Ponz: Nice explanation. I couldn't see the vacuum working that way.
I especially enjoy the part where I describe the cylinder as going down. Cylinders don't go down, pistons do. Oh well.
As the ported vs non-ported vacuum article explains, the lower the initial timing, the hotter the engine becomes. With the ignition occurring close to top dead center, the combustion continues long after the piston is travelling back down.
The 351w in my 69 Mach 1 always knocked due to pre-detonation even under moderate acceleration. Conventional wisdom dictated I should retard the initial timing. Before adjusting the timing, it was set to 9*. So I warmed up the car, and proceeded to lower the initial timing one degree until I was at 6* BTC. Pre-detonation was still slightly occurring. And the engine was not idling smoothly at all. I was curious at what setting the engine would idle best, so I turned the distributor cap, advancing the timing until the engine was running oh so smooth. It turned out to be around 15*. Well, I knew that would never work, as the engine would knock itself to death when accelerating. I ended up cleaning the distributor which had a gummy/sticky substance inside and on the fly weights; removed most of the lash from the heavy fly weight spring, and connected the vacuum advance to the full time vacuum, which yielded 13* of timing at idle. Using full time vacuum for spark advance does not affect the rate of advance under acceleration because as soon as the driver mashes on the gas pedal and opens the throttle plate in the carb, the amount of vacuum provided by the ported and non-ported ports in nearly the same. After several hard acceleration runs, (adjusting the play in the fly weight spring), I got the rate of advance adjusted just this side of pre-detonation. The engine idles super smooth, steps out with face tightening acceleration, and runs much cooler. How much cooler? Before the adjustments, the temperature gauge was sitting a little below half way. After the adjustments, it moved a good 3/8” to the left.