09-10-2013, 10:23 PM
I think the perfect exhaust would probably start at 3" (turbo end) and gradually come down to about 2.25" at the exit as this would take into account the contraction of the gasses as they cool down and help maintain a constant gas flow speed. This would help avoid any resonant 'booming' where gasses expand and slow down within the system. Obviously in the real world this would need to be done in steps though which isn't ideal.
In reality, assuming the rest of the system is free flowing and doesn't have a standard baffled backbox, the most important part of the system is the very first turn where the gasses exit the turbo as this is when they are hottest (most expanded) and need as smooth and easy a path as possible to enter the exhaust system without introducing further turbulence (and also maximise turbo efficiency). If this bend has a smooth radius and is around 3" in diameter with a smooth transition to the next section it's unlikely that the bore size of the rest of the system will make a noticeable difference to on the road power, particularly on a Peugeot of this age as the exhaust has such a smooth route from downpipe to tailpipe with no tight bends necessary. Unless they were starting to push closer to 300bhp a 3" pipe isn't necessary.
That said a 3" system always looks really impressive under a car!
In reality, assuming the rest of the system is free flowing and doesn't have a standard baffled backbox, the most important part of the system is the very first turn where the gasses exit the turbo as this is when they are hottest (most expanded) and need as smooth and easy a path as possible to enter the exhaust system without introducing further turbulence (and also maximise turbo efficiency). If this bend has a smooth radius and is around 3" in diameter with a smooth transition to the next section it's unlikely that the bore size of the rest of the system will make a noticeable difference to on the road power, particularly on a Peugeot of this age as the exhaust has such a smooth route from downpipe to tailpipe with no tight bends necessary. Unless they were starting to push closer to 300bhp a 3" pipe isn't necessary.
That said a 3" system always looks really impressive under a car!