Twin Tip Iridium Plugs - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 2 (permalink) Old 05-20-2020, 11:18 AM Thread Starter
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Twin Tip Iridium Plugs

I REALLY want to try a Twin Tip Iridium spark plug (https://www.denso.com/global/en/prod...lug/iridiumtt/)
But they don't have a cross-reference for the plugs in this bike.
Does anyone know how to interpret plug types well enough to see if they make a compatible plug?

Why twin tip?

From Wikipedia:
"The central electrode is usually the one designed to eject the electrons (the cathode, i.e. negative polarity relative to the engine block) because it is normally the hottest part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces. In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, from a sharp point or edge rather than a flat surface (see corona discharge). Using the colder, blunter side electrode as negative requires up to 45 percent higher voltage, so few ignition systems aside from wasted spark are designed this way. Waste spark systems place a greater strain upon spark plugs since they alternately fire electrons in both directions (from the ground electrode to the central electrode, not just from the central electrode to the ground electrode). As a result, vehicles with such a system should have precious metals on both electrodes, not just on the central electrode, in order to increase service replacement intervals since they wear down the metal more quickly in both directions, not just one."

In our bikes, a given plug will always fire in the same direction every time (I think), but the two plugs in each cylinder are on opposite polarities of the same coil so one is always firing the opposite direction from the other in the same cylinder. The narrow tip plugs (platinum/iridium) are only advantageous on negatively fired plugs.
I suspect that in situations where the ignition voltage is marginal, the positively fired plug fails first.
That means if the voltage is just barely enough, our bikes could be running on only one plug and most of us wouldn't notice.

For example, I ran for awhile (unknowingly) with a bad plug wire that shorted on the block. I noticed that moving the cable increased my idle RPM and my off-idle throttle response. Loosing one plug in dual plug systems typically results in 5-10% performance loss (based on articles I've read). New plug wires made the bike faster, but I can't say how much faster.

Additionally, Denso claims reduced fuel consumption by 2.4%, and a faster flame growth (effectively like an ignition advance).
But that's marketing so who knows if that's true. Their claims are at least plausible.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #2 of 2 (permalink) Old 05-21-2020, 11:33 AM
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Well, I'll swear the Splitfire plugs increased mpg on my Pontiac. Never bothered to try them in anything else. Not sure they're even in business anymore.

There's a lot to consider on spark plugs. Thread size/pitch, tip length, and heat range, all critical.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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