Which Bosch spark plug will run hotter in an engine: W9DC or W7DC ?

Reason I ask: I remember "knowing" that higher numbers = hotter plug. Then I heard that the opposite is true, due to some method of spark plug classification.

I see from a Bosch catalog that early M117 motors take the W7DC plugs, but later M117 motors take the W9DC. Did EFI motors require a different combustion temp than the CIS ones? Is this a no-catalytic vs catalytic requirement?

All M117 motors take the 75C thermostat, AFAIK. So will an early motor run at "cooler" combustion temps? Hotter?

Who can set me straight on all this?

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Michael Stoic

1979 450SLC
1989 300SE