vielside,

I'm a bit new to all of this as well, but I think I can come up with an answer for you.

Anyone out there, please correct me if I'm wrong.

What it comes down to is allowing the engine to "breathe" easier, provide better throttle response, along a tad bit more horsepower and fuel economy.

Most exhaust systems are designed around deadening noise, with little regard for airflow. This adds something called "Back Pressure", making it more difficult for your engine to empty it's cylinders on the exhaust stroke. The engine works harder, making it less efficient.

Most performance exhaust systems balance noise reduction, with airflow. Less back pressure providing a more efficiently operating engine, but with a slightly higher noise level.

Regards
Spence

Still no Xterra (hopefully this week).