I've verified both vacuum timing schemes: ported and manifold.
GM HEI distributors from late '70s till computer control took over utilized manifold vacuum advance. You had full advance until you got into the throttle, then as manifold vacuum dropped off, the timing retarded, decreasing the chances of spark knock. Verified with a timing light on a 1977 Chevy 1/2 ton pickup with 305 small block.
Earlier, before emissions became a concern, most manufacturers used ported vaccum (as used by Excaliber here). Increasing airflow through the carb causes increasing ported vacuum via the venturi effect, slowly pulling in more & more vacuum advance. Verified with a timing light on an '81 Jeep 258 six and a '66 Mustang 289. Sort of tough to do, because it CANNOT be checked with the vehicle sitting still and opening the throttle. The engine MUST be under load to create the necessary airflow in the carb venturis required to create ported vacuum.
For one of the best descriptions of various timing schemes (including switching from ported to manifold vacuum, delaying advance, etc.), try to find a Petersen's book on carburetion & ignition from back in the mid 70's. You'd be amazed at some of the methods that were used in the pre-computer days.
Oh, and regarding German fighters... Would water/methanol injection be justified in a Cobra if you had a BMW on your tail?
