both. steel with a ceramic lining to keep heat down lighter too. yo the ceramics are steel also and they do rust. any posted hp gains are not definite it varys with each application. i hear dcs,comptechs, and several others are good 4 street use there are some headers best used for serious allmotor power. and if you plan to do an engine swap or turbo any time just forget the header alltogether because it would have to go.