I wento to: http://www.theevo.com/?p=evolution,7,gsr
a very nice site for Evo's information and read this about my Evo 7:

"Back pressure in the exhaust system has been reduced through the use of a spherical joint for the front exhaust pipe and a straighter exhaust pipe. The fitting of a variable back pressure valve in the main muffler provides better noise reduction at low engine speeds and lower back pressure at high speeds. Resistance to corrosion and strength have been improved with the use of stainless steel in all exhaust pipes. "

I didn't know of a variable back pressure valve... Where it is located? In the rear muffler? So if I put an aftermarket cat-back I lose this valve?

Someone here could help me with this information?

 

turbo cars don't need backpressure. NA cars do for torque. The best exhaust for a turbo car is a straight through pipe.

 

It is used to control noise at low loads.

 

As indicated by Eroishi, the intent is to reduce noise at idle (sitting in traffic) and low load. In all cases, this design increases the pressure drop accross the muffler which affects turbo performance. That 90 degree elbow into the muffler is a mechanical engineer's nightmare if you are looking for lower flow restrictions. The factory tune accounts for the exhaust design.