Heh. It was used with certain cleaner air systems (CAS) in the early 70s.
This MTSC says your 400 was the engine that would get it in '73.
http://www.imperialclub.org/Repair/Lit/Master/302/page06.htmNow that one says the purpose is easier starting.
I don't know if that's a completely accurate explanation. Dodge FSM shows 10* BTC and I don't find any TSBs that change that.
10* BTC should not be a problem starting.
1971 MTSC shows a similar (or same?) vacuum adavance for 383 and 440s.
It disconnects the vacuum advance on deceleration.
http://www.imperialclub.org/Repair/Lit/Master/287/Page11.htmThat explanation makes sense. When the throttles are closed, manifold vacuum (under the throttles) pulls the fuel. Restrictions in the passages are set to provide an air-fuel mixture at idle speed. But on deceleration, the throttles are closed at rpms above idle. The manifold vacuum will be higher than at idle and mixture richer. Richer mixtures take less time to burn so vacuum advance is not needed on decel.
This is often a non-issue because the vac advance was often connected to a manifold vacuum port at or just above the throttle blades. So vacuum advance wasn't added at idle,
My guess is the 383 and 440 (and 400?) engines either use manifold vacuum, or have the blades right at the vacuum port. The latter is most likely. Emissions packages generally ran leaner idle mixtures than their earlier non-smogged conterparts. Also cams sometimes had slightly more exhaust dilution at idle. These factors required higher idle speeds to runs smoothly (compared to their earlier counterparts).
