Come on man, never seen the Y pipe? Don't assume... I've had it off the truck 4 times this year because of transmission problems and replacing the exhaust system. But, I didn't expect you to know that. Yes, the parts of the Y pipe are smaller in diameter, but only three cylinders are dumping into each. Oops, I already said that. Yes, a true cat back (almost none worth mentioning exist) with larger diameter pipes mandrel bent could be better, but why haven't many of the manufacturers fabbed something all the way to the cats? It's possible, that the gains were negligible. Maybe it decreased performance because of too little back pressure. It's all speculation because none of it has been documented. I just know that increasing the diameter after a restriction, is going to allow the air to move faster through the restriction. I'm not trying to start a war here, just pointing some stuff out...