For the person who wants a subtle custom appearance with all the benefits that a Flowmaster Cat-back exhaust system provides. Single Side Exit. (Single out side) systems are the perfect choice. American Thunder systems deliver a moderate to aggressive interior and exterior tone and are suited to the enthusiast who wants their performance to be heard. Like all Flowmaster systems, the benefits include improved throttle response, power and mileage. Designed for an easy fit, these systems include all necessary parts and hardware for easy installation.