An E-core coil has an improved magnetic circuit path. The canister coil magnetic path is a rod, so it is like a bar magnet. The E-core wraps around and has a small air gap, so there is less leakage inductance. The E is shape of core, the windings go on the center bar, that is slightly shorter than ends. An I bar closes the ends, the air gap is in the center. The windings, often on a bobbin, are shorter, and fewer turns, less resistance, so less copper losses. So the E-core is more efficient, a higher percentage of energy gets to plug.
E-core coils are not oil filled, they are in a potted plastic housing. Dwell needs to be properly controlled, by time/current limit. An HEI ignition has that built in.
