Actually the exhaust flows both ways, when the exhaust valve opens on the cylinder that the port is connected to on the right side a small amount of exhaust crosses through the intake passage to the left head and out, when this event happens on the left side cylinder the flow is reversed and goes to the right side. This exhaust passing through the intake heats it up to help prevent icing in cold climates, In warm climates and performance apps it's better to have these passages blocked off.
I take two small pieces of tin and trim them to fit in the gasket openings on each head, add a dab of sealer to hold them in place then install the intake trapping the pieces of tin in the ports.

 

I know this is a very old thread but does anyone know where I could get this plate? Im rebuilding a 390 and have a 4bbl manifold with the egr hole under the carb.

 

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