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Quoted " Some say the oil filled canister coils are superior because the cooling oil keeps the temperature and internal resistance stable especially with a CDI ignition, which makes for a more reliable coil. The drawback is that the windings are not epoxy encased, so they are more prone to failure. This design also extremely old and not efficient by design.Some say that the E-Core coils are superior in spark output because they are a more efficient design. The drawback that I see here is that epoxy is an awful heat conductor, and with a high output ignition, the windings are going to get hot and the internal resistance will go up. The heat could make the coil more failure prone or put more strain on the ignition box if the resistance gets high enough.One thing I noticed was the E-Core coils had a lot less resistance than their canister counter parts, which to me means that they would generate less heat.
The lower resistance not only means less heat it also means that the coil can charge faster as well, which is benificial at higher rpm. "
Hope that helps you a little more as it covers both sides of the street.