Goon has a point... IF the OEM exhaust is truly "tuned." Straight exhaust systems do a good job in clearing exhaust gases from the cylinder, but their vacuuming effect can be a mixed blessing because they don't know when to stop vacuuming and can pull a sizable portion of the fresh charge right out of the cylinder. And since horsepower is a direct function of the air/fuel mass trapped in the cylinder at the onset of the compression stroke, this aspect of a straight exhaust system can have a negative impact on power. Here are some excerpts from the Wikipedia entry for "tuned exhaust:
"A tuned exhaust system is an exhaust system for an internal combustion engine which improves its efficiency by using precise geometry to reflect the pressure waves from the exhaust valve or port back to the valve or port at a particular time in the cycle...In a four-stroke engine, tuned extractor manifolds are used to promote scavenging of the exhaust gases by delivering a pulse of negative pressure just before the exhaust valve closes...the efficiency of the system is greatest at a particular engine speed, which is determined by the exhaust system geometry. Tuning the system for best effect is typically done both by calculation during the design of the exhaust system, and by trial and error during engine development. An important recent development is variable geometry exhaust systems, both for two-stroke and four-stroke engines, which allow the tuning to be effective over a wider range of engine speeds."
There's an interesting animation in the Wikipedia entry that illustrates the tuned exhaust principle: http://en.wikipedia.org/wiki/Tuned_exhaust
That said, you would still need to convince me that the S turns in the OEM exhaust have more to do with tuning than with muffling the sound. Like any exhaust system, the OEM system is a compromise/tradeoff between various interests including 1) the desire to maximize horsepower while 2) muffling the sound. An aftermarket exhaust system like RPI that is less concerned about muffling sound and more concerned with maximizing horsepower (and reducing weight) might do so by removing the S turn restrictions inside the muffler and allow freer flow of exhaust gases. The same can be said for any aftermarket exhaust system including Eisenmann, Stromung, etc. They all tend to be louder because they are less restrictive than OEM.
