Spark plug heat range

[tparker]

I went out to get some stuff at the parts store and picked up some champion spark plugs at the last minute for my rebuilt 340.But I couldn't remember the right part number. I did a quick search on this site and saw someone that says they use RN14YC, I believe. When I got home I double checked and the manual said N9Y, which is what I recall in the back of my mind using back in the day. I did a bit of research on the numbers to figure out the code and the main difference is the number. THe higher the number the hotter the plug. I assume the 14s will be OK. But does anyone mess with different numbers? Any reason too? It seems various places on line suggest cooler is better and could translate to more performance. Any thoughts? Should I ditch the14s or keep them for a bit then change later?

Tom

[Mrbill426]

With Champion plugs the higher the number the hotter the plug and 14 is on the high side for sure.   Lots of factors go in but you won't know for sure until you run them a while and then read them.  Plenty of comparison charts with photos out there.

[Plange]

14 is pretty high when you really just want a 9 or 10. Not much more than an 11.