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So, my guess is that the hotter plug would be better in order to combust all the fuel more efficiently?

Soren



Not really. Plug heat range has to do with the ability of the plug to preclude heat at the tip. Too cold a heat range lets deposits build up on the plug. Too hot a plug lets the tip burn up from combustion temps. You can see that cold plugs have very little porcelain around the center electrode exposed to the combustion chamber, so it has less ability to pick up heat. Hotter plugs have more porcelain so they see more heat. On a modified engine, you may need a slightly colder plug, as the engine, making more power, will be making more heat.

A too hot spark plug can get to glowing and cause pre-ignition (not to be confused with detonation). A too cold plug can possibly lead to misfire, especially under a greater load, such as hard acceration, or going up a hill with your oversized sister-in-law on the back.

To get a proper heat range plug, start with a new stock one and "read" it after a hard, full throttle blast down the road (and shutting off the engine, pulling in the clutch, and coasting to a stop). The center electrode porcelain should be colored somewhat. What you are looking for is a medium brown deposit way up inside the plug, which sometimes takes a lighted magnifying glass to see. If the plug is pure white, it's lean; if it's really dark brown to black, it's too rich.

At least modern gasoline, though lower in octane rating, has less "junk" in to to foul plug gaps. That's why new cars can get away with such long service intervals on plugs.

What helps combustion is a large gap and a higher output ignition box (to be able to jump that larger gap). This gets the flame front going better. A winter bonfire is a good analogy: it's easier to start a big log fire with a plumber's blow torch than a small wooden match. Same in an engine with small and large plug gaps.