The exhaust manifolds found in the 1967-1987 Chevrolet Pontiac Buick and Oldsmobile engines were designed to combine the exhaust gasses from each of the cylinders quickly with a manifold that was cost effective to produce. Our 4-1 Mid-Length Style off-road headers are designed to allow the exhaust gasses to flow more freely from each 4 cylinder bank and combine in a more efficient manner. Tubing has been optimized for the Small Block 265/267/283/302/305/307/327/350/400 cubic inch engines. This creates a reduction in cylinder backpressure and improved exhaust gas velocity. These improvements free up lost horsepower and torque. An added benefit is a more performance oriented exhaust sound. Replace for Chevy 265-400 Cu. In. Small Block GEN I V8