Forgive my ignorance, A/M ignition

[sv51macross]

So then, a CDI module would go between the ECU and the factory coils? And this works on stock wiring? [well, as you mention, it just increases the voltage. I assume amperage goes down].

[RETed]

Let's go back to basics...

The (stock) ECU fires a low voltage signal to the ignitor.
The ignitor fires a higher voltage signal to the coil.
The coil - using a design of a step-up transformer - steps that signal up to voltage levels exceeding 10,000 volts.

There's some reasons for such a complicated design.
1) You don't want too high voltages running through the delicate electronics of the (stock) ECU. High voltage (pulse) emit EMI. EMI wreaks havoc on low voltage signals - which is what you mostly see internally in the (stock) ECU. Also, high(er) voltage signals require high(er) power amplier circuits that that use more juice - having lower power requirements allow the (stock) ECU to run cooler with less power consumption.
2) Having the higher power components outboard of the (stock) ECU allows you to replace them a lot easier and faster. Higher power components will tend to have less reliability and longevity. Would you like to keep replacing the (stock) ECU if the on-board (high power) ignitor circuit died?

The stock ignition system is called an inductive ignition.
This is typical of most OEM ignition systems.
Inductive ignitions systems have the advantage due to the fact that 1) they are very reliable, and 2) give good combustion burn due to the relatively long spark dwell.
Downside of an inductive ignition system is that it has harder time firing at higher RPM's (and combustion loads).

Let's look at higher RPM's...
As RPM's rise, the window of opportunity of firing the spark gets smaller and smaller.
This is due to the fact that the rotor is spinning faster and faster, and the ignition system has to fire within a narrower time window.
The inductive ignition system has to "charge" or "rise time" of the circuit before it can effectively fire a spark.
If the coil does not get the minimum voltage charge, it cannot fire (or discharge) a spark.
This is called a "misfire".

This is where capacitive (discharge) ignitions come in...
MSD (multiple spark discharge) built their company on this concept.
By using (a bank of) capacitors, you effectively eliminate the problem of charge time at high revs.
Capacitors can build up voltage in very short time - way faster than purely inductive systems.
Now, this is a double edged sword...
Because the capacitor charges so quickly, it also discharges very quickly...
A CDI (capacitive discharge ignition) box disadvantage is that it can only fire a very short duration, high powered spark.
This is where the inductive ignition has an advantage - the longer duration spark is better for combustion.
This is where MSD comes in - multiple spark discharge...
By firing a bunch of sparks at low RPM's, it tries to compensate for the disadvantage of the capacitive ignition.

Most CDI boxes can input a signal from ignitors, intercept that signal - step up the voltage, and then output a much higher voltage to the coil.
Typical CDI boxes can step up the voltage up to 300...400+ voltages at output!
Inductive ignitions have very hard time getting voltages that high.

Now, CDI technology sounds real good, so what's the downside?
If this box fails - and it will - it will knock your entire ignition system out.
Due to the high voltage it runs at, it will eat spark plugs.
This can be somewhat compensated by running colder plugs.
Also, the fact that the CDI can fire large spark plug gaps, the spark plugs will last longer - relatively - because an inductive ignition system would probably misfire due to the wider gap.

Now with that out of the way, let's take a look at the stock RX-7 ignition system...
Some of you might already know this, but it's worth repeating.
I'm ignoring pre-FC3S ignition systems that all 1st gen's use - twin coil, distributor systems.
This applies to direct fire systems that the FC and FD (and possibly RX-8?).
This stock ignitions systems use a twin tower coil to fire the leadings and a pair of single tower coils to fire the trailings.
This system allows for a wastespark system which effectively fires three spark events per (rotor) chamber.
That 3rd spark event is also known as a far trailing spark.
This extra spark makes for more complete combustion.

What does this have to do with CDI?
A stock ignition system will have the characteristic brrrrrrrrr....brr.....brrrrrrr idle.
Anyone with an unmolested stock ignition system can relate to this.
That small hiccup is due to the twin tower leading misfiring cause the stock ignition system cannot fire reliably.
Remember that due to the wastespark system, that twin tower coil is firing into one freshly charged combustion chamber filled with fresh air + fuel + pressure *and* one highly ionized charge combustion chamber that has just fired and mostly filled with exhaust gases.
So which one do you think coil will want to fire easier?
1) high pressured and filled with air + fuel?
2) less pressurized and filled with highly ionized burning gases?
Yes, #2.
The twin tower coil will tend to fire easier in the wastespark.
This is where the misfire occurs.
Mazda knew about this, but it doesn't really matter cause it doesn't really affect performance.
Slap on a CDI box on the leadings, and this "problem" disappears.

Any comments or corrections welcome...


-Ted