Depending on the type of vehicle you may want to start by changing out the muffler. Just changing the muffler can give you potential gains of 2-4% over stock on most vehicles. Changing the muffler will also give you more sound by enhancing the sound frequencies coming from your engine. Often in older vehicles pipe diameter is not adequate and piping is rusted or has holes. Replacing old pipe with mandrel bent product along with mufflers has the potential gains of 6-8% over stock on most vehicles.