Let me say this. Compression won't make a dam bit of difference where it comes to CDI, multi spark or any other method of getting a spark from the plugs ! When the compression is raised it requires a higher voltage to jump the gap, amps makes no difference. Big coils are mostly for show, with lot of surface for cooling, this is a waste IMO. We can see this by looking at an HEI coil. These buggers put out over 65kv without getting hot. Those huge ACCEL yellow coils only put out around 40kv but make a hotter spark with high amp current and make a lot of heat. I can prove that even under 12:1 comp, if your coil can put out 35kv and everything in the ING system is as it should be, then you have nothing to complain about. Of course if you are using a set of Champion Resistor plugs, you can usually trace it to those. Some people will swear by them, but not this guy. I have spent too many years dealing with pulling bad plugs and installing good plugs, most of them right out of the box and either back to the store or into the trash. There are 2 ways to raise the voltage output from any coil, first is with a CDI (MSD IS CDI)which does not increase the amps but increases the primary voltage to the coil. When using a CDI, you better have a coil that designed to work with CDI or you will fry that coil period ! The primary windings are not designed to withstand that high voltage and WILL short out internally which WILL take out your CDI unit if not for good, temporarily, in the case of having a resetting fuse inside. Instead of sending a nominal 12v in cases no ballast is used which with a CDI the only purpose would be to limit current supply to the box, and this makes very little sense since the current supply must be high to charge the Capacitor rapidly. With a CDI the primary voltage sent to the coil is increased to between 300 to 500 volts at a very low amperage. Opposite scenario is you are using a CDI coil without a CDI and the output will be excessively low and will make the engine run like carp, a very weak spark that will not fire the cylinders under load and or higher compression. The duration of the pulse is in milliseconds. This makes the coil cycle without saturating the magnetic field which does nothing but build heat inside of it. Old point systems only worked when the points opened and the magnetic field collapsed which would produce the high voltage. MSD type systems obviously build up and collapse the magnetic field multiple times in you guessed it milliseconds. The simpler way is to change the coil turn ratio inside. For more insight from someone that has been educated in electronics and had hands on experience with this, the first thing is to make sure you are getting a proper supply voltage to the control box. Then make sure your system is not loosing power or function on account of defective component that is suffering from heat related failure, such as a bad solder connection or even from solid state component problems which are common. It could even be a problem with something simple as a connection in the junction box or ignition switch. Old MOPARs are notorious for these. Now on to crappy running issues, if the primary voltage to the coil is too low you will still get a spark to the plug but under compression it will not be enough to fire the fuel which will make it run like ****, been there seen it and it was my fault for connecting the dual ballast incorrectly. Everything looked right but wasn't. It would pop and backfire and set the carb on fire, it would idle crappy and not rev but as low as I was increasing the throttle with a very rich fuel ratio it would act as if it wanted to run better but not best. The thing I found and had to deal with was the correct phasing of the rotor to the tower terminals, as the coil was firing for each cylinder the rotor was pulling away from the correct terminal causing the coil voltage to require exceeding the capability of the coil output max. This made the engine stutter and stumble under acceleration and power was pathetic. I ran a 383 with 13.5 compression ratio a dual point dist. and a stock electronic dist. with a stock control box parts store replacement and didn't have any problems getting a spark at the right time with enough to keep things happy. I also ran an 18 with a stock point dist. with a Napa aftermarket conversion that was wired into the stock electronic controller used in the mid 70's that used the dual ballast for electronic Vreg Alt and ignition. One can get into issues here also with the isolated field alternators MOPAR used. Which is how having the wrong primary supply to the IGN box happens which is what causes popping and backfires at idle and anywhere else unless the fuel is super rich. If things were happy with the previous motor and not happy with the replacement, take this to the bank, it's not because of higher compression! It because you messed up, or the system is not happy on account of account of a impending fault in the electronics part. I have seen coils that would work fine cold and crap out when hot, I have seen control boxes do same, I have seen wiring problems at connections, junction boxes and even inside harnesses where the wire had been crimped, bad connections inside harness splice blocks, damaged insulation that let moisture inside and the wire had corroded. The list is long. Be assured that any controller box made by any electronics firm has been required to pass a quality control program that includes a battery of tests to insure the circuit is a viable design and will perform under any condition to be found in the environment it was designed to work in. I have opened up stock type MOPAR chrome boxes and blue boxes and orange boxes, I have even been inside MSD controllers to repair the electronics. Once repaired they were resealed water tight, and put back into service. None of this is to say electronics don't fail, because they do! It's up to you to decide if the problem is something you caused or it was caused from a screw up or something else. Another thing I can pretty much count on is there is usually a protection device inside most every circuit that will stop the electricity in case of a short or overload, some automatically reset some don't. Lots of fuses are installed in fuse boxes for this purpose but you all know that already. The MSD has a fusible link inside it that will pop if a mistake is made in installation. At least they did years ago. Some electronic systems have polarity protection that will pop a fuse inside if connected backwards, some just don't work backwards and will be fine when corrected. Other are designed to work either way. Not usually the case with auto stuff though.
