'71 vs '72 340 Air Cleaners

Dartswinger70 said

I seem to recall the trap door was for emissions or drivability not so much performance is that correct? something about letting warm air in or something? people think it is a performance air cleaner I'm not sure that was the original intent but it is a 1 year only piece right before the strict emissions started showing up.

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I think you are thinking of the plate in the snorkel that closed and caused the engine to draw warm air off the exhaust manifold during warm up (I think?).

The trapdoor was setup to be pulled closed by vacuum and only open when vacuum dropped enough and the spring forced it open, like at full throttle. Pretty sure it was intended as a performance thing, just not sure drawing air right off the hot intake was the best idea.