Put a vacuum gauge inside the car -- you'll see quickly that any significant throttle opening (one that results in noticeable acceleration) is accompanied by a related and proportional drop in vacuum. It has to -- it's atmospheric pressure that actually fills the cylinder with air. The beauty of using vacuum to assist with timing control is that it offers up a great signal that is proportional throttle opening and load.

These 4/5/6/8/10/12 cylinder engines don't know if they were made in the USA or Sweden or Germany or Japan. The principles are similar for just about all of them that are operating in a similar rev range - let's say 600 to 6000 rpm just for grins. Running lean saves fuel and reduces CO2 and unburned HC - hurts NOx because of higher combustion temps. Running lean can cause problems UNDER LOAD, but at very reduced load - idle, near-no-throttle cruise - lean doesn't hurt a thing. And, of course, it saves fuel. General rule of thumb -- something in the order of 14-18 degrees BTDC static ignition advance at idle; add another 15-25 degrees before via the vacuum advance (which will only be there under light load); centrifugal advance adds in another 16-20 degrees advance (preferably all in by 3000 rpm - most stock springs don't allow full advance until well past 4000-4500) for total advance when accelerating hard of 32-36 degrees. This almost always result in great performance and responsiveness and the best fuel economy you can get with that package.

Most recently I put all this to work on this....1.8L pushrod 4 cylinder; twin sidedrafts. Initial timing at 18 BTDC; centrifugal adds in another 16 degrees for a total of 34 degrees -- centrifugal is all in at 2900 rpm. My vacuum advance canister is adjusted to add another 15 degrees when vacuum is present. The details of these changes and how I made them are documented here. https://puristmotorsports.com/abeja-roja-the-red-bee-1973-toyota-corolla/ignition-timing-and-valve-adjustment/ This motor very similar to the B18/B20 Volvo engines - main difference being a hemispherical combustion chamber. Usually the hemis can get by with a bit less ignition advance than a wedge.