In response to TwoTired Response: Since two cylinders fire at once, those coils are wired together. You would have four coils total. Do you see what I'm getting at?
Older motorcycles biggest weakness is the ignition system. The alternators have minimal output and so do the coils. An automotive alternator transplant would be more difficult than a coil transplant but could be possible to accomplish also.
I'm not trying to necessarily accomplish anything more than a mental exercise to see if anyone has any thoughts on the subject.
Does anyone know what the impedence of automotive coils are? Are they close enough to bike coils to be utilized?
One coil per spark plug method.
The stock SOHC4 coils have four connections and two windings, primary and secondary. The primaries are where the low voltage from the points are supplied. This is usually the impedance or resistance referred to with coils and determines how much current is going to drawn by the coils during operation. The coil's triggering device must be able to handle the demands of the coil primary. The secondary's two output wires attach to a spark plug each. The spark gap characteristics of both spark plugs determine the voltage needed for both to bridge the gap. As they are wired in series, both must have met the conditions before either can fire.
The automotive coils I'm thinking about, also have four connections. But, not all of the connections are exposed externally. This limits applicability for other installations. Sometimes a primary and secondary connection are tied together and commonized to ground. No big woop on the primary, same triggering considerations apply. However, that arangement only leaves one spark plug connection. If this single output is divided into two and subsequently connected to two plugs, only one may fire unless the spark gap conditions are EXACTLY the same, which is highly improbable. A sparking plug represents a short circuit to the coils output energy. If one plug needs a higher voltage to spark than the other, the one with the lower requirements dumps the energy before the other can spark.
This is true with the standard points type triggering system. The parrallel ourput scheme may possibly work with other methods of triggering the primary that don't rely on the energy stored in the coils to develop voltage at the spark gap. There are many different kinds of these, but they don't appear on the stock SOHC4.
A coil per cylinder method.
First drawback is the weight and space penalty. Ignoring that...
Assuming the same points type triggering? This is a key assumption.
High output voltage coils usually have a lower resistance primary. This is because in order to develop higher voltage for the 12 v electrical system the truns ratio of the coil must be larger. Adding turns to the secondary also introduces wire resistance related energy losses. Removing turns from the primary accomplishes the same ratio increase. However, this reduces the resistance, and increases the current demands from the triggering device. I haven't calculated what the change should be, but the condenser or capacitor value used to reduce arcing should also be changed. The points will, nevertheless, increase their heating from such an application. To the points further detriment, we would need to drive two coils from the same points set. This will double the current the contacts have to handle leading to further heating and shortened life. To maintain 12v to each coil primary they would have to be wired in parrallel (double current). You could halve the current by wiring in series, but each coild would then receive only 6V, halving the coutput voltage.
Perhaps now you understand why I asked previously about the triggering method. The coil is only one part of the ignition system. If you intend to use the points, do you care about their durability? They will trigger automotive coils, for a while anyway. You can measure the primary resistance of the coil with an ohmmeter. The stock SOHC4 coils are about 5 ohms. At 12V, the points routinely handle 2.4 amps, 3 ohm primaries would require a switch capable of 4 amps, 2 ohm primaries, 6 amps. Ohms law: I=E/R. Two high output coils in parrallel could require 12 amp switched handling. I'm thinking this shouldn't come through the stock ignition switch.
You still haven't identified the big weakness in the ignition system. What is this big weakness you are trying to overcome?
If you had 100Kv going to the spark gap, what benefit to you envision this would have?
Do you want a larger spark gap?
Do you want to eliminate points adjustment/life?
I'd sure like a description of the specific goals you are trying to achieve. Do you have some auto coils laying around looking for a home?
Cheers,
