Hi, all.
In light of the fact that my OEM ignition coil is weak, and since I'll be doing experiments that'll require a pretty hot spark (ie: alternate fuels and lean lambda), I've bought a Pertronix FlameThrower HV 60,000 volt 1.5 Ohm coil and a MagneCor R-100 CN 10 mm ignition wire rated to 80,000 volts.
The OEM coil looks like the first attached image. The red lead comes from the battery's (+) terminal via a fuse and the keyswitch, and the yellow/black lead goes to the ECU's pin 12, and branches off to the tachometer as well.
The ignition on this bike isn't really CDI, it's more akin to the old-style Kettering ignition with solid-state "points" (which is actually an SCR triggered from a Hall Effect sensor that gets its signal from the flywheel magnet). The "points" merely short to ground, causing current to flow in the primary coil, which induces a high voltage in the secondary coil.
You'll note there's a diode on the "upstream" side of the secondary coil, to prevent back-EMF from feeding back into the primary coil and blowing out the tach or the ECU.
The new coil doesn't have that diode, so I'll have to come up with some other means of preventing bEMF from smoking the tach or ECU. I considered putting a series of high voltage diodes on the "downstream" side of the secondary coil (where the spark plug wire connects), but finding diodes that can withstand that kind of voltage isn't easy.
So, I've come up with the idea of putting essentially the same setup as is in the ECU (an SCR) downstream of the ECU and upstream of the coil, except one that can handle much higher voltages than the ECU can. I'll call it the "isolation unit".
The ECU's SCR will trigger the isolation unit's SCR, which will then shunt the primary coil to ground, causing current flow, which induces the secondary high voltage.
The tachometer will connect to the upstream (protected) side of the isolation unit. When the ECU's SCR dumps to ground to trigger the isolation unit's SCR, the voltage drop that triggers the isolation unit's SCR will be read by the tachometer.
In this way, we can use hotter coils (and we're not limited to merely coils for scooters... the FlameThrower HV is for car engines up to 10 cylinders), we can wire them up without worrying about blowing out the tach or the ECU, and the tachometer will still work.
After my electronics guy gets a circuit mocked up, and after I get the new coil installed, I'll offer the circuitry plans and instructions for sale for $1, to recoup my design costs.
