Quote:
Originally Posted by nubbzcummins
....Ceramic body of plugs from hex head to maybe 1/4" toward top of plug was turning a burnt rusty brown color though........
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You are hitting on the entire purpose of having a full length insulator.
This weekend, I pulled the original plugs from my wife's 2007 Honda Fit, and the insulators had a very light brown color to them, which is perfect.
Back in the 1970's, motorcycle ignitions were notoriously weak in voltage, so if the ceramic insulator was too short (i.e. cold), and you happened to ride a two-stroke, the insulator would get a thin film of oil baked on it, then that cylinder would stop firing all-together.
Why?
Because the insulator is *supposed* to run hot enough to keep cylinder deposits from building up on the insulator, and keep it from conducting the voltage from the tip, down the ceramic to the metal casing.
To make the most of the available spark energy in the ignition system, I would want the insulator to be the highest resistance (to ground) item inside the cylinder, and keep it that way.
With a strong spark, most of the available fuel will be more fully burnt, and thus give better fuel economy.
Placing a plug with a shorter insulator only compromises the spark system, and increases the chances that the spark will leak off some of it's potential voltage before jumping the gap across to the electrode, because when it's colder, it can not burn off the carbon deposits more readily.
The objective is to keep the insulator hot enough to keep it clean, but not to the point of over-heating.
Cycle magazine had some excellent articles from the 1970's, on how to read spark plugs. These would go a long way in helping to the get the proper spark plug range for each running engine with it's specific requirements.
Jim.