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Originally Posted by L8*Apex
This is wrong. 1960's engine theory does not apply here. An engine is an air pump. More in more out more power.
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The (simplistic) analogy to an air pump is the true 1960's reference.
Two engines sucking in the exact same amount of air. One changes ignition point based on rpm (essentially VANOS), and the other doesn't. One generates more HP than the other.
There's so many little tweaks a modern engine can make... to call it simply an air pump is naive.
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Originally Posted by http://en.wikipedia.org/wiki/Aftermarket_exhaust_parts
Free-flowing exhaust systems may maximize peak horsepower; this is typically accomplished by making the path as straight as possible, using a consistent diameter throughout the system and eliminating obstacles such as catalytic converters and pre-catalytic converters or by replacing the muffler with a less restrictive (and usually louder) one. In a naturally aspirated engine, this may result in the use of oversized piping which may negatively affect low-end torque (forced induction engines are not as susceptible to this side effect). Furthermore, despite the potential (albeit generally slight) performance increases possible by eliminating catalytic converters and other emissions control hardware, doing so will often render the vehicle illegal for street use in many jurisdictions. Other aftermarket modifications that bypass the catalytic converters and/or mufflers, such as electric or cable activated exhaust cutouts—which open and reroute the exhaust flow to bypass restrictions in the system—can similarly increase performance, though would likely be illegal in areas where tampering with emissions devices is prohibited, particularly when placed in front of the catalytic converters.
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Show me your source.