A little something I found, from a long article on sparkplugs.
One of the biggest improvements in spark plug design in recent times was the
projecteded-nose configuration introduced (if memory serves) back in the
'60s. These plugs have what their name says: Both the ground wire and the
center electrode extend farther into the combustion chamber, and move the
spark gap with them to a point about .200-inch beyond the plug shell.
A projected-nose plug looks like its electrodes and insulator are terribly
exposed to the fire inside the combustion chamber. But these extended bits
also are out there where the incoming air/fuel charge can do a fine job of
cooling them. In an engine, the projected-nose spark plug runs hotter has
better resistance to fouling at light throttle openings, yet is cooled by the
blast of air and fuel it gets when you crank up a bunch of throttle.
The down side (isn't there always one?) of projected nose plugs is that in
some engines there just isn't enough clearance for them among the other
hardware, valves and piston crown, also occupying the combustion chamber.
It's a point to watch, but where there is room, and assuming the appropriate
heat range is available, projected-nose type plugs are the best choice.
An engine's ignition advance must be tweaked a little to adjust for the burn
rate changes produced by the different nose configurations. The old retracted
gap racing plugs needed more ignition advance because they started the fire
back in a hole. Projected nose plugs start the blaze out where it can spread
easily, so an engine fitted with such plugs need less advance.
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