Direct Fire Ignition
I started this discussion in a different thread, but that section of this Forum seems to have disappeared. ???
I've started designing/building a direct-fire ignition setup for my GTS. While in general the stock ignition works OK, it will start having problems trying to ignite a spark under boost. In fact, at 7500rpm, the stock ignition is only delivering about 30% of the energy that it has at 3000rpm. (Note that the '98+ 626 KL engines have gone DIS - no distributor, uses a 3-coil pack and waste-spark setup.) Also, the whole distributor-cap operation seems grossly inefficient to me...
My idea depends on getting hold of some coil-near-plug ignition coils from a GM LS1 engine. The ideal is coil-on-plug, but there's no way we could fit an ignition coil module into the access points for the rear cylinder bank on our engine. The GM CNP system uses very short high voltage spark plug wire from the individual coils to the plugs. The coils are fired using standard 12V wiring. There are numerous advantages to this kind of design.
The main thing I'm looking for is to get 100% of available spark energy, all the time. At 7500rpm there is a spark event every 2.67ms. The typical inductive ignition coil achieves full charge in 3ms, and needs to cool down for 1ms between firing. As you can see, a standard single-coil ignition with a distributor is a total loss at high RPM, it needs 4ms to operate at peak power but only has 2.67ms available.
My approach retains the distributor, but replaces the single coil with 6 individual coils. I am modifying the distributor cap so that it no longer distributes high voltage. Instead, I am mounting six Hall switches inside the distributor cap, one next to each electrode, and putting a magnet on the rotor. When the rotor swings by a Hall switch, it will select which ignition coil to charge/fire. The switches will be carrying only 12V, on standard grade automotive wiring, out to the individual coils. There will be no EMI/RFI from these wires to worry about. These components will never wear out, the distributor cap and rotor will become 100% maintenance-free.
Just by itself, this approach gives me back my 1ms coil cool-down interval at max RPM. But still there's not quite enough time to get a full charge on the coils. So in addition, I'm adding a Capacitive Discharge Ignition (CDI) kit. I ordered the kit from Altronics in Australia. The kit comes with two inputs and two coil outputs, I will be modifying it to accept 6 inputs and 6 outputs. A CDI can charge and discharge in under 1ms. This particular CDI kit generates 370V on the primary side (still low enough to not have to worry about RFI/EMI, but kinda high) which will produce 37000V at the spark plug. Some other ignition systems claim up to 50000V at the plugs, but in reality they deliver much less than that after you've added up all the losses going through a regular distributor cap and resistance spark plug wires, and the drop-off as RPM increases. My setup will deliver a consistent 37KV at all engine speeds. When I've got this all assembled I'll have an ignition capable of operating well beyond 15000RPM. (Not that you'd ever need it.)
Of course, this is all hot air at the moment. I'm still waiting for the CDI kit to arrive, and I've just begun modifying a Bosch distributor cap...