Originally Posted by Scott Fisher

I have run both C series and F series plugs in my onroad engines. I am going to ask Jim soon, but I believe you guys are both right and both wrong. The C plugs are the long body, and the F series are short body. (Although in the package they look like they are the same length.) The length of the body will make a difference in how "hot" they are. With the same diameter wire the C series is a hotter plug. I believe C stands for Caliente which is Spanish for Hot. I believe the F stands for Frio which in Spanish means cold. I don't know if one or the other has anything to do with on or off road engines. This is the first I have ever heard of that. I am not sure of all this, but that is what I think.