Evan, Peter, and interested parties---When the vacuum retard is properly
connected, it only retards the ignition advance when the throttle is
closed. This would be at idle, and during deceleration, when Big Brother
says the engine's NOx can be lowered. Properly connected would mean the
vacuum port used is the one right at the throttle plate, between the
intake manifold and the carburetor flange. As the throttle plate is
opened, there is a decreasing vacuum sensed in this line, so the
ignition retard is cancelled, and the distributor's mechanical advance
does its thing.
Using a distributor such as found on the early TR6 that has a vacuum
advance, can mean the timing can be automatically further advanced with
the right conditions. The most advance would be under steady state
throttle positions when high vacuum is being produced, and a line to the
distributor connected to a carburetor port that sees such vacuum. This
type of ignition advance gets greatly reduced when the engine in under
load, and little if any vacuum is sensed in the intake manifold. This is
good, as one does not want too much advance under this condition.
It would be quite a project to convert a distributor that has only
vacuum retard, over to vacuum advance. It is easier done by installing
an electronic ignition that offers an in-cockpit timing control, such as
the MSD.
Generally speaking, performance cams will tolerate much more ignition
advance.
Dick T.
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