly enhances airflow while increasing filter height by only ¾”. Oiled synthetic fiber material traps the smallest dirt particles and provides years and years of service. Special longer mounting hardware is included. Fits H-D® Big Twins equipped with factory Stage I air cleaners including 2008 throttle-by-wire bikes. Simple, affordable and effective upgrade you can feel – installation takes all of about 5 minutes! (Legal in California only for racing vehicles which may never be used upon a highway). Zippers Performance.