Re: Surface Gap Spark Plugs?
Bob: Once you understand the engineering behind sparkplugs, then it all becomes simple.
Sparkplugs are designed in Heat Ranges for different applications. The heat range indicates basically how hot the plug will run in the engine.
The center electrode and porcelain insulator of the sparkplug are exposed to combustion temperatures and with each cycle they heat and cool.
The amount of center electrode and porcelain exposed to combustion temperature determines how hot they will run. Surface gap plugs have almost no electrode and insulator exposed to temperature AND the steel shell of the plug body provides a short path for cooling through the cylinder head. These plugs run COLD. In a 2 cycle engine where there is one power pulse per revolution, (twice that of a 4 cycle) the plug is exposed to MORE heat and less cooling time, so frequently, these plugs are specified for outboard engines.
On the positive side, these plugs protect the engine from damage. Yes, I have seen engines designed for surface gap plugs with holes melted in the piston crowns as a result of using bent electrode plugs --Evinrude QL77 type--(which run much hotter). On the negative side, these surface gap plugs require the extremely high voltage of an electronic ignition. Most will not fire at low speeds with a magneto ignition or a regular points and battery ignition. The do tend to foul and misfire with prolonged trolling or low speed operation.
Many people say that there is no danger in substituting bent electrode plugs and the idle substantially improves, BUT: Do you think it was an accident that Mercury engineers specify the L76V surface gap plug for their engines?
As far as multiple electrode plugs like Splitfire or Bosch, all a bunch of marketing hype. B.S. and hogwash of the first degree.
Electricity takes the shortest path OR the path of least resistance. With multiple electrode plugs like the 4 electrode Bosch, the spark will preferentially jump to only one electrode until that electrode has higher resistance than another one. There will NOT be multiple sparks at one time. (Note that MSD ignition modules DO generate multiple sparks at the same ignition cycle but I do not have an opinion on any benefits outside of racing)
Hotter spark? Phooey! Blue-white is blue-white. What is a couple of hundred degrees difference (if any at all) in a couple of thousand when there is more than sufficient temperature to ignite the air/fuel mixture? Again, marketing hype! Why use a propane torch to ignite a cigarette when a match will do it just as well?
For your engine, use the BRAND of sparkplug that you like in the STYLE specified by the manufacturer.