So the F8DC4 run one notch "colder" than the H9DC0's...Under what conditions are colder/hotter sparks favorable?

Also, can you explain the last # in the part number (Special Shell or Electrode)?

Just trying to understand which would be best for me as it looks like this spark plug has a lot of sources compared to the H9DC0

I switched from the H9DCO's which were fouled to new Denso non resistor and I cannot be 100% sure, but I feel like it seems to be idling a tad rougher...That is why I want to get a spare set of Bosch plugs to test it out as the old ones I had were fouled up from past leaking stem seals.

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