Originally Posted by John CC

Let me first say that I am not an expert on this particular ignition system, but....

The system works by detecting ion flow through the spark plug gap prior to the ignition event. Ion flow is allowed by the presence in the gap of combustion byproducts caused by preignition. The "resistance" if the spark gap is huge compared to the resistance of the plug and plug wire, to the point where these other components are insignificant to the task of detecting preignition. If this were not the case, the natural increase in the plug gap over time, and the predictable variation in setting the gap in the first place would have a much bigger effect on the system than the properties of the plug.

So, IMNSHO, the requirement to use only Harley Davidson plugs originates from the marketing department.

TMSAISTI!

Originally Posted by Germansheperd

That is not correct. The Harley Davidson 6R12 plug is exactly a Champion RA8HC not an NGK. Put in a pair of NGK DCPR7E and take the gap down to .038 and the bike will run better/smoother.

Originally Posted by RIPSAW

Do you know what years Harley uses this system?