Air Filter (NU-4091ST) by UNI Filter. Uni Filter street bike air filters are a direct factory replacement air filter and are designed to fit perfectly into your stock airbox. They provide unsurpassed airflow dirt stopping and overall performance. For over forty years Uni Foam Air Filters have been unsurpassed in both engine protection and overall performance. That s because Uni s actually work best in the worst of conditions. And their unique two stage design produces MORE HORSEPOWER and MORE TORQUE than any other air filter.