[gone-ot]

Normal spark plug (electrode) temperature is around 1,000ºF, while arc temperature is vastly HOTTER; so, guess which one actually ignites the A/F mixture (short of knock)?

Flame propogation, however, is more a function of the gasoline composition, its octane value, the compression ratio, and A/F mixture movement (swirl) and homogenity (mixture evenness).

Some SAE documents on this subject:

• SAE 690018 - Inlest Manifold Water Injection for Control of Nitrogen Oxides--Theory and Expedriment.

• SAE 700081 - Spark Plug Design Factors and Their Effect on Engine Performance.

• SAE 710832 - Effect of Compression Ratio, Mixture Strength, Spark Timing, and Coolant Temperature Upon Exhaust Emissions and Power.

• SAE 710835 - Effect of Engine Intake-Air Humidity, Temperature, and Pressure on Exhaust Emissions.