Here is some of my experience with plugs.
First, there IS A DIFFERENCE in plugs and it pertains to the how and where in their operation.
As has been said, the plug is the final conduit for the voltage generated in the coil. The gap is the exposed part of that conduit. The "quality" of that gap, where the gap sits in the combustion chamber, how that gap affects the voltage of the spark and the quality and size of that initial ball of flame that starts the combustion process will have an influence on the affectiveness of that plug.
I had a GSXR1000 in the shop a few years back that had headwork, milled head and a set of cams. The work was done by a popular race shop for sportbikes. During the dyno work here at my shop we pulled the plugs to inspect them. We found a set of Brisk plugs. I had never heard of them and was very interested in them as they were very different from what I had seen before. I am guessing they could be classified as a "surface discharge plug" as they had no ground electrode. This is the "how" the plug design affects the spark quality.The spark had to jump from the center electrode to the base of the plug, and that distance was at least .100", quite large. A gap this large would have the affect in raising the voltage required to jump that gap but also shortening the duration of that spark.
Now the "where" aspect. Since the spark must jump "back" to the base of the plug, that initial ball of flame is now starting on the side of the plug. That flame front now has the protruding spark plug in it's path somewhat blocking it's path across the combustion chamber. Such a large path for the spark would suggest a HUGE ball of flame and good and complete combustion.
The plugs were clean and in good shape, but as my curiosity of thier performance grew I decided to install the plugs we use in the GSXR's for a back to back comparison. The plugs we use are platinum tipped dual ground electrode plugs. NGK brand. The dyno runs were less than one hour apart and the results were impressive.
The NGK plugs started to show a power gain at 5000 rpm. There were gains of 2.6 hp @ 7000, 3.4hp @ 8000 and 9000 rpm, 2.9 @ 10,000rpm with a 1.3 hp gain at the peak! The brisk plugs wre no match. I have also seen no gains in performance with E3 plugs when compared to the plugs I sell.
Bottom line, where the plug gap sits in the combustion chamber, how much of the air/fuel mixture is exposed to the gap and how little the plug "blocks" the flame front from spreading across and thru the combustion chamber is all important. The 1700's use an "extremely projected tip" Iridium plug that is one of the best designs available and I have dyno results and customer feedback documenting that fact.
Iridium is a very hard metal so electrode wear is minimal as compared to a copper or platinum center electrode plug. These plugs can probably last 50,000 miles!
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