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.
Though Oak and I had corresponded several times about the project, and it was Oak that I had hoped to engage at the National, Tom Cutter provided two key facts: The first is that he welds aluminum washers (left) above the second spark plug port which provide enough thickness that standard reach and diameter Bosch W7DC plugs can be used top and bottom. This avoids keeping track of which plug goes where, and simplifies spares and reordering. Tom also indicated that, through his years of familiarity with the process, he has refined the location of the bottom hole to be offset as far away form the exhaust port as possible, minimizing the chance of cracking the head. I was a bit leery of Tom’s suggestion that a compression bump would give increased power without pinging, but both he and Snowbum agreed that with the timing settings and advance curve alterations needed to do the job right, 9.5:1 wouldn’t be a problem. And as Tom pointed out, the compression increase was easy to achieve by simply swapping out the pistons for a set that were originally specified by BMW for non-American GSs. Best of all, they’re still available as new parts from the factory.