Like you said, the vacuum gauge can tell us "what" the motor is doing, but it can't "make" the motor do anything.
My pre 1972 high compression 440 and 413 engines give best mpg and drivability (off idle and part throttle response) and best full throttle power with ported advance.
I set the initial advance down low @ about 5 btdc, then reduce (as needed for pinging) the available ported vacuum advance at the can, then delay as needed (for pinging) the centrifugal advance, then "increase" the available centrifugal advance to about 30 degrees (as tolerated) and arrive at about 35 degrees total @3500.
All because of the very poor 93 gas that is available here. Winter to spring season changeover is the worst leftover blend here, it's the pits I tell you.
Idle speed is set at about 700 rpm in gear with the a/c on so that the radiator and a/c condenser get plenty of air flow in repeated traffic stop light conditions. Idle air ratio is set just a touch rich to cover up the hole when the choke opens early, and the engine is still a little cold. So, the hot idle is a touch richer than necessary for a hot engine, but you wouldn't know it to look at it and it doesn't affect idle speed, idle quality or driving performance at all. But the nose knows.
And the vacuum gauge says whatever it says.
What you have pointed out is how you compensated for the fuels in your area by tuning the advance curve to allow for such. Additionally the need for LESS advance to decrease pinging on acceleration. Whereas all of the "drag racing"-oriented advance curves are about MORE advance SOONER and a bit more power from that. These distinctions ARE important with modern, lower Research Octane fuels!
I also understand your orientations on the idle mixture and idle speed changes from OEM stock specs. My experiences in the Texas summers, in the later 1960s, as supported by our service station guy who also drove mid-60s Fury IIIs with factory a/c, was that a Chrysler cooling system if in good working condition did not have any over-heat issues as similar GM vehicles could. So hot idle speeds were set to 550 in D with the a/c running and no issues as to cooling performance for the engine or the factory a/c, even in the slightly thinner air of Lubbock, TX compared to DFW.
For the difference in how a 10.0CR 383's distributor calibration differs from a 9.2CR 383 2bbl, look at the 1965 distributor specs. In 1965, the new-for-1966 256/260 "standard cam" for the 4bbls had not yet appeared, so the '65 383 4bbl still used the 252/252 cam as the 2bbl did and had single exhaust. Which can make that comparison valid as to CR. This was when normal Regular-grade fuel was usually 93-95 Research Octane and Premiums were 98-100 Research Octane. In modern definitions, that would relate to 89-91 Pump Octane for Regular and 94-96 Pump Octane for Super Unleaded. At sea level or thereabouts, as octane numbers decrease as altitude increases, due to the thinner atmosphere of the higher altitudes.
Personally, I'm not sure the slightly higher idle speed or richer idle mixtures help to the degree they are perceived to help. Considering the higher idle speed would not offer better cooling to a significant degree as it also loads the engine harder against the torque converter, which to me, might generate more heat in the cooling system as the richer mixture seeks to cool things down.
On our '66 Newport 2bbl, I also played around with the way the air cleaner snorkel was pointed. Pointing it out of the hot air from the radiator (and any "ram air" affects) did not help hot idle characteristics or power, so I put it back "factory", with the orientation that the hotter air would result in better atomization of the fuel for better overall results. FWIW
In our younger (generally) years, we can tend to get the idea that "Those factory engineers didn't know how to get the best power from their engines, BUT I DO" as they follow a magazine article on how to allegedly do that. Back then, that meant lighter springs to get the centrifugal advance "all in" by 2500 rpm (rather than 4500rpm) and more initial timing such that a full 38-40 degrees BTDC was reached. No body really mentioned the additional need of higher-octane fuels to do that, though. The magazine article usually included some sort of chassis dyno power figures to support what they had done, implying that anybody could do the same. In some cases, the article advocated for "no vac advance" and "full mechanical advance only, but with a quicker advance curve". Sounded good and made a bit more power on the chassis dyno, but NOBODY mentioned a loss in cruise fuel economy. The quicker curve could offer a bit better throttle response in normal driving (compared to the "emimssions-oriented" retarded/delayed advance curves), so all was well. All that mattered was what happened when the throttle was opened. I know that one clue the vac advance on our '66 Newport's vac advance "had gone" was a mpg drop to 10mpg from a normal 14-15mpg, with no significant loss in drivability.
The reason I keep mentioning the advance curve on the '66 Newport 383 2bbl is that it hits the generalized "number" at to total advance with the spec 12.5 degrees BTDC base timing. Vac and mechanical seem to compliment each other, too. Only thing might be that it could be a bit quicker than all-in by 4500rpm (which is the approximate horsepower peak). From NEW, that car did not like Regular fuel like it should have. It clattered. The fix? We used Premium fuel in it from then on. Which later allowed me to advance the base timing to 15 degrees BTDC with no problems. It also ran better on Phillips 66 Flite-Fuel premium gas, which had a pump octane of 95.5 (101 Research) in 1974. The same fuel in DFW was rated at 95 Pump Octane and didn't offer any performance advantages down here.
Sorry for the length. Just my experiences and observations over about 50 years of fun.
CBODY67
