Designing a exhaust system is not just a question of a taking bit of tube and bolting a silencer on to it.
A typical family hatch car system will incorporate a de-coupler to isolate engine motion from the static section of the exhaust, 5-6 rubber hangers to take up thermal expansion and reduce engine noise in the exhaust gas flow from being transmitted into the body structure. Then at least two boxes - a resonator to tune out undesirable frequencies and a silencer to reduce overall noise. The whole system will be tuned to give an engine sound which is acceptable to the customer - sporty for the GT version, or quietly refined for the Luxury version. It will also be tuned to reduce back pressure and improve performance. Finally, the system has to be manufacturable and be easy to assembly. It must weigh nothing, last for ever and cost peanuts.
And thats before you even start thinking about a catalytic converter and zero-lead fuels.