My car clearly has an issue with KR, too much backpressure it seems, plenty of fuel, just at the 1-2 and 2-3 shifts it goes from 0 to 20*. So therefore I'd say it seems there is a lot of exhaust buildup when the RPMs go from 5300 or whatever to whatever it is after it shifts gears.
On startup and if reving, my muffler clearly makes a loud obvoius rattling noise, something is broken in there, but in my experience a muffler looks like this:
I assume the stock muffler on the ville'* is just like this.
I dont see how it would be possible for anything to be broken that would cause problems unless one of the pipes inside there is broken and rattling around.