How Exhaust Gas Flows best!
The hotter the gas is, the faster it will flow through pipework.
Hot gas is much less dense than colder gas and takes up a lot more space inside the pipework.
It is important to keep this gas speed as high as possible, through proper exhaust size selection. The faster it is able to escape the exhaust system, the less work the engine has to do to pump it out.
Based on these facts, the perfect exhaust will start off large because the gas takes up a considerable amount of ‘space’ when it is hot, and will effectively shrink as it cools and passes through the exhaust.
So dropping the size down towards the back promotes high flow.
This type of exhaust tuning is used heavily in the higher end of production performance vehicles, but not to often on mainstream or diesel offerings.
Overly large exhaust systems really don’t offer any benefit to engine performance and on occasions can actually hinder it.
Once you get to a point where there is zero psi of back pressure after the turbocharger, the exhaust has done its job.
If you can achieve this with a 2.5″ system, you will not gain anything going up to a 3″ or 4″ system on the same vehicle.
The difference in what the vehicle sounds like however can be significantly altered by using a larger than necessary system.