That is the air INLET port. The PCV valve on that system pulls the air from the hole in the block where the road draft tube would normally go, from a standpipe that goes up to about the level of the cylinder head to avoid pulling liquid oil into the PCV valve. If you're going to suck air and crankcase vapors OUT of the engine, you've got to put air IN there first. Those systems used a sealed oil fill cap and drew air from the "clean" side of the air filter.

Why does a home HVAC system have a "return air" duct? Would it work if there was no way to get air into the system so it could be heated/cooled? The PCV system works the same way.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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