One of the main features which car tuners come to when they decide to make a change is the exhaust system. Special powerful sport exhausts are very popular these days. The main purpose of an exhaust system is to take away the chemicals created after the fuel burned in the combustion chamber. It also muffles the sound coming from the working engine. Some young people like to install wide pipes, but this change does not always improve the work of the system. Exhaust pipe affects Blow phase of combustion letting the gases out of the system. The less pressure there is in the exhaust system, the higher the BHP is produced. At the same time the rate of the flow and velocity depend on each other too. Extra large exhaust slows down the flow, but thinner pipe increases it. So, the exhaust has to be balanced out with the intake and engine capacity and resist backpressure. The ideal exhaust pipe should be based on the amount of air that engine needs to put out. Standard motors vary greatly from turbo and highly tuned ones. The bore size of the pipe is determined pretty easy for standard engines: small ones under 1.6-liter work fine with the maximum 1.5-2-inch pipe, under 2.5-liter motors go with 2-2.5-inches size. Bigger engines work best with twin exhaust system and half the engine size determines the most appropriate pipe size: a 3.0-liter needs a twin 1.5 inch bore and a 5.0-liter requires 2.5 inches size. The shape of the exhaust manifold is important. Although there might be a variety of offers, it is better to leave this part up to the manufacturer because the automakers usually hire highly qualified specialists who know this better than ordinary people. One can also look for aftermarket performance exhaust headers which always do a good job. The systems of exhaust manifold vary. An ordinary street car does best with 4-2-1: the four pipes come together into two and then merge in one. This system helps to lower RPM torque. Race cars do better with a two-step manifold: 4-1 or 4-2. This allows showing higher numbers and coping with high RPM easier. A reduction of bhp is typical for cars with catalysts, but unfortunately all modern vehicles are required to have them. An improvement for this case could be achieved by high flow sports cat or an off-road de cat pipe. Some high performance and sport cars are equipped with a sport cat from the manufacturer by default; the other standard cars need an upgrade. Sport catalyst will give very little or no advantage to small-engine cars. The same result as with sport cat can be achieved by a 4-2-2 exhaust manifold. The majority of standard exhaust headers are cast. Welding rarely goes smoothly and leaves imperfections on the inside of the pipes. Very often one can see ridges where the pieces were put together. If the car owner wants to improve the work of the exhaust, it is advised to smooth out the inside with a grinding wheel of a drill. There are also stainless steel units in car shops designed specifically for this purpose. It is recommended to polish the inside of the exhaust too because it increases the exit of gases curing the airflow and improving the work of the whole system. High performance cars have stainless steel exhausts, so they are smoother on the inside and lighter. However, some attention should still be given to the joins and bends of the system.