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Strange SBC heads?

I have a pair of heads off my sbc specs are: 1968-69 3998991 307 1.72 1.5

one of the center exh valves vents (or connects) to the intake manifold.

is that a preheater or something? (I have only had BBF and SBF’s apart before, i’m clueless on the SBC stuff).

That’s the exhaust cross-over which heats the center of the intake manifold to provide carburetor heat…SBF’s and BBF’s have them too…The flow is controlled by a “heat riser valve” located at the right exhaust manifold / exhaust pipe joint…

For performance applications, you can block that passage off. But that will make winter warm-ups long and tedious…

I figure it was something like that. Thanks!