Small Block & Big Block Chevy Specific - ehxaust gas crossover block

in a set of sbc heads there are chambers in the center two exhust ports that go up to the intake, there is a chemical or putty that blocks them off, ne ideas.

the first pic is the ports in the heads, im sure you guys know what im talkin about. and the second pic is where it meets the intake. apparently this is an old school way to gain some horse and helps with exhaust flow. by filling this passage way in the head and not just blockin it off by using an intake that doesnt have this passage is what helps the flow. if ne one has ne idea of what the chemical or putty is, it would help, thanks