Selecting spark plugs with the correct heat rating

Warm for the street, one step colder for strip and another step or two colder for high compression. Not your engine combo but when I built my 273 (which went from 10.5 to 9.6/1 compression, the engine didn't run clean. It didn't run bad it just ran rich and blubberly especially at idle. I messed around switching carbs and ignition systems and never changed anything. I found out that the N9Y Champion converted to a N8Y which is colder than a 10.5/1 engine needs and way too cold for my 9.6/1 mill. If you can follow me, my compression was in the middle of the 10.5 and the 8.5 so I settled for a N12Y. As soon as I changed the plugs out and adjusted the timing and carb, it ran much better and idled smoother. Bottom line is....warmer plug for the street. It will run like poop with too cold a plug in there. tmm