1. CDI spark's short duration is disadvantagous in certain situations. This is why MSD has "multiple discharge" CDIs.
MY TAKE ON THIS: Not valid if you tune correctly. The point of a spark is to initiate the flame front, not sustain it. If you are having problems doing this because of a short duration, you are running too much advance at low RPM. I feel this is backed up by the fact that all the multiple discharge units I know of switch to single spark at relatively low RPMs. I have personally worked with vehicles that have HUGE cams and Motec, M&W, Autronic CDI's and none of them have multiple discharge at low rpm's and NONE of them have disadvantages due to the short spark.
2. FROM ELECRTOMOTIVE'S WEBSITE:
" According to the BOSCH automotive handbook 3rd edition...
Page 460... "The major advantage of the CDI is that it generally remains impervious to electrical shunts in the high voltage ignition circuit, especially those due to spark-plug contamination. For many applications the spark duration of 0.1 ... 0.3 ms is too brief to ensure that the air-fuel mixture will ignite reliably. Thus CDI is only designed for specific types of engine, and today its use is restricted to a few applications only, as transistorized ignition systems have virtually the same performance. CDI is not suited for aftermarket installations."
MY TAKE ON THIS: Plain untrue. A certain amount of activation energy is required to ignite a mixture. The activation energy of a combustible mixture is NOT time dependant. The reason it apears to be is that the longer time of an inductive spark means its more likely that the mixture nearest the spark plug will achieve a low enough activation energy. I've seen how the instantaenous A/F at the plug changes quite a bit as the piston approaches TDC. Not to mention the effect of other properties including temperature and pressure. When ignition occurs as a result of a long-duration spark, you're effectively retarding timing. In reality, a properly timed CDI spark should never be "too brief" to ensure the air-fuel mixture will ignite reliably. And since the energy of a CDI spark is so much greater than inductive, a proper and complete burn is more likely to be established over a greater range A/F and cylinder pressures.
In the last year, there were several occasions where we converted cars with fully sequential coil-on-plug applications from Bosch multi-channel ignitors to both EFI and MOTEC CDIs. With no other optimization besides a larger spark plug gap, we saw peak torque and power climb 5-15%. Besides a more complete burn, I believe that a higher effective advance number contributed to the power gains.
MORE REAL WORLD INFO: On a factory Porsche 964, 993 street engine, and 964/993 RSR race engine, they use a simple Bosch ignitor, coil, and twin distributor. This means that at redline of 8000rpm there is 7.5ms of time for 1 crank revolution, and there are 3 ignition events within that time, or 2.5ms available for coil charge time. Now, we know that the coils that are on that car require about 3.0-3.2ms of dwell to be fully charged. Sooo, what do we have here? Porsche isn't dumb, Bosch isn't dumb, why would this design be used, or more importantly, be used in a racing application? Perhaps it was rules that required race systems to be the same as street systems, I don't know. But at the end of the day, it APPEARS that the coils are not fully charged when the engine is operating over about 6000rpm on either race or street version!
Ok, so lets talk about the physics involved in the two types of ingition systems.
CDI Ignition operating principles:
In a CDI igntion, the 12v battery power is stepped up by an inverting powersupply to about 450v and stored in the Capacitor. This occurs within about 1ms of time and about 105mJ of energy is available. The formula for CDI ignition is W (energy in Joules) = 1/2C (capacitance in Fareds) * Vsquared (voltage). Since the energy is not stored in the coil, the coil only needs to have low resistance to step up through the secondary windings. These coils are designed to handle the high energy level (105mJ or so)
Inductive Ignition operating principles:
Unlike the CDI ignition, the energy is stored in the primary windings of the coil. The formula for Inductive ignition is W (energy in Joules) = 1/2L (primary inductance (Henries) * Isquared (Amps). Most inductive coils have a maximum spark energy of 63mJ and require 3-3.5ms of charge time (Dwell). You can increase the energy by increasing the primary current or primary inductance, however, the time required to charge the coil (Dwell) will increase.
So, can you use inductive coils in a CDI or vice versa?
All formulas point to decreased energy capabily in Inductive ignition or increased energy choking and absorption in CDI. In addition, most inductive coils can handle only about 65mJ of of continuous energy before running the risk of overheating if the 105mJ of a CDI ignition is applied.
The generally accepted maximum required amount of energy required in most applications is about 100mJ. An interesting point is that coil on plug applications may need as little as 50mJ. The biggest problem I see is that people buy a CDI and mate it to inductive coils. They are storing the charge in the CDI, and in the coil. You loose half of the current doing this. You need a coil with very low resistance. Inductive coils will saturate quickly when charged with 450V from a CDI.
SOOOOOO, to close, is CDI nesicarry? Simple answer NO. But it does work exceptionally well at what it claims to do. Also, remember, there is a HUGE diference in CDI technology from each brand. You DO get what you pay for. There is a reason that Nascar runs not one but two CDI ignition systems, and thoes units are HAND BUILT, and BEEFED up specifically for Nascar ... yet they still need dual CDI's. You won't see MSD on a FIA Rally Car, F1 or a Cup car.
Many Thanks to Ian, Geoffrey, and Steve for a headache from thinking to much.