On distributor curves, the mechanical advance is based on the best power under load without detonation, this basic idea goes way back as far as the moveable advance system whether it was a level on the steering column or a centrifugal advance mechanism. The Windsor V8s can take 36-38° total mechanical advance at around 5000 rpm, mechanical advance is 2 stage, a quick initial, up to around 3000 rpm for a street engine advance of 28-32°, the remainder coming in slowly to max rpm.
The vacuum advance came later as an economy measure and it makes up the difference between under load and light load conditions. There are two main styles, a vacuum brake system used by Ford on the flathead V8s and 6s with the front of the cam distributor and used by Mallory on their street distributors for years. The second is a vacuum can that has a diaphragm attached to a rod which moves the breaker plate (except on the old Chevy in-line 6, on those the whole distributor moves) this system has either an adjustment screw in side the can, or on older Fords a removable cap under which was a spring, shims and a hollow stop tube inside the spring. These are pretty straight forward, at X inches of vacuum you get Y degrees of advance.
What happens with the EGR is not that you can get more advance, it dilutes the mixture enough that the vacuum advance does not cause detonation. A specific example, the mid 80s Oldsmobile built 307 ci V8, if the EGR wasn't opening, you would get a light throttle spark knock at roughly 35-40 mph in 4th gear lockup, a bit more throttle and it would vanish when the computer added fuel by decreasing the down time on the metering rods on the E4ME primaries.
Now, to further confuse everyone, for a long time, Ford used Holley distributors, with no mechanical advance, only vacuum, called a Loadamatic distributor These had a 2 stage set of springs on the breaker plate, one very light one for the venturii signal, the other a heavier one for the throttle port signal. Depending on the year and application many had a spark control valve on the side of the distributor. This blocked the throttle port so that the very weak venturii signal didn't leak, when the throttle signal was there the valve moved in under the higher vacuum to allow it to the distributor diaphragm to receive the full vacuum. Some carburetors simply used a small check ball in the throttle passage. Because this system ran at maximum advance at cruise conditions, Ford found that it was necessary on some applications to use a second diaphragm on the back of the main one connected directly to manifold vacuum to quickly retard the main system under sudden WOT conditions. A quick way to recognize these systems, the diaphragm "can" is very flat on these as there are no springs inside it.
The next piece came with the emission systems, 1966 in California, and 1968 nationwide. One of the problems with many engines was idle emissions, unburned hydrocarbons and carbon monoxide (CO) also oxides of nitrogen. It was found that if the carburetor idle circuits were at the transfer slot that the idle could be set lean enough in the jetting that the engine could run cleaner. In order to do this, the initial timing needed to be reduced. The other solution was air injection. Each of the big three used a different approach, Ford used two systems, IMCO for IMproved COmbustion and Thermactor or air injection. Both systems used a lazier distributor curve and in some applications a lower initial (static) timing. It also was the death knell for the Loadamatic distributor as it's advance would vary with altitude and to some degree barometric pressure differences. The leaner low speed mixtures also would cause the Loadamatic equipped engines to misfire at low speed cruise due to the advance being fully in.
I hope this isn't too confusing, but hopefully it will help understand how the spark advance systems relate to other systems. All have to work together in harmony for best power and economy.
Bill AKA "LOBO"
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