Hey, YR, just to be clear for the OP, isn't a NGK GR4 plug already fairly hot??? (yeah, I hate NGK #'s too!) Doesn't seem like it would cross to a Champion 11, but these plug numbers scramble my brain. And I agree about the platinum plugs!!!! I had a 440 with stock Mopar ignition that I could never get running right until I finally pulled the new-ish platinum plugs and replaced them!!!
If this helps anyone:
"NGK plugs are colder the higher the number, hotter the lower the number. Starting with part # BKR6E-11 (heat range 6), a colder plug would # BKR7E-11 (heat range 7), a hotter plug would be # BKR5E-11 (heat range 5). For NGK Racing Plugs, the heat range is located AFTER the hyphen."
"NGK GR5 (one step colder than GR4)"
