neubie need engine size explanations
If heat wasn't key, you wouldn't need a glow igniter. The metal composition has to do with it's ability to absorb and radiate heat (as well as longevity properties in the harsh environment in which it is subjected to).
If you take a new glow plug, while holding it upside down, take a thermal temp on it (I use my raytec temp gun). Measure the temp of some hobby fuel. They will both be at ambient temperature. Put a drop on the glow plug coil and measure the temperature every couple of seconds. You will find the temperature doesn't change. I don't buy into the chemical catylist theory.
With respect to traxxas calling the new engine a 2.5, their previous generation engine (which they still sell) was called the traxxas .15 pro. In my opinion, they used it to differentiate between the two.