Citroen DS3 cat back resonated exhaust (quieter).
The least restrictive exhaust system allows hot gasses from the cylinders to exit the car quickly. Since gasses can be slowed by turbulence the faster you can support them to travel the least restriction you will have. Large diameter bore size can also help with this as can the amount of packing in the rear muffler (which is effectively keeping the noise levels down). A catalytic convertor has a number of cells typically from 400 to 600, the gas passes through the hot cells and this creates a chemical reaction whilst helping to ignite the gas once more. As the gas passes through the cells it naturally becomes slowed and is why you may of read about sportscat exhaust which has 100 to 200 cells in them. This reduces the restriction further whilst a remap is required so that the ecu understands the emission levels are different than what is expected on its standard factory map. A decat however which also requires a ecu tune so that fuelling is correct has no catalytic convertor and is used for off road, track and rally events allowing minimal restrictions. The remaining restriction mainly coming from the turns in which the downpipe has to route itself so to bolt on to the turbo and connect to the cat back down the cars exhaust tunnel.
Scorpion cat back exhaust.
Citroen DS3 Performance
Citroen DS3 156hp
Citroen DS3 Racing