Hi guys i want to get an after market header and exhaust on my dc2 vti-r but im a little confused with the sizing. The previous owner changed the exhaust to an oem audm ITR exhaust and i was just wondering what size piping are they? Also im not sure if he also changed the cat converter to the ITR one but are they any different? And when ive been reading around most headers have a 2.5" collector so how does that fit to the remaining of the exhaust if say i had a 2.25" exhaust? they are different sizing? and does anyone know why full exhaust systems don't include cat converters? just makes all the sizes confusing to understand.. :S could someone please help me with some answers? Thanks in advance! sorry for all the questions :S