The kit completely replaces everything from the air meter forward and can be installed in 15 minutes. The increase in horsepower is coming from both the increase in total airflow through the package as well as from a resultant boost increase of up to 2 additional PSI. Due to this increase, the air/fuel ratio of the stock vehicle is adjusted by one point from 11:1 to a more aggressive 12:1. You must make sure that any aftermarket tune used in conjunction with this assembly DOES NOT remove any fuel, which could lean out the air/fuel ratio too much.