Hi Vernon
A
distributor with vacuum advance is not a full 3D system. A 2D system is where you have just two variables, engine speed and ignition advance. You can plot these against one another in a 2 dimensional graph - the advance curve. This system was modified for most cars with the addition of a vacuum advance which, rather crudely, moved the distrutor plate to advance the ignition in conditions of high vacuum, which means low throttle opening hence low load on the engine and thus no risk of pinking/detonation. The vacuum advance meant a lower static timing advance could be used which makes starting easier, and part throttle running at low revs would be more efficient because more advance could be used without risking excessive advance at high revs and under load which would cause pinking/detonation. It gives a much more economical engine on part throttle, better idlling and easier starting than the non vacuum advance option. A full 3D system uses electronics to vary advance with engine speed and either throttle opening or manifold depression (MAP or vacuum - all the same thing) to deal with different load conditions as well as engine speed. To represent the advance on these systems you need to use a 3D diagram, showing how advance varies with both engine speed and load - a 3D map. With these systems (eg Megajolt) you can plot the relationship of the variables into any complex topography that suits the engine's needs. So with the ford Crossflow in my Westfield we spent a couple of hours on the rolling road modifying the map for a whole range of load and speed conditions to maximise power output at each point - hence such a sweet running and relatively economical engine.
My concern is that with pre electronic
distributor systems the advantages of adding vacuum advance to a 2D
distributor system were well known and not expensive, but it was not done with engines using Webber DCOE carbs or the similar Dellortos. There must have been a good reason which could not be solved by just tapping a vacuum take off from one inlet tract and I believe the reason was excessive pulsing at low speeds. My question for 123 would be will it work with the vacuum off just one inlet tract and if it does not can I have my money back. Strombergs and a number of other carbs did use balance pipes, but again, Webber installations did not so I'm not sure there will not be ill effects if you cross connect the inlet tracts.
Last point. The Dellortos on my Westfields Crossflow have the sort of tappings shown in Simon's diagram and they are used for balancing as his diagram shows. But they are not the air bleed screws, which the Dellortos also have, these are different drillings and are for balancing between the two chokes of a single carb, they bleed air into the idle mixture I think.
Finally, just to say thanks for raising this, I'm very keen to hear both people's views and the outcome
Vernon(2)