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Dual Plug BMW Cylinder Head

Dual Plugging the R100GS

What is dual plugging? It’s the practice of installing an additional spark plug into the cylinder head, with the goal of promoting quicker combustion and preventing detonation of the combustion charge (what most people call pinging). All sorts of wild claims for additional benefits are made as well, from wheelie popping horsepower gains and double digit mileage improvements to increased robustness (after dual plugging, you’ve got a second ignition system to fall back on in case anything goes wrong with the first one, or so the story goes). In my own case, I was looking for a reliability gain, and some additional power would be nice, but only if it didn’t sacrifice the ability of the motor to happily digest low octane third world gas.

A great deal has been written about dual plugging, with the Airheads Beemer Club’s Oak Okleshen representing perhaps the most experienced perspective. Oak was kind enough to forward a copy of his seminal paper on the subject, which provides not only an excellent introduction to dual plugging, but also dispels the myths as well. Robert Fleischer (aka Snowbum) has an extensive series of web available tech articles, some of which cover dual plugging, and that occasionally capture the pearls of wisdom cast about by Tom Cutter, another Guru that frequents the Airheads Mailing List. Ultimately, the best resource was a tech seminar given by Tom and Snowbum at the 2004 BMWMOA National Rally in Spokane (Oak was also on the agenda, but a health problem prevented his participation). During the Q&A that followed their talks I was able to get the latest story on dual plugging, and by the end had a strategy for moving ahead.

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Spark Plug Caps

On the early Airhead points type models, spark plug caps contained a resistor of 1000 or 1200 ohms. Using a resistor inside the caps is a better method than using highly variable (between manufacturer’s especially) “resistor” type spark plugs, in which some of those resistors were actually coils.  The resistance caps reduced Radio Frequency Interference, and helped form the proper type of electrical spark itself.   Later Airhead points models and all electronic ignition models used 5000 ohm caps for even lower RFI, with added safety benefit for the 1981+ electronics ignition…described later here.

The spark plug cap resistance has more than one purpose:
It reduces spark plug tip and ground electrode erosion, and therefore greatly reduces any gap change over the life of the spark plug, and does this by reducing the electrical current flow. It does NOT reduce the applied voltage. Reduces some types of radio interference. Works in a complicated electrical way with the coil to produce a quality spark at the spark plug for good fuel mixture igniting.   There are some other beneficial and rather complicated effects as well. Note that if the resistance is too large a value, then the EFFECTIVE spark ENERGY (combined voltage, current, and with SOME time effect) will be lessened too much, and, this means that using resistor spark plugs WITH resistor caps is a BAD IDEA.

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