i've noticed what you're saying first hand, lloyd, after pulling clean plugs after a 1/4 mile run on the same stretch and same throttle in different temperatures.
Perhaps this will help. Probably just bore some people, though.
A liquid cooled engine has a temperature governed thermostat to vary the cooling effectiveness of the device being used to keep the combustion chamber temperature constant. It's not perfect, but when it is working, it does confine the range of temperture variance and provides an fairly consistent path for combustion heat to flow towards.
Our air cooled engines have cooling fins. They don't get larger or smaller when outside air is hot or cold, and this effects their ability to transfer combustion chamber heat to the surrounding air. Worse, headwinds and tailwinds, or the amount of airflow moving past the fins is variable under street conditions.
The perfect fuel mixture conditions we seek, are when all available oxygen in the chamber finds all the hydrocarbons during the combustion cycle. It's called Stoichiometric Combustion.
The oxygen content is governed by temperature, barometric pressure, and humidity in the air.
- Humidity has volume and as such it displaces oxygen in the combustion charge. It also slows the flame front as the charge ignites.
- Barometric pressure, either from wandering air masses about the planet, or changing altitude, effect how many oxygen and fuel molecules can be present in a given volume, due to the compression weight of the all the air molecules stacked above it in the atmosphere.
- Temperature effects the spaces between molecules, and thus the total population of the fuel and air molecules in the volume of charge being burned.
While the above factors effect a water cooled engine as well, the cooling fin heat transfer efficiency is also effected by these factors.
Cooling fins don't transfer heat as rapidly to hot, dry, thin air, as they can to cool, "wet", heavy air that contains many more molecules to retain heat while passing in contact or nearby the fin.
Heat is not picky about its avenue of conduction. Engine heat will transfer into the incoming fuel air charge also. This is significant not only from a charge density aspect, but also from a fuel flashpoint aspect as well. Start with the fuel temp, add ambient temp, add compression heating effects, add combustion chamber latent heat, and all these must tally below the flash point of the fuel, or the charge will all detonate before the spark initiates the fuel burn. If the piston is still on it's way up, it crashes into the explosion before it can be repelled by it.
This is one reason why our air cooled motors have to run on the rich side. "Rich" helps cool the engine and helps remove latent heat from the chamber. The incomplete burn keeps combustion temps down, too. It gives a "cushion" whereby when the engine is operated in the "corner cases", such as when the engine is hot, the air is hot, the humidity low, and the barometric pressure is high, the engine won't begin to detonate and self destruct.
It is not as efficient as an engine operating in Stoichiometric conditions. And, it is one reason why so few air cooled motors meet EPA mandates.
If you want to read plugs like a drag racer with a liquid cooled motor, you'd better do it on a 110 degree day, with a tailwind, low-to-no humidity, below sea level, with a deep high pressure area firmly entrenched throughout your test period. Then realize that at all other times, your plugs ought to look on the rich side.
Because the SOHC4 engine is actually overcooled by the fins in all but the most extreme conditions, the base ring will always be black/sooty. The engine head and thread base must keep well below combustion temperatures to maintain a significant heat draw from the combustion chamber. However, you will need temperatures sufficient for the plugs to self clean. The center insulator porcelain near the electrode tip should be near white. Graduate to tan as you look down into the plug body, then dark brown and black near the bottom where the cylinder head maintains a temperature below that of combustion. This is what a plug should look like after 500 miles in varying conditions. New plugs on a brief plug chop run will begin to show this deposit pattern, but it will not be as developed as one with 500 miles of varying conditions.
Another factor in the plug chop method is power output. The plugs give a good initial indication, but time and distance calculations tell about the power output. If you start rich and gradually lean the mix on successive runs, the times will diminish until you reach a peak for that motor under those conditions.
One final note, drag racers are concerned with engine life under max power for 1/8 or 1/4 of a mile. Do you really want your street bike to have a rebuild schedule equivalent to this? People that live on the edge, sometimes fall off.
Cheers,
