From what I'm reading the dual diaphragm units are a little different than a vac-advance / boost-retard unit.
Dual diaphram: base port uses manifold vacuum to retard the timing at idle ( not pressure )
and the top port uses ported vacuum to advance at throttle...
vac-advance/ boost retard unit: top port uses vac to advance and the bottom port uses boost to retard the timing...
Apparently porche used the vac-advance/ boost retard unit in turbo 911 models around the mid 80's and ford used the same unit on the turbo pinto :lol:
