These Tri-Y design headers fit vehicles with manual or automatic (except C6) transmissions. Will not fit models with power steering or air injection heads. Doug's headers have a metallic ceramic thermal barrier coating (MCTBC) which is simply the best header finish on the market. MCTBC protects and extends the life of the headers by creating a thermal barrier that significantly lowers under hood temperatures and protects the substrate metal from thermal fatigue. Tube size: 1-5/8 inches to 2 inches. Collector size: 2-1/2 inches.