Simplifying CDI

[mcinturff]

Couldn't you use the signal generated by the distributor in conjunction with a diode to trigger a transistor that would cut off current to the coil? Basically whenever current is generated by the pulse coil at TDC for each cylinder, this would send a (positive, due to the diode) current to the transistor, which would be normally closed and connected to the coil. When the transistor "opens" then magnetic field of the coil would collapse, rather than sending high voltage to the coil via CDI. If I'm not mistaken, the increased voltage output of this generator at high rpm would result in a lower impedance of the transistor, and (correct me if I'm wrong) decrease the time it takes to "rebuild" the magnetic field of the coil. Any thoughts?

[HawgRyder]

The quick answer is yes.
But....the coil for a CDI unit has higher turns (bigger number of turns) than a regular 12V coil....so....if you just put in 12V and interrupted it with a transistor operating at 12V then your spark output would be less than needed for ignition.
So...by all means...change to a transistor type ignition if you like...but replace the coil with a standard 12V type to get the correct voltage of spark.
Or....why not just get a working CDI unit?
Bob

[mcinturff]

Do you mean a CDI coil has more primary and secondary windings? Or do you mean it has a lower inductance? I wasn't aware that coils for CDI were very different, though I could see this being the case as the inductance of a CDI coil wouldn't need to be as high as a regular coil as CDI puts out about 450V (I believe). Any idea whether a NPN or PNP transistor would be appropriate?

[HawgRyder]

CDI uses about 400 volts to fire the coil.
The regular coil is about 12V of course.
So....the primary of the coil (number of turns in the input of the coil) is more than in the 12V version.
Think of it this way...if you put 12V into a 400V coil...you get a voltage of about 1050 Volts....obviously not enough to fire a spark plug.
So....depending on the voltage you are switching with the distributer you must use the correct coil.
OK?
Bob

[304065]

The reason we don't use kettering-ignition anymore is not due to the latency of the points switching the coil-- when the distributor shaft cam pops the points open the field breakdown begins instantaneously. The problem is that a single coil does not have time to charge between ignition events at high rpm. Hence, CDI in 1969 (from the factory, things like the Sydmur Fireball and Delta Mark Ten were used before then) which effectively stores energy while the points are closed, then dumps it into the coil when they open- the capacitor charges so fast that high-rpm weakness is not an issue. The sacrifice is spark duration.

A CDI coil differs from a conventional one in that it has a fairly low turns ratio and low impedance compared to a kettering coil. The issue with these things is heat: if you try to use a kettering coil with a CDI, applying a 450V pulse to a coil with a 200:1 turns ratio will blow it up pretty quickly.

So that's my answer- it's not the switching, it's trying to use a single coil on a high-revving six cylinder engine that requires a lot of ignition energy due to high dynamic compression that's the problem.

[mcinturff]

DIY TCI + CDI Trigger Article