All carburetor small block factory intake manifolds have the exhaust passage. That passage runs from one head, under the carburetor, to the other head. As HRL noted, it works in conjunction with the exhaust manifold heat riser to heat the intake manifold plenum floor. Depending on year, model, application, the intake manifold gaskets have a changeable restrictor that will tune the amount of exhaust that is allowed to pass through. New intake manifold gasket sets typically include the restrictor and some instructions on the application of the restrictor plate.
