Crumpp is right that +12 boost is about 3 times higher than the maximum continuous rating (+ 4 1/2), which is the highest rating that is not considered a overload condition (see attachment). I don't know if this was the rating the engine was designed for.
IIRC we know that +12 boost reduced the life-time to about 20 hours instead of 100 hours at maximum continuous rating.
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