Ok, I've already read tons of articles on how reduced backpressure is the key and that you need the 2" pipes for <2L engines to keep the waves moving to help scavenge the exhaust. Yes, I got that.

So... the argument is (which I believe) is that if you totally take off your entire exhaust system, you'll have horrible low end torque. So we put a long pipe. But... How long is really necessary? After all, the muffler eventually vents to open air where the velocity suddenly must slow down due to the sudden increase in volume.

What I'm getting at, is if the muffler is at about the point where the pipe is long enough, then it doesn't matter that you have 3", 4", 6", or 12" outlet on a 1.6L civic **so long as** the muffler inlet is < 2.5" or so. Right?

 

Ok, exhaust theory is a great big subject in of itself.

In general:
* Exhaust diameter should be large enough to evacuate all the exhaust from the engine at the peak of horsepower, while maximizing exhaust velocity, no matter how long the pipe.

* Exhaust length should be long enough to provide the strongest vacuum during vavle overlap where you want your peak torque to be. Shorter length piping will move your peak torque up the powerband, longer piping length moves it farther down the powerband.

Both diameter and length can be tuned depending on what you're after, but you add a whole 'nother wrench in the mix when you start changing exhaust diameter before the end of the pipe. Sometimes a large tipped muffler can act as a reversion chamber or expansion chamber, both of which can play havoc with the sound waves and spacing of said waves in the exhaust. It's that reason that I really despise mufflers that have a larger outlet than inlet. The muffler is usually short enough that it doesn't adversely affect anything, but it just really isn't necessary to have a tip any bigger than your piping.

I could go farther in depth, but in order to follow all the physics behind exhaust theory, a thermodynamics and fluid dynamics class would be beneficial.

 

in theory sounds good, you are correct, if it's already been throught the muffler and the pipe is fairly lengthy(rear of car) it wouldn't matter what size tip is on it, should only effect the sound,