The crank turns twice for every one turn of the distributor. And on a V8 distributor cap, there's 45* between the spark plug terminals.
Yup. So far, so good.
How about several semi-obvious explanations?
Sooooo if you have a large centrifugal advance with vacuum advance up to, say 52* total at no-load cruise, that's 26* advance on the cap. But with only 45* between the terminals, the previous cylinder's terminal is only 19* away. Why doesn't the spark jump backwards to the closer terminal?
Just wonderin'. There has to be an embarrassing obvious explanation that I am overlooking.
I'd think that would make you feel a bit better.
1. Depending on the distributor style, the rotor contact and the distributor cap contact are each ~1/4 inch wide (more or less). That provides something like 1/2 inch of rotor tip travel where the rotor is still metal-to-metal aligned with the terminal on the cap (except for rotor-to-cap air gap, of course)
2. At base timing, the rotor is firing "ahead" to the closest distributor cap terminal; at full centrifugal advance it's firing "behind" to the closest distributor cap terminal. In other words, the arc of advance degrees is somewhat centered on the distributor cap terminal, and therefore that terminal is always the closest one. It's not like the rotor is exactly aligned with the distributor cap terminal at base timing, and goes farther and farther away from alignment with additional advance.
3. Centrifugal advance causes the rotor and reluctor (HEI) or point cam to advance in relation to the cap. Rotor moves forward--in the direction of distributor shaft rotation--during advance.
4. Vacuum advance moves the pickup coil or point plate AGAINST distributor shaft rotation, the spark jumps off the rotor earlier (in degrees) than it would without vacuum advance. Therefore, the centrifugal advance tends to move the rotor ahead, but vacuum advance makes the spark jump earlier--the two "kind of" offset each other in terms of rotor position vs. spark timing.
Feel better now???