Dodge Vipers, Shelby Cobras, 1960s AF/X drag cars and NASCARS have them. Of all exhaust configurations out there, few scream "racy" like side exit pipes and for good reason. Exhaust pipes are, for the most part, a necessary evil. Past the headers/manifolds, all those pipes can do in terms of performance is reduce it. Side exit exhausts are the ultimate in minimalist design and are kind of like a bicycling garden gnome with a shotgun--short, fairly impractical for the street and dangerous in the wrong situation.
Side exit exhausts can sound wicked when used on the right car, but will make most commuter cars sound more like a John Deere than a Ferrari. It's all about where you stand. For most cars with a rear exit exhaust, a listener will be subjected to the seamless sound of all cylinders firing together. Unless you duct the exhaust so that it all exits through the same side, an observer will only hear half the engine no matter where they stand. If you have anything smaller than a V8, you might want to avoid traditional side exit pipes.
Unless you want your V8 to sound like a four cylinder and your V6 to sound like a Geo Metro, use an X-pipe crossover in the system. A well-engineered X-pipe will not only make a great deal more power than an old-school balance tube (a.k.a. H-pipe), it will help to seamlessly blend the engine cylinder noises into a single, unbroken note. It's simple to engineer, too; just run a pipe diagonally from the passenger-side header to the driver's side exhaust and vice-versa the other end. Use flattened, oval-shaped tubing if ground clearance is an issue.
Many would-be customizers have found out the hard way that using a short exhaust system gives the exhaust gases less time to cool down and a really short one can easily spew foot-long flames under certain circumstances. These hot gases are more than capable of melting tires, paint, plastic bodywork and almost anything else it regularly comes into contact with. And remember to use some sort of cover if you're running the pipes along the underside of your doors like the 427 Shelby Cobra and Dodge Viper. Fire makes metal hot and ankle burns are extremely uncomfortable.
Angle your exhaust so it points slightly downward and is near the back of your rear-most window. A downward angle will help to reduce soot build-up on your car's paint and the rearward placement will help to keep exhaust gases out of your car when the windows are down. If at any point you smell fuel or start to feel oddly sleepy while idling in traffic, park your death trap and move the pipe exits.
