Hi Sign,
No problems about help.
MikeMi - You correctly opined that "more spark" at the sparkplug doesn't do anything. Just need enough spark to bridge the plug's gap, and that gives ignition. You're right, but this is an antique kick-start motorcycle. Extra capacity allows for ignition even under poor conditions; fouled plug, drained battery, eroded points, wet wires, etc. Don't want to be stuck on some lonely road.
The characteristic of the spark can have a radical affect on engine performance, but it very much depends on the type of engine. A high capacity 'lazy' engine would normally be less affected.
The fact that the spark is important is evidenced by the availability of high performance sports coils.
But there is also a vast area of study of combustion in internal combustion engines, and the spark plug and spark characteristics play a big part. Some engines even have two spark plugs, not just for safety, but to improve the flame propagation and to encourage a more complete burn.
And on some older piston aircraft you get 'mag drop' where the engine RPM drops when you turn one of the magnetos and hence plugs off. This was used as a quick and dirty test to ensure both magnetos and the other ignition components were working.
I have designed and commissioned a number of ignition systems, and going from a traditional Kettering system to capacitor discharge, on the same automobile, brings a noticeable improvement in the torque and general running of the engine.
There is an inherent problem with the traditional ignition system, which works by charging the coil during the dwell time (points closed) and releasing the energy as a high voltage spark when the points open.
This works OK at low RPM, but as the RPM increases there is less and less dwell time and thus less charge in the coil and thus less spark energy. And this is made worse by points bounce which further eats into the dwell time. So at high RPM, right when you need a high energy spark, the energy in fact drops off. This is where low resistance (& inductance) coils help because they charge more quickly.
Dr Pepper - You pointed out that modern coils are constant energy, with an "on time" equal to max rpms, and I could end up burning out the coil. This is beyond my expertise, but I think I'm safe, in that this is a small 350cc motorcycle, and aftermarket performance coils are usually designed for larger bikes or cars. My bike has a 6v 60 watts magneto and a 9amp-hr 6volt battery, and an aftermarket coil should be safe w the limited system. But this raises another question; is it possible that the system won't adequately charge a larger performance coil?
DP raises an important point- if the coil is designed for constant energy or CD (same effect) you could well overheat the coil as DP says.
There would be two approaches to fix this:
(1) Convert your ignition to constant energy- this would involve fitting a timer which is not as complex as you may think.
(2) Limit the current to the coil. This could possibly be done with a ballast resistor as is fitted to some coils.
Of course the fact the your bike has a 6V system, rather than 12V helps a bit, but the current is still high at around 8A.
Is it possible to get a data sheet for the new coil that you plan to use?
I hate to say this, but for a Rolls Royce job you would be far better off going for a contactless CD ignition system (no more starting problems, no more points bounce, and no more points adjustment/replacement)
By the way, what is the bike (Honda 250cc)?
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