ORIGINAL: Chex
...last I heard, you match the ambient temperature to the plug rating, ie cold plugs for cold outside temperatures.
I think thats how its shown on the back of OS plug boxes, and I think thats what Rody said on the RB forum.
so unless I/we are wrong then I would the plug rating to the ambient temperature, more than nitro content
What he said...
And also depending on what you want from your engine.... hot plug, more ignition advance, more bottom end less top end... and vice versa...
