Wish I still had the link to the page , but years ago an engineer for an auto company explained about exhaust system design.
IIRC the exhaust travels downstream in pulses. As each pulse moves it creates a scavenging effect ( a vacuum behind the pulse) which helps pull the next pulse behind it.
If you look at a set of headers , each tube is a different length on "tuned" headers.This is so the pulses travel in an orderly fashion down the exhaust.
In a manifold type exhaust the ports in the manifolds are different sizes (diameters) to get the same effect.
A hole in the pipe after the headers or manifold reduces the scavenging effect , making the system, well , pile up on itself.One pulse slamming into part of another.
Hope this makes sense.
Archimedes screwed us !
I knew a blonde dyslexic agnostic insomniac - she'd lay awake at night, wondering if there really was a Dog.
2003 Ranger Edge 3.0 manual 2WD