Twin Spark Firing using single CDI Box

[mysocal911]

Originally Posted by winders

It's not that simple.

Here is an interesting read:

Ignition Systems - Basics to High Performance

Inductive Discharge vs Capacitive Discharge Ignition (CD)

All of the above descriptions are examples of Inductive Discharge type ignition systems. The first high performance ignition system design was the Capacitive Discharge Ignition which uses high voltage stored in a capacitor to induce the few hundred volt pulse into the secondary instead of generating it in the primary as with an inductive system. In a CD system a high voltage oscillator circuit charges a capacitor. When it is time for a spark, another circuit discharges the capacitor into the coil primary. The voltage produced from the secondary is in the same proportion as with an inductive system. For example if the capacitor is charged to 400 volts, a 100:1 coil will produce 40,000 volts (or there abouts). CD ignitions have certain limitations depending on design. As an example if the oscillator is not powerful enough, the capacitor will not recharge quick enough and the ignition will have an RPM limitation. As you engineer more powerful oscillators, power consumption also increases. As the size of the capacitor is increased, the output of the ignition also increases, however the oscillator must be made more powerful. Certain tradeoffs must be made in design for a particular application.

Compared to inductive systems of the day, CD systems were more powerful since higher primary voltages could be discharged into the coils. One inherent limitation to a CD system is that the spark duration is short due to small capacitors used. As mentioned earlier, spark duration does the actual work burning an air fuel mixture, the voltage is used to initialize current flow across a spark plug gap. Due to the high primary currents of an inductive system, spark duration is naturally longer. Some Manufacturers of CD ignition systems provide multiple sparks to increase overall spark duration. Of course, as mentioned before, capacitors take time to charge up so there are limitations as to what RPM levels a CD system can maintain multiple sparks.

Today with improvements in technology, inductive ignition systems can be made more powerful with special transistors (Mosfets) switching the coil. These devices have higher breakdown/clamping voltages which allow the primary voltage to increase ( 550+ volts) resulting in higher secondary voltages. This coupled with the inherent long duration makes for a powerful ignition which has the capability of also going multi spark. Inductive ignition systems are emerging as the new high power ignition systems.

Good analysis! It's unfortunate that some may not fully understand it or fail to except valid electronic theory. As you imply, early CDI ignition analysis theorized that the typical spark load
was highly capacitive necessitating a fast spark pulse rise time, and that an inductive discharge ignition (IDI) with early transistors, i.e. germanium, were inadequate - poor spark dv/dt (coil voltage rise time).
Ignition coil designs of that era also contributed to IDI being inadequate. Present day COP systems provide more that adequate spark rise times for capacitive loads.

As noted in your post, some CDI ignitions like the MSD provides multiple sparks, which basically infers that a single CDI spark pulse (typically less than 100us) is inadequate.
Given that and the many posts in this thread, some continue to ignore valid electronic theory and reality, i.e. the CDI spark inadequacy. As the saying goes, "You can't have it both ways."
A typical COP IDI system produces spark pulse widths approaching 10X a CDI's with very fast spark rise times - nanoseconds.

The MSD Ignition - 'Exposed'

[Neil Harvey]

I'm going to throw my oar in the water again.

It was never about which was the better system, in theory. For me, its all about which one is better in these large hemispherical chambers with domed pistons and offset plug(s).

Pent roofed chambers with volumes down in the 40's, with centralized plugs built for cars that have to comply with emission requirements need a long duration. And the fuel today does not burn as quickly as fuel from a few years ago.

Its all about the application, for me. Nothing else. I read where the discussions get heated over which ignition is better, but I have not yet read anything where the application was included in the argument.

We can disagree on anything. Simply, as a business building some Porsche 2V high performance engines, I choose what I feel is better, based on testing done, back to back. If others have found differing results, then that's ok too.

[Henry Schmidt]

Originally Posted by Neil Harvey

I'm going to throw my oar in the water again.

It was never about which was the better system, in theory. For me, its all about which one is better in these large hemispherical chambers with domed pistons and offset plug(s).

Pent roofed chambers with volumes down in the 40's, with centralized plugs built for cars that have to comply with emission requirements need a long duration. And the fuel today does not burn as quickly as fuel from a few years ago.

Its all about the application, for me. Nothing else. I read where the discussions get heated over which ignition is better, but I have not yet read anything where the application was included in the argument.

We can disagree on anything. Simply, as a business building some Porsche 2V high performance engines, I choose what I feel is better, based on testing done, back to back. If others have found differing results, then that's ok too.

agree 100%
With these old air-cooled Porsche engines, maximum horsepower is rarely the end all beat all. With these old Porsches, performance based on specific criteria is the goal.
To date, I haven't seen a COP system the works with a 12 plug distributor. Old school tech? of course but honestly, how much in the world of Porsche restoration is about that old school feel. People aren't spending $200K + trying to maximize the horse power in an RSR resto-mod. They are searching for that nostalgic feeling.
50 year old MFI, 2V, distributor engines don't make maximum horse power but there is certainly a wood factor.

[Dpmulvan]

I can fire those coils with 3 boxes
I can fire those coils with 2 boxes
I can fire those coils with 1 box. Oooh aah clapping ��
Fire those coils.

[rswannabe]

Reviving an older thread to ad a new option. I have a twin plug 2.8 that I am efi converting. I want to keep CDI, but triggered by the ecu (separate cam and crank trigger sensors for the ecu). I was thinking I was going to run two of the new MSD Ultra 6a units to keep the footprint small, but then found the Ashlock Tech (Bob Ashlock) Twin Plug cdi. It is two modern parallel cdi units in one old school Bosch CDI case. The cost is comparable to two MSD units.



https://ashlocktech.com/ashlocktech-cdi

I think this is a great option for those of us who want to keep the old school look, but take up less space.