Modern ignition coils

[boylesg]

What is the optimal frequency and duty cycle for these?

 

[chuckey]

It depends on the driving circuit to a large extent. You have to charge the primary inductance with current , so this has a time constant of L/R. So there is no point in going to a shorter charging period or you won't get the peak current. The duty cycle is determined by the internal temperature rise over the ambient temperature. So its all about the mean power in the coil. In the old fashioned Kettering system (points+capacitor), it was about 30% . With a capacitor discharge system it is likely to be <5%. I suspect the modern point less systems are some where in between, at maximum revs.So at lower revs, same pulse length the duty cycle falls. In the 1990s, VWs (golfs and Passats )were very prone to the coil catching fire. They are a highly stressed piece of kit.
Frank