2 hours ago, human-potato_hybrid said:I think I understand how a dual point distributor works but it's not obvious to me why a single point distributor can't have the same functionality in terms of dwell timing.
As RPM increases, or the number of cylinders increases, eventually you run out of time to charge the coil in between firings, and weak spark or no spark is the result.
As the dwell gets higher, the points gap becomes less. At some point it becomes impractical to go any narrower with the gap. Dual points (as commonly used in the postwar era with one coil and the same number of lobes as cylinders) extend the possible dwell by using one set of points to do the opening and the other set to do the closing.
Dual points with two coils gets confusing because there is more than one way to do it. In a big picture view, if you fire every other cylinder with the opposite coil you gain a bunch of coil charging time (dwell) because you are firing half as often.
Imagine if you will that you can make a points distributor for an 8 cylinder reliable at 5000 crankshaft RPM. You can, by the way, but it's fiddly and most 8 cylinder distributors never had to deal with speeds like that. The implication is if it were a four cylinder distributor you could turn 10,000 crankshaft rpm. If it were 16 cylinders, maybe you could turn 2500 crankshaft RPM. More cylinders and or high engine speeds make it more likely you see some exotic solution like dual points, or dual points with dual coils, or dual magnetos, etc. As m-mman mentioned, sometimes it was done when not strictly necessary for the number of cylinders or rpm involved.