Piston aircraft engines typically have two plugs per cylinder. One reason is because they have two independant ignition systems for safety. The vast majority of these are magneto type ignition.
As with car engines, the twin plugs also improve the combustion process inside the cylinder. They burn the fuel/air mixture more completely and also create a more even flame front of the expanding gases acting upon the piston crown. At least that was the answer in all those powerplant exams I wrote years ago.
This is well illustrated in an airplane because you can turn each ignition system on and off independantly when doing checks. There is a corresponding drop in engine RPM when one mag is switched off, showing that two plugs do work better than one.
This reminds me: Does any of the pilots on this board have or know of one of those Porsche powered Mooneys that were around years ago? That would look good in the garage beside your Porsche farm tractor.
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