Spent the last day day or so looking at aftermarket ignition stuff and reading a lot of tech articles.
It seems that CDI ignition is only really needed in the most extreme conditions, such as super high compression or when running nitrous. MSD has done a great job of marketing and convincing folks into believing that CDI is best way to go. It's kinda like the Angus Beef Association that has convinced the public that a hamburger made from meat from a black cow tastes better than one from a brown or white cow. Propaganda and marketing.
In any case there are some relatively inexpensive aftermarket ignition modules that can work fine on a mower engine up to 10,000 RPM.
One key to using these modules is to match the coil to the module. For example, you don't want to use a coil for a CDI ignition with a GM HEI module. The Module will operate in current limit mode and get too hot.
Coils are rated in primary resistance, secondary resistance, primary inductance and, secondary inductance. The turns ratio generally determines the voltage step up. But consider the primary resistance. This is what determines the saturation current. Saturation current is the current drawn after the coil has been "charged up" ( coils don't really charge but it's the best way I know to explain it). This current can be calculated by Ohms law where Current = Voltage / Resistance. For example: a 1 ohm coil and a 12 volt system. 12v / 1 ohm = 12 amps. If the ignition module limits current at 5.8 amps then it will get very hot, especially with a high dwell angle. But if the coil primary is 2.4 ohms then the saturation current become 12v /2.4 ohms or 5.45 amps. Now we're close to matched.
Some of the after market HEI modules can go up to 8 amps before current limiting kicks in. So with onE of these, an ideal coil will have a primary resistance of about 1.5 ohms. 12V / 1.5 ohms = 8mps . This makes sure the coil is fully "charged" so that when the current is quickly removed (HEI unit triggered) it can provide the energy needed to step the voltage up to 30 - 40KV . (None of this really applies to CDI as it works differently)
HEI modules can be triggered with a magnetic reluctance sensor, or a Hall Sensor. A Hall sensor is typically more accurate.
Dwell is defined as the number of CAMSHAFT degrees that current is applied to the coil's primary. In the older points systems it was the when the points were closed. On an 8 cylinder it was about 22 degrees and on a 4 about 45 degrees. Too much dwell and the coil gets hot. On an electronic system like HEI, dwell is managed in the triggering system. But since we may be using a rigged up triggering system on the crank instead of the cam some kind of interface between the trigger and the ignition module is needed for ideal performance. (A cam trigger would be nice as it would allow elimination of the waste spark and help keep the coil and ignition module cooler.)
This is where I will be focusing my efforts rather than redesigning the wheel. A control box between the Trigger device and the Ignition module can retard/ advance timing, provide a tach output, do rev limiting , etc.
Harold's Junk Yard is only 5 miles away so I am gonna get a GM HEI module, some Bosche modules and whatever else I may need to do a bit of testing and see what I can come up with.
Bill