Ballast resistor with EI conversion

Oops, meant ditch the resistor

Here is a good explanation of why EI doesn't need the ballast resistor - the 'coil on' time is constant regardless of RPM, therefore the coil doesn't get too much current at low RPMs:


From this Link


One of the many compromises with contact ignition is the fact that the coils saturation time will reduce with increasing engine speed. In the illustration shown in Fig 1.2, the engine is running at approximately 1000 rpm and the points are closed for 16.3 milliseconds. This results in an induced voltage of 286.3 Volts. As the engine speed is increased to 3000 rpm the coils available time to fully ‘saturate’ will be reduced pro-rata. Illustration Fig 1.3 shows that the time available to charge the coil has now been reduced to 5.6 milliseconds. As a result, the induced voltage has been reduced to 275.4 Volts and the coils HT output reduced accordingly.

primary ignition - figure 1.3
Figure 1.3

The dwell period on an engine with electronic ignition is controlled by the current limiting circuit within the amplifier or Electronic Control Module (ECM). The dwell on a variable dwell or constant energy system will be seen to expand as the engine speed increases, compensating for the shorter time period.

The term ‘constant energy’ refers to the available voltage produced by the coil. This, regardless of engine speed, will remain constant as opposed to contact ignition where an increase in engine speed means the contacts are closed for a shorter time period. The coils saturation time can be seen in Fig 1.4, where the time available to saturate the coil is a constant 3.0 milliseconds regardless of the engine speed. The saturation time is considerably lower than that of a contact system due to the coils supply voltage being approximately double that of a ballasted contact system and the coils primary resistance approximately halved. This will result in a far higher current, saturating the coil with amperage that would not be possible on a contact system.


Keep in mind that not all EI systems are designed with the same robust semiconductors, which is why some are more prone to failure. You get what you pay for!

Hollywood wrote: We are only talking .8 ohm resistors typically. I'm not sure I have a strong enough electrical understanding to know what .8 ohms does to 12-14V. Perhaps it is impossible to know without knowing the current. I can't seem to figure it out. I must measure my ignition circuit, this is really bothering me.

Wood simple Voltage Divider Rule.

It all depends on the resistance of coil. Sure 0.8 ohms is not much, but if the coil has a resistance of 0.8 ohms. It will only have 6V volts applied in a 12 volt series circuit.

Good question. That's where it goes over my head.

Good call. Edited.

Is your MSD coil epoxy filled? I went through 2 oil filled coils I'm assuming because of the mounting position (horizontal rather than vertical) or due to the fact that I didn't have a resistor in place. You can mount the High Vibration/Epoxy filled coil any way you'd like.

From what I read the Prestolite EI kits don't require a resistor, so it would come down to whether or not the coil requires one. The instructions for the MSD High Vib. state that it only requires a resistor if you have points.

I've seen quite a few knowledgeable posts from you on here, so you probably know a good bit more about this subject than I do, but I figured I'd pass along my experience with it all. I will say that the MSD coil is the best $55 I've spent on my boat.

Yeah, I think you're good w/ no resistor. I have the same setup at SNobsessed. Prestolite EI conversion w/ and the same coil you have with no resistor. Runs great. From what I understand the resistor is only needed if you have true points.

The instructions for my Ignitor II EI module say that it can handle the full 12V at all times, therefore no ballast resistor is needed, although it will still work with the ballast resistor in place.

The coil I'm using ( MSD High Vibration Blaster ) says to USE a ballast resistor IF using a points-style distributor.

Well, I have a points style distributor but no points with the EI conversion. Should I wire in the ballast resistor or bypass it? Is the purpose of the resistor to save the coil or the now-gone-points? I've been running it bypassed for several hours of runtime with no problems so far but figured it'd be a good idea to check.