Maybe you could tell me why....Japanese Companies make Headers with 2.5" collector, but they make exhaust piping 2.25"? To preserve torque or what? Wouldnt a 2.5" collector be better matched up with 2.5" exhaust piping?

Posted twice, because the ITR fruits are crying.

 

Not always. One thing to consider is that the restrictions farther back in the exhaust present less of a problem than to the front. As the exhaust moves toward the rear, it is cooling down and therefore is more dense, needing less volume. It is usually a matter of tuning and, of course, economics. If they don't need to use the larger piping, they won't.

 

Thanks, makes sense.