average coil current. I see the module as being a switch with no intended impedance. So the dwell or on time is longer on the MSD module. Other than that, I don't see how a switch can force a coil to take more current than it's impedance dictates.
True, you can't violate Ohm's law, but coils are a combination of R and L (resistance and inductance), and most coils are built with low winding resistance, and depend on the inductance and dwell time/angle to keep the current from spiking too high. If the MSD module is, for example, expecting to drive the coil to 8 amps (typical with many newer coils) then the module (if a smart module like the original HEI) will keep increasing the dwell time (or dwell angle if using a dwellmeter instead of an oscilloscope) until the coil hits 8 amps (as long as the coil has less than 1.5 Ohms resistance, then 8 amps is possible from a 12 volt source). What's the coil's instantaneous wattage at 8 amps? Remember P = I x V= 8 x 12= 96 watts. Small primary wires might fuse at this instantaneous current and wattage heating. The only saving grace here is that the coil might only need 20 degrees dwell to reach 8 amps, so after the coil/plugs fire the coil has a bunch of time (45-20= 25 distributor degrees) to cool down before it is turned on again. Is the 25 degree non-dwell time long enough for sufficient cooling? Depends on the coil and the ambient temperature.
Now, say the coil resistance is 1.75 ohms. With 12 volts (lets ignore the actual alternator voltage again for easier math) the dwell current would max out at 12/1.75 = 6.85 amps. The module would however be looking for 8 amps so it would keep increasing the dwell time/angle in an attempt to give the coil more time to ramp up to 8 amps. However, all that's going to happen is the coil gets turned on for max dwell (about 40 degrees for an eight cylinder) and sits there cooking at P = 6.85 x 12 = 82 watts. This is less instantaneous wattage than the earlier example, but this coil only gets to cool down for 5 dwell degrees before it's turned back on. This coil will tend to get hot and stay hot, which is not a good recipe for durability. Different combinations result in different instantaneous and average wattage numbers.
So essentially it's the combination that determines the performance and reliability, and not just current levels or dwell time/angle numbers. Without knowing the specs of the coil and the module it's just a crap shoot if the system puts out more energy than stock, or if it will survive.
Short and sweet. Hope it made sense.
