On the left - stock NGK Iridium spark plug. On the right, NGK Race surface gap spark plug.
NGK has a description here: http://www.ngk.com.au/spark-plugs/product-information/what-is-a-surface-discharge-spark-plug
However, there's no real indication as to why these are better or not. From what I can gather talking to the engine builder, the race plug allows us to run much leaner/meaner A/F ratios (I assume this is because of the protruding electrode on the stock plug creating a heat riser in the combustion chamber).
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