Our Foam Prefilter is the best way to keep dirt out of the engine. Captures almost all of the dirt before it can reach the cotton element of the air filter and saves time because it allows 3 or more races in between cleaning the air filter. We sell three versions. 600-5153 fits the flat filter air box and 600-5155 fits the std oval box and 600-5155-2 fits the Guhl oval air box. To use, spray the foam prefilter with 77-260 Klotz Foam Air Filter Oil and simply place it between the outerwear and the air filter. We performed many back to back dyno tests showing no loss of horsepower.