What symptoms would you see (just driving the car) if you are running to cold a plug?
You would begin to get carbon on the insulator of the plug. Due to plug dispelling heat at a faster rate than a hot plug the insulator would not stay hot enough to keep the carbon from building up. Eventually the plug might foul if the range was too cold. Fouling occurs when carbon coats the insulator portion of the sparkplug from the electrode in the insulator down the insulator to the threaded body inside the sparkplug. When the plug receives power from the coil the electricity runs across the top of the insulator in the carbon which is a conductor. At this point, there is no visible spark when the plug 'fires'
What would the spark plugs look like if you are getting either pre-ignition or deposits?
The plug could get a buildup of various deposits or have the electrode melted away based upon the cause of the pre-ignition. Cross-firing through plug wires can cause dramatic after effects on spark plugs after extended use under those conditions.
What does a plug running in the butter zone look like for a car engine, and also for a small two stroke engine like a snow blower or lawn mower?
It should like tan and the electrodes should have an intact look to them. In other words, physically they should have the same appearance as new but the slight combustion buildup and color are the give away that it's a used plug.
Different engines require plugs in different heat ranges. What characteristics of those engines create the need for the different ranges?
What drives sparkplug heat range selection
Characteristics of the engine such as cam lobe shape, crank type and amount of cylinders don't typically drive plug heat range selection. Things that do drive heat range selection are frequently on the list below.
Forced induction engines with a turbo or supercharger will drive a higher volumetric efficiency which can account for a colder plug selection to dispense heat more rapidly from the combustion chamber.
Naturally aspirated vehicles with a reduced quantity of fuel and oxygen could justify a cooler plug due the decreased need to dispense with heat due to reduced oxygen and fuel in the overall combustion charge.
High RPM engines that run in extreme rev ranges will have more combustion cycles per minute than a low RPM engine, due to increased heat a hotter plug would be required to, again, dispense with heat being converted from more air and fuel in a shorter period of time than a low RPM engine.