The BMW Internet lists have been ablaze lately over spark plugs! Why has a simple item been such a popular subject of discussion? Here's why
Many of you are aware (sadly so!) of the surging that is common in many of the R1100/850 series BMW Oilheads. This subject itself has been one of much discussion and I took some time to investigate it myself on my personal '94 R1100RS. My work led to my four part series on "R1100 Fuel Injection/Surging/Fixes", and later my "Zero=Zero" effort on getting the Throttle Position Sensor (TPS) optimized for best performance. However, along the way in doing this I learned that spark plug performance and selection can play an important part in overall performance of the R259 Boxer.
Being a thrifty person (a nice way of saying "cheap"!), I have habitually looked for aftermarket service item parts that meet or exceed OEM requirements but are more readily available and less expensive. Wear items like brake pads, oil filters, air filters, and spark plugs are open-discussion items for me and it wasn't long after my Dec '95 purchase of a used '94 R1100RS that I began the process that had ended with the sale of my '87 K75S.
I'd had good luck with Autolite 4153 plugs in my K and I was eager to avoid the extravagant price of the OEM Bosch plugs of about $10 fame. So armed with my Autolite and Champion spark plug catalogs, I cross-referenced to the point of the best possible match: Autolite 3923 and Champion RC9YC plugs. But wait; here's the difference.
The '94-spec OEM Bosch plugs for the R1100RS are FR5DTC, a unique three ground electrode setup that surrounds the center electrode from the side. The Autolite 3923 and the equivalent Champion RC9YC plugs are standard single ground electrode "over" the center electrode as many of us are familiar with. Would these "standard" plugs work?
I took the dive, bought Autolite 3923s, set the gap to the BMW specification of .031" or .8mm, and went for a ride. They worked great! I even noticed smoother power characteristics and somewhat less surging, and this was in early '96 before I began my above-mentioned Injection/TPS experiments in earnest. So you see, I've been using non-OEM plugs in my oilhead since about March 1996 (about 28K miles) with TOTAL success.
By the way, BMW changed their own plug spec to a two-electrode variety in July 1996 under the auspices of the "Low Friction Throttle Cable" service bulletin #2748 which was an attempt to solve the surging I was investigating at that time. In so doing, BMW revised the specified plug to the two-electrode Bosch FR6DDC, a slightly hotter running tip. Some have reported the newer Bosch plug is an improvement, but many other riders I have communicated with say the Autolite or Champion plug is, in most cases, quite superior in operation and surging minimization.
What's my favorite? Autolite. They exhibit minimal wear during the 12,000 mile specified service life and the center electrode color (on my RS) is a perfect medium to light tan on the insulator. My experience with Champions (in K-bikes) is satisfactory--quite good actually--but the Autolites seem to win out in life expectancy, electrode wear, and have a performance edge that I "think" I can feel. Perhaps this is just a prejudice, though.
Some riders have reported that the non-OEM Autolites or Champions are 1/8" longer than the original Bosch plugs. I don't find this to be the case with the Autolites. The measured length from the crushed gasket to the tip of the electrode is 22.5mm on the original 3-electrode FR5DTC and 23mm on the Autolite 3923. Perhaps the length of the FR6DDC is slightly shorter and/or the Champion RC9YC is slightly longer equating to the reported difference. I haven't measured the Champion, but in any case there have been no reported interference problems and all of these plugs are 14mm, ¾" reach and take a 5/8" socket. Use a 12 point thinwall socket to clear the small cylinder head access hole.
So here are your options, gapped at .031" or .8mm:
Bosch FR5DTC (3-electrode)
Bosch FR6DDC (2-electrode)
Autolite 3923 (1-electrode)
Champion RC9YC or RC9YC4 (1-electrode)
(Note: If you are hard-core Bosch, their 1-electrode Autolite 3923 equivalent is the F6DC)
One VERY good thing about these non-OEM plugs is their availability and price. For example, you can purchase the Autolites at a store such as Auto Zone or Checker for about $1.20 each and then get a $.50 rebate each on top of that. What a deal for better performance!