Exhaust gas contains a large percentage of water vapor, which over time will rust and degrade the exhaust system. Components in the exhaust system may have detachable flanges to facilitate their replacement.
Despite the high water vapor content, exhaust also contains noxious gases. Holes in the exhaust system may allow exhaust gas to leak into the vehicle. If inhaled, these gases are harmful to people.
Mechanics' Corner: More Technical Detail
The exhaust pipe begins where the exhaust manifold ends. The first section is called the header pipe because it has a unique shape and is the "head" of the exhaust pipe system. It begins like the wide end of a funnel and can be four to five inches in diameter; it then narrows down to around two inches where it connects to the catalytic converter. The header pipe is usually short (one to three feet in length) because the extremely hot and still-burning exhaust gases must stay so as they enter the catalytic converter. It is here where they are "after burned" in the presence of specific rare metals and converted back into non-poisonous gases. Ideally, when the exhaust gas leaves the vehicle, it should only contain carbon dioxide (CO2), water (H2O), and oxygen (O2).
The next section of the exhaust system connects the catalytic converter to the mufflers and tailpipe. By this time, the high-temperature and poisonous nature of the exhaust gases have been greatly reduced. The main function of the exhaust system is to decrease noise and route the exhaust gases away from the passenger compartment toward the back of the vehicle where they are expelled.
It is important to remember that the exhaust system is part of the emissions system and therefore regulated. The exhaust manifold, header pipe, catalytic converter, mufflers, and all the connecting pipes are part of the emissions system. Even high-flow muffler systems can change the "scavenge" rate of the exhaust gas flow rate through the catalytic converter (which causes "dirty" exhaust emissions to be released). A change in the header pipe can cause the exhaust gases to be pushed too fast through the catalytic converter, which results in less efficient "scrubbing" of the emissions.
High-performance "header kits" must have federal, state, and local approval because too many of these kits alter the volumetric flow of the intake and exhaust gases by changing the "back pressure." In addition, these kits often change the critical location points of the oxygen sensors, which can alter the way the engine management system controls the fuel injection. In some cases, entire high-performance systems need to be removed because they cause the vehicle to fail emissions testing. Attempting to obtain a "dark side" or illegal emissions certification is not only expensive, but a felony in most states—for the vehicle owner and the supplier of the certification. A state highway patrolman, policeman, or city police officer can pull over and impound a vehicle if the exhaust system has been illegally modified. Only use the approved high-performance systems when modifying an exhaust system.
When replacing an exhaust component, the vehicle is usually raised on a lift where the underside of the vehicle and the exhaust system can be accessed. Whenever possible, it is better to use bolt-on components because these are designed and built to factory specifications. Welding in components is less desirable because the actual lengths in the sections of the exhaust system are altered. Welded-in components are usually generic one-size-fits-many and tend to be much less efficient in their operation. Use these as a last resort when the factory-specified component is unavailable or too costly to be practical.