EXHAUST

A series of pipes are used to connect the various exhaust components such as the monifold, catalytic onverter,
resonator(s) and the mufler together. The lengh and cross-section of the pipes, as well as the type of junction used, influence the vehicle's performance characteristics and sound.The junctions of the various components are connected by means of insert connections, flanges or welds. Exhaust system for vehicles with larger displacements ( e.g V6 0r V8 engines ) are often fitted with twin pipes.

PROBLEM
Stock CAT-BACK  system are often welded into one complete element for faster mounting - not good. The joint from welds affect the efficient exhaust flow. Moreover , the pipes are thinner with narrow angle bends causing restrictions hence introducing backpressure. Backpressure robs your engine of power ; instead of directing energy at the crankshaft, your engine has to work to push the exhaust gas out under pressure; the greater the pressure , the bigger the power loss. Some backpressure  is inevitable , but should minimize in order to scavenge power that is available.

PIPE SIZING.
Many mistakenly believe that a bigger exhaust is a better exhaust . Exhaust gas is hot , and we'd like to keep it        
hot throughout the exhaust system . This because cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large pipe will slow down the flow, which will in turn give the gas plenty of time to cool off en route - no good.. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which would nullify the headers purposes. This is because the pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulating material, such as  header wrap or a ceramic  ( thermal barrier ) coating would mitigate this somewhat. Albeit, there is know way of accurately calculating the optimal exhaust pipe diameter. This is mainly due to the complex nature of an exhaust system; like bends in the piping , temperature fluctuations , differences in muffler design etc. make selecting an optimum pipe diameter difficult.

SIZING RECOMMENDATIONS.
If vehicle is stock or if you are limiting yourself to bolt-ons', keep to the stock size or limit your pipe diameter to no more than 2" or you will lose some bottom end with no usable gain on top. Naturally aspirate vehicle in the 150-200 hp range should use pipe diameter between 2 to 2.25 ince s.Turbo's and V6's in the 200-250 hp
range can go for 2.25 - 2.5 inches. For engines making 250-350hp, a generally acceptable pipe diameter of between 3 to 3.5 inches is quite enough already .Extremists using 400 to 500 hp should limit the pipe size at 4 inches and no more or you may unexpectedly find a small dead dog sleeping in your exhaust one day.

ANCILLARIES.
The entire exhaust system should be connected to the underbody via insert connections and flanges with flexible suspension elements. Flanges and insert connections are preferred instead of welds to enhanced flow dynamics. The welds reduced the pipes diameter on the insides and affect the smooth exhaust flow.Avoid the typical crush bends and instead opt for mandrel bends which is more linear and do not alter the pipe's diameter. The fixing points must be carefully selected, otherwise vibration can be transmitted to the bodywork
and thus generate noise in the passenger compartment. Ideally use heavy duty rubber mounts with a generous standoff from the bodywork. The exhaust-system noise at the exhaust-emission point ( tailpipe ) may also cause bodywork resonances. A larger diameter designer tailpipe that adds on to the muffler tips is recommended to give the vehicle that race finish look. The design of the tail pipe and length of the muffler end will also affect the sound of the overall exhaust system. For twin exhaust system , an active lateral pipe joining both sections together after the resonator boxes is recommended to achieve some positive backpressure to help in torque - try this and be surprise.

CONCLUSION.
If you see yourself using nitrous or forced induction in future, go with the larger diameter piping now to save from double spending. If you plan on lowering your ride , make sure your new exhaust system won't cause any clearance problems ( beware of the humps ). If budget is a consideration a header will be the first upgrade option which provides the greatest bang for the buck. Next is the free flowing muffler, otherwise operate on your stock one or look for one with a bulit-in valve that allows it to be free flowing under high speeds. The valve type mufflers helps positive feedback at low speeds for acceleration like the latest generation one  design for the  CTR ot ITR's.