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?

 

I dunno, maybe something to with emissions.

 

Notice I said "Japanese Companies"

 

Did you mean to post this in the ITR forum?

How is this ITR related?

 

Well maybe because ITR owners have B18C5/R, which most of the headers with 2.5" collectors are for.