Several reasons. One is the material. The 100k plugs are usually platinum.
Platinum electrodes do not have the transfer of material like steel plugs.
Two. The spark. EI gives a much hotter spark which also affects how the
electrodes wear. The length of the spark also has an affect.
Three. mixture. Modern cars run lean meaning less deposits on the
electrodes.
Can we get 100K on the plugs of an MG. I doubt it. First of all you would
need a modern electronic ignition for spark and then run it on the lean
side.
The manufacturers went to the 100k plugs for emissions. New cars are
required to have an emissions warranty for at least 50k. I think there is,
or will be, a movement to a 100K emissions warranty.
All that being said, I would doubt whether I would keep a set of plugs 100k
miles. Also, if I had a new car, I'd pull the plugs and make sure there was
antiseize on the threads. Any of us who have worked on aluminum heads knows
what can happen if the plugs gall.
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