Originally Posted by madscientist
Originally Posted by Cab_Burge
The 1968 to 1971 340 motors came with N9Y plugs originally work
They are not available any more I've been told so the closest one to them is the RN9YC resistor plug and then the resistor 5/8 body RC9YC in the Champion line.
I had a 8.5 to 1 compression 350 Chevy iron head motor in a 21 ft.inboard out board ocean boat that would ping with the stock RJ12Y plugs in it and wouldn't ping with a R11Y plug in it work
H twocentse is at 10.+ to 1 compression ratio so I'm thinking a #10 or #9 champion should be just about right at his altitude



Cab, if you look up what Champion calls out in a newer catalog you’ll see it says a 12 heat range for that stuff. I can’t remember what year that changed, but IIRC there was an industry wide change in pump fuel blending (I think it was before OBDII but I wouldn’t swear to that in court), probably because of a federal regulation (wouldn’t swear to that in court either but that’s kinda what I remember) that required a heat range change.


I think they have upped the heat range on the older engine listings because they assume they are old and need it to keep them clean from deposits of a high mileage engine.