Advance vs. Advance & Retard 74 2.0 vs 76 2.0, Vaccum Throttle Dizzy ; Advantages? Options

[detoxcowboy]

Ignorant to the advantages of vaccum advance and retard advice needed.

Situation;
My 914 is a stock fuel injection 74 2.0 which uses only vaccum advance off the dizzy to the throttle body (single ported). I also have a throttle body (dual ported) from a 76 2.0. I have correct dizzy's for both years and correct vaccum canisters for both years..

Question;
Is there any advantage to switching from the 74 2.0 advance only set up to the 76 2.0 advance/retard set up? Do I have to also switch the dizzy's??

[pbanders]

This question comes up often, should a FAQ. Geof has the right year/configuration info.

The purpose of the vacuum advance is to improve combustion under low load conditions (e.g. cruising at light, constant throttle on a flat road), improving fuel economy and reducing emissions. It may also help throttle responsiveness. Here is the quote from the Bosch "Ignition" Technical Guide:

"Early (advance) adjustment system: The lower the load, the earlier the air-fuel mixture needs to be ignited since it burns more slowly. The percentage of burned but non-exhausted residual gases in the combustion chamber increases and the mixture becomes leaner. The vacuum for adjustment in the advance direction is taken off at the intake manifold (my note: the diagram shows it being taken just above the throttle plate, so that when the throttle is partially open, it gets maximum intake manifold vacuum - this is where it is positioned on the '73 throttle body). As the engine load decreases, the vacuum in the vacuum advance unit increases, causing the diaphragm and vacuum advance arm to move to the right. The vacuum advance arm turns the movable breaker plate in the opposite direction of the rotation of the distributor shaft. The ignition point is advanced further, i.e. in the "early" direction."