K&N's claims you get more horsepower from your Twin Cam engine when this extra large "RK series" filter is fitted. Built around the legendary K&N High-Flow Air Filter, the new RK Series assembly includes an innovative powder coated billet aluminum back plate that eliminates external breathers and features a built-in, dyno-tuned velocity stack. The result is a significant airflow increase to the engine without sacrificing filtration. This RK Series assembly retains a factory appearance by utilizing the stock air cleaner cover, yet boasts superior filtration and performance with a K&N stamped laser etched beauty ring. All models feature internal breathers to replace the external OEM breather set-up. Available for all Twin Cam 1999-2017 and XL Sportster 2004 to present.