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...
In case you're wondering: MSD makes some OK products, but there's one serious problem: they produce all this high voltage spark energy, which is great, but it has to go through your distributor cap before it gets to your plugs. The dizzy cap wasn't designed to handle that high a voltage; you will start to have internal arcing problems and other such nonsense. The cap will burn out in a short time. Also, the actual design that the popular MSD-6 series is based on was spec'd out in the 70's and is really quite primitive. The Altronics kit is totally up to date, using high speed/high power switching transistors and other good stuff. Plus, it's a kit, which makes it easy to make it do exactly what I want, dirt cheap. (Costing me about US$65.)
My Altronics CDI kit arrived this morning. Looks like it's going to be a few hours to assemble just that. I even have to wind the coils on the power transformer myself. Sheesh, why couldn't they have used a self-contained DC-DC converter. Oh well. This kit is a multi-spark, putting out 8 sparks per cylinder at 600rpm, down to 2 sparks at 6000+. That works out to a minimum spark duration of about 11 degrees (at 800rpm, 6 sparks) and a max of around 20-something depending on engine speed. It always outputs sparks in pairs. It's not exactly what I expected, the first spark is a negative pulse, then the second one is a positive pulse.
Anyway, I need to do some more research now to figure out how I'm going to switch this output. I was only expecting a single polarity output, not this AC stuff...
Somewhat. Now that I've had a chance to look it over and think about it, I could probably have just gone with an off-the-shelf CDI box after all. The switching circuitry will be the same regardless. I will probably build my switches into a separate box, so that it can be interfaced with anything else. The actual switching logic has gotten a little bit more complicated than I originally envisioned, but it's still not too bad.
The Hall switches are only on for a moment, so I have to wire them up to a flip-flop. No biggie, a pair of CD4044 ICs (4 gates per chip) will do the job. The flip-flop outputs will turn on the high voltage switches, which actually connect the capacitor output to an individual ignition coil. These switches will be six TRIACs. This switch setup is really very simple, very reliable...
If I get too frustrated from building this CDI kit I may look at another off-the-shelf unit. Thanks for the tip.
I don't like the specs for the Jacobs ICEpak. The multispark only fires across 10 degrees of crank, which may not be a long enough spark duration for good ignition. The output voltage of 65000volts is excessively high (maybe it makes up for their short spark time) which will put more stress on your plugs and plug wires, and possibly cause crossfire between plug wires as well.
That 1380mJ of spark energy is quite an impressive figure. I'm sure it will work extremely well assuming your distributor and plug wires are in top condition.
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