I know the 'manual' lists a N13Y for the 8.6 360 and a N9Y for the 10.5 340. When we machine and rebuild these engines, what plug range should I use? I have a 9.5 340 and a 9.4 360. My 360 is missing occasionally and I am running the N9Y Champions. I think they are too cold a range for the 9.4...