This dual-plane was designed for 283-400 ci small-block Chevrolet engines using the new vortec cylinder heads P/N 12464298 or 12558060. This high rise manifold was designed for maximum horsepower and a broad torque curve to give you great performance. This manifold accepts square bore carburetor and includes extremely plumbed hot water crossover passage. Vortec heads were released on 1996 RPO L31 truck engines. This manifold only has four bolts attaching it to the cylinder head instead of six like all earlier models. Check for proper to carburetor clearance, especially on corvettes. Use manifold gasket P/N 12529094 and attachment bolts P/N 24504713.