MSD Coil replacement

MSD Coil replacement

I like a friendly debate too. Your response raised a couple interesting questions about the Mopar ignitions.

Again, we have apples and oranges here. A MSD HEI coil can't be compared to a Blaster II and a Mopar orange box with a ballast resistor delivering only 9V on the primary. A coil can't draw any more then an inductive Mopar ECU can handle.

The ballast resistor is the biggest factor that I'm unsure of here. It does seem like it would do more to limit current at low RPM than at high RPM, and an aftermarket coil could possibly heat it up more from more current draw, making the coil self-defeating until you bring the RPM up. Would it be more accurate to say the coil can't draw more current than the ballast resistor allows?

I agree that an inductive style ignition has a longer duration spark than a CDI, but I wonder if your statement holds true in regards to the box vs the coil. If it did, it would be much more apparent at high rpm vs. low rpm, yet the multiple spark is discharged only below 3K. Mallory even rates their coils on expected rpm use.

It does seem a bit weird that the ignition would have an easier time lighting off the mixture at high RPM than at low RPM, but that often seems to be the case. With no ballast resistor and no dwell - just emptying a capacitor - the spark energy is the same across the rev range, or at least it should be.

ramcharger said
Matt, I also want to say that your input on this subject is greatly appreciated and as you can tell, I love a good debate. :) Great graphs and the MSD handily outperforms the stock HEI coil in both consistency of spark and power delivered. What I found was seen on a Sun oscilloscope way back in the mid 80's with a Accel super coil vs. a stock coil and a dual point distributor. We both know how a transformer works and the more current that can be pulled across the primary windings, the more potential power at the secondary winding given the same ratio of windings. I wonder if this increased pull across the primary could also cause longevity problems with the Mopar box?

It wouldn't surprise me if that was the case. Unless there's a thermal shut down or the ballast resistor puts an absolute limit on the current, most transistors can be damaged if you overheat them from too much current. Or, depending on the internal construction of the box, something else may get damaged - it's possible that some of the internal connections may be the limiting factor.