How about relationships that used to be? Ford owned the majority stake in Mazda for many years then sold that off. Ford motor in a Mazda legal? Mazda motor in a Ford legal? Ford owned Volvo. Volvo motor in a Ford okay? VW/Porsche are the "same family now", but what if Porsche suddenly sold off VW? Or Audi? How long does the "in the same family" last after the companies diverge?
Personally I believe the "in the family" rule to be a bunch o shit. Want to run a 2L Mopar in your Honda? Do it up. The class has displacement/weight rules, who cares what company makes the engine?
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