I think CR plays a big factor. I was using regular tip NGK -7 plugs in my 10.3:1 470 street car, and have been having an issue keeping them clean and would have sporadic misfires driving around town. I changed to some used Autolite 3923 (projected tip) and the plugs look better and I don't notice misfires now. I need to get new plugs and have been pondering what to get. Given my recent tests, I plan on going with -6 but still need to decide exactly which type to get. The "super projected tip" plugs look interesting. Also thinking about trying the fine tip iridiums as an experiment.