Cat back exhaust systems are the easiest way to free up horsepower that is already there. Most car manufacturers use extremely small 1 3/4" - 2 1/4" piping in their exhaust systems because it is easier to come by and much less expensive than performance exhaust systems. 90% of all cars will see a signifigant (10-45 horsepower depending on vehicle) increase simply by changing out the restrictive factory exhaust and replacing it with a free flowing 2 1/2" - 4" system with fewer and less crimped bends. Want to see the automotive equivalent of the plastic flex drinking straw? Lots of sharp turns, pipes made out of the smallest, cheapest material available, then bent into shapes that make you wince at the mere sight of them (right photo). Or, you can have an exhaust made from large diameter, highly polished, stainless steel with the straightest paths possible (lower right photo). Which one would you rather have on your car?