Replacing or modifying your engine exhaust is an easy way to spruce up the life and look of your car. Stock exhaust systems often use unattractive steel tubing to ventilate the exhaust from the motor. The tubes are often too thin, and suffocate the engine. Often they are also poorly insulated, which causes your engine exhaust to cool before exiting the system, which causes it to exhaust at a lower velocity. This reduced speed will cause lower engine volume, and inferior engine airing.
Stainless steel exhaust tubing utilizes the material properties of stainless steel to offer better engine exhaust performance. The most common exhaust material is aluminized steel. Aluminized steel has a specific heat of around .11 BTU/lb*F which is lower than stainless steel at .12 .11 BTU/lb*F. This means that for the same amount of stainless steel, it will absorb and less of the exhaust heat, and therefore your exhaust will remain at a higher temperature as it exits the exhaust. Higher temperature means high velocity, which means more sound, and is generally considered healthier for your engine. This is especially when using high octane fuels.
Aside from superior thermal properties, using stainless steel as exhaust tubing provides other benefits as well. Stainless steel was originally developed because of its resistance to rust. It is mixed with chromium, which when exposed to air reacts and forms a thin film of waterproof alloy, that prevents water from contacting the iron and forming rust. When gasoline combusts it creates heat, water, and soot. Because the air ventilating out of your engine contains significant amounts of water, it is important to be using a material that is resistant to oxidizing via water.
Of course all physical properties aside, the most important reason people love using stainless steel as exhaust? It looks great. Stainless steel is just that, stainless. It is easy to clean, does not corrode, and will last for a long time. Other materials will gather soot, and all sorts of other debris and are hard to clean. It also easy to coat with other materials, namely chrome. Chrome plating is popular in the automotive industry because of its beautiful shiny look and how it protects the underlying metal from corrosion. Stainless steel is a bit more expensive than alternatives, but nearly always the extra money with worth the better protection, improved car performance, and great new look that it will give your newly modified car.