If you’ve ever turned wrenches on a car or a bike, chances are good that you’ve upgraded an exhaust system at one time or another. The cheap and easy way to do this is to replace the muffler, or (better yet) the exhaust system from the catalytic converter on back. The real horsepower gains come from replacing stock exhaust manifolds with headers, then replacing the entire exhaust system with a less restrictive one. Dyno data shows this to be an effective way to add horsepower to almost any vehicle, but do you know why this works? The video below shows how it works and even (briefly) explains why; think of it as science class for those of us with dirt under our fingernails.