the FBO system deletes the ballast resistor with a jumper wire so that takes that out of the discussion. the FBO requires 12 volts to the coil and is supplied with a Pertronix coil. with the ballast gone the coil will now get whatever voltage the charging system provides; at least mine did. now there's 14-15 volts to the coil. i check the dwell on boxes and usually it will be somewhere between 35-40+ degrees; depending on the box. the FBO box i used had 15 degrees dwell,....
. so, i called Don at FBO and asked about that and he explained it to me and the truth is most of the explanation blew over the top of my head; basically didn't understand it all. i got to thinking that a mopar kit used a 1.25 ohm ballast (vs none with FBO) and will run 6-7 volts at the coil with twice the dwell (orange box is around 36 degrees). is this a case where less dwell time is needed to compensate for twice the voltage? is the spark voltage the same to the plugs; just done differently? i don't know but another one of those more questions than answers thing. anyhow, the soft rev limiter worked well which is what i needed but the driving situation stifled me. maybe i'll re-visit this at a later time.
It still is a series circuit. I don't know what is in the FBO box, but the ballast could have been moved into the box so the external one is no longer required. Regardless, the transistors and diodes in the box also have a voltage drop effect. So despite 12V at coil input, the ground terminal won't measure 0. There would be a voltage between 12 and 0 at the negative of the coil. The box drops that voltage to zero battery ground. There is now micro chip technology to control current flow to coil, but the old tech the current was set by the selection of the box, ballast and coil. But besides coil resistance, the inductance matters more. Determines how much current coil can handle, how fast it can charge. Wrong coil selection that charges too slow for the set current, would drop off in energy as rpm rises. Regardless of your dwell, you are firing a spark plug every 2.5msec at 6000 RPM engine speed with a V8 single coil. No time to charge coil with the old tech. Which by the way the MOPAR boxes had RPM limits around 5K for the HP BB 4spd cars. Think it was 5200 or 5400 for an automatic.
If you allow more current to flow the coil charges faster which is good for high rpm, but street driving the coil saturates too fast and overheats. Damage to coil. So it is all trade offs.
The GM system is mostly better because of the coil in my opinion. They could send more current to the coil all the time.
MSD is capacitive. Charge an Inductor (coil) in mill secs. Charge a capacitor in micro seconds. Hence MSD can discharge the required Voltage at much high rpms to get sufficient KV to the plug. It is a different process.
There are some really good tech articles on how much voltage and for what duration is required to jump the gap and ignite the mixture.
The other funny aspect especially with multi spark is that if the ignition misfires at normal fire time, if you look at how fast the piston is rotating at high RPM, the second spark to ignite mixture provides no useful power, just better emissions
