The dyno headers were 1 7/8 , I made mine 2", I tried 2 1/8 but wasn't enough room. So the modeling kit was 2", they don't make 2 1/8. If your going with merge collectors call Coen Eng as theirs are build your own and very easy and nice stuff! I would've used stainless but its way more $$ and more involved to weld, needs back purged.What size/type of header did you use when you dyno’d you’re engine? My plan was 2 1/8 primaries with a welded merge collector. I might be interested in the ice engine works kit.