What are the advantages of getting a 2 barrel Intake manifold for a Weber 38 vs getting an adapter plate?

(1) two barrel intake will flow better through the opening for the carb
(2) hood clearance (may or may not be an issue depending on your application)

You can also modify a one barrel intake to be a two barrel intake.

I agree with Reed. The 1bbl intake will probably work okay, but the 2bbl intake will be less restrictive.

And, as Reed mentioned, hood clearance could be an issue.

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