Quote:
Originally Posted by JoeCB
PS, as a side note, it is curious that . 22 micro farad (or there about !) is a common size for many many engine applications, small 4 cycles, outboards and Model A's
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It is curious but this may help.
The coil on small engines is usually located inside the flywheel along with the other ignition components. The coil is therefore limited in physical size. A coil of certain physical dimensions can only be designed to put out so much voltage and current.
These small engine all have approximately the same room inside the flywheel so the various coils and caps are similar in size.
A certain size coil will require a certain mfd. size capacitor. It is the inductance of the coil that determines this.
Capacitors can vary all over the place in size PHYSICALLY but still be made with the same capacitance.
A certain coil needs a certain capacitance to tune it to resonance. This results in a high impedance primary circuit which limits the average current so the points will live.
Electronic controlled primary circuits don't have points so the primary circuit can be designed to carry more current in the same physical space, thus hotter spark.