Originally Posted by DoubleWasp
Let's get one misconception out of the way. Back pressure does not help an exhaust do anything at all. Zero, zip, zilch.
When testing an exhaust for blockage, anything greater than 1.25 PSI at idle is a fail, and anything greater than 3 psi at 2000 rpm is a fail. Mind you, this is on a factory OEM exhaust with cats, mufflers, and all. A catless exhaust will generate almost 0 psi unless fitted with a grandpa-car "can't hear it running at all" muffler.
There is no specific minimum for back pressure. If an exhaust were to have 0 psi at both speeds it would be considered a pass with flying colors.
Pipes that are too big, or exhausts that are made wrong cost low end power due to a lack of or incorrect exhaust
velocity or scavenging.
The back pressure theory has been tested in a million different ways and is a solid failure every time.
The gains of an OEM dual exhaust system over a single is simple. Take an OEM muffler. Designed to flow as much as it can while maintaining a certain sound level. Let's just use some arbitrary numbers here and say that "OEM Muffler X" flows 400cfm and emits 69 decibels. "OEM Muffler Y" flows 500 cfm and emits 75 decibels. Let's say the target noise level is 70 decibels. What's an engineer to do? Just split the exhaust, put two of the "X" mufflers in, and now you still have 69 decibels (or maybe less) and 800 cfm flow.
Of course, sometimes the OEM will decide that their "aggressive" dual exhaust should also sound different, and will be louder despite being a dual. Complete style issue there.
But anyone in the know is aware of the fact that a single exhaust with a big single muffler makes the most power. This is mostly due to the Dynamics of combining dual banks without splitting the exhaust again. Let's call this "OEM muffler Z". Why use two "X" mufflers rather than a single "Z"? Packaging and styling.
Thank you! I was waiting for someone to post this!
Also, in regards to different mufflers, here's some OEM ones from a Charger. One is an R/T or v6, the other from an SRT. SRT is a lot more open and free flowing while being louder.