Here's some more info on header tuning:
http://www.hondaprelude.to/forums/sh...ad.php?t=51963
So adding an opening will affect the negative expansion wave but if you put it before/at the cat you will also have the benefit of reducing the back pressure as much as possible. According to grassroots motorsports magazine, each psi of back pressure will reduce engine output by about 2% but will get progressively worse with bigger cams. At what point having another negative pressure wave (bad for peak output) is outweighed by less back pressure is a good question and I have no idea what the answer is!!
