What size pipe should I use when I build my turbo manifold? It is for a GSR. I figured about 1.75 so I could flatten it a little till I get the oval shape or should I just use 2 & leav it round? Also what thinckness whould I use? sch 40???? Please let me know so I can get this project on the road. I am takeing a short cut though Im buying a used cheap header & use the flange from that instead of spending 60 for the exhaust flange. Do you see a negative in this?