Motorcycle Modifications, Repairs, Restores, and Upgrades:

inferno

General Spark Plug errata
The type of plug that works best in a particular type of engine depends on a lot of things - like the existing ignition mapping and the
combustion chamber design and a host of other factors.

GENERALLY, multiple electrode spark plugs provide better ignition "quality" at the beginning of the burning of the a/f mixture. That will
produce "peak cylinder pressure" earlier in the stroke. Effectively it's close to the same effect as "advancing" the ignition timing a little bit
(like a degree or so for the engines that we've looked at.
According to long proclaimed, often repeated, alleged fact, the spark will only spark across one of the electrodes each time it fires - whichever one has the least amount of resistance. I surmise that that means that the spark will be able to spark earlier with multiple spark points (in general) as compared to the single electrode plug that has to wait for the ignition system to develop enough voltage to jump a randomly occurring, high resistance gap - effectively "retarding" the timing!

Will a multiple electrode spark plug ever fire across 2 electrode at the same time? Seems pretty tough to imagine, as you'd need ~2x the spark energy to jump two separate gaps.

GENERALLY, part throttle requires more ignition lead than required a full throttle, so installing multiple electrode plugs will make the part
throttle power better (snappier) - responding as if the ignition timing was manually advances a degree or so with a standard, single electrode
plug.

GENERALLY, "lighting" off the a/f mixture in the general "middle" of the combustion chamber produces the quickest burn rate and requires the
least amount of ignition lead time which translates into less pressure pushing down on the piston as it is rising and approaching TDC on the
compression stroke - which leads to the power stroke, where the heated gases produce pressure that should now be pushing down on the piston to produce power that eventually gets to your rear wheel, pushing you down the road.


However, "extended tip" spark plugs sometimes run into simple geometry problems - like the electrode hits the piston and I'm a bit hesitant to
install "projected tip" spark plugs - with their long heat path from the tip of the side electrode as that electrode will get extra hot (causing preignition) and structurally weaken the electrode.
A spark plug with an overheated, weakened side electrode working as a glow plug, causing preignition, coupled with the harsh shock of detonation can lose the side electrode - with dire consequences.
I've seen, umm, let's see, umm, single electrode spark plugs with cleavage providing 2 spark origination points actually melt the side electrode under race conditions - not recommending them, at least for extended high rpm use.

Multiple electrode plugs are a good compromise for many bikes (but not all bikes) that respond to extending the spark point into the combustion chamber.

OFTEN, changing from resistor to non resistor (and vice versa) spark plugs alone will slightly affect (.25-.5hp) hp output - sometimes in surprising ways - I wouldn't even hazard a prediction!

If you are trying to find the best power using different spark plugs, you MUST also change fueling rates to optimize power and adjust ignition timing for each spark plug geometry change!

CBR900? Dual electrode plugs work better as a rule. We use the Denso plugs. We are also a Denso distributor - but, I'm pretty sure that someone with patience and access to other spark plug brands could do the same comparison tests with other brands of plugs if they were so motivated.

CBR600f4, there's a Denso plug that makes .5hp increase at high rpm as compared to the standard Denso or NGK plug (I don't have my notes here as to which one it is.) I do remember that it's NOT a dual electrode plug, though!