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port 2 on the distributor is vacuum retard. it pulls back the timing at idle and deceleration to lower certain exhaust emissions.
there is no "boost retard port"
under boost there is no vacuum anywhere, just air pressure and it saturates both vacuum lines so vacuum advance and vacuum retard cancel each other out or they are equalized on both sides of the diaphram in the vacuum pot so it is in the middle position, not advancing or retarding the timing.
so, without any vacuum advance or vacuum retard going on you only have centrifical advance happening and that maxes out around 3000rpms in most distributors.
that said, without total igntion advance (centrifical and vacuum advance together) with boost pressure in the intake system this scenerio is called boost retard.
hope that makes sense to you.
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