Spark plug gaps

RustyRatRod said

You have it backwards as a rule of thumb. Normally more compression will tend to need a colder plug. More compression = more heat and in order to keep that heat in check, you will want a colder plug. But I always go by the "how it runs" and how the plugs look" deal, because there are exceptions to every rule.

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Just like tires. You need to read them and then adjust. It's true what you say about the higher the compression the colder the plug. Hence, early 340's used N9Y champion with 10.5 to 1 compression, while the later 340's with its dropped compression around 9.5 use a N12Y or N14Y plug. The plugs wear can give you an idea on your compression ratio.