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Old Wizard
3,000 Posts
Discussion Starter #1
Regarding the use of platinum and iridium spark plugs in a Ducati:

Platinum or iridium plugs will give you worse performance than a conventional plug unless you use a larger gap than is recommended for the steel electrode plug equivalent. One by-product (and benefit) to having platinum or iridium as an electrode material is that the harder material erodes more slowly and consequently allows you to reduce the size of the center electrode and still have a long-lifetime plug. Re-gapping is infrequent or eliminated. In fact, the initial reason this type of plug was developed was an attempt to meet the 100,000-mile durability/maintenance requirement mandated by the US EPA for exhaust emissions, not because they offered any improved performance over conventional electrodes.

A smaller electrode, however, will arc at a lower voltage. This is good because the lower arc-over voltage is not as demanding on your less-than-new ignition coils and wires so the firing is more reliable. But this is also bad because a lower arc-over voltage presents a weaker spark kernel (lower arc current and duration) that is less likely to light off the air/fuel mixture.

Consequently, dyno testing shows a performance gain with specialty plugs only when their intrinsically lower arc-over voltage has allowed users to increase the plug gap above that possible with conventional steel electrode plugs. A larger plug gap needs a higher arc-over voltage to fire, and a larger gap, combined with good plug wires and coils, will span more fuel molecules resulting in a more reliable burn with fewer misfires. So you get better throttle response.

Not more power mind you, better throttle response.

The transition between throttle positions involves a wide range of fuel/air mixtures and the ability to fire these less-than-ideal mixtures with a minumum of misfires is what throttle response is all about.

When it comes to spark plug gaps, bigger IS better. The larger the spark kernel that is generated by a spark jumping the electrode gap, the more likely and complete the fuel burn will be, and the smoother the engine will run. That is, the larger the spark gap that’s exposed to the air/fuel mixture, the easier it is to initiate combustion. This translates directly into improved throttle response.

Conversely, I have seen several examples of Ducati throttle response problems cured by replacing platinum/iridium plugs that were gapped too small (i.e. the 0.024 in. Ducati recommends for conventional plugs.) Both NGK and Denso pre-gap their Ducati application specialty plugs to 0.035 in. This should be considered a minimum gap for this kind of plug.

If you have a older bike, you may arc-over the plug wires before you can fire an optimized larger plug gap. If the spark plug wires have inadequate insulation, the wire cannot maintain a high enough voltage across the insulation and will arc to ground before firing the plug gap. The factory spark plug leads are stranded wire covered with an EPDM jacket and although the wire itself will last a long time, the insulating jacket will start to break down after a couple of years which is why most good aftermarket wire is insulated with silicone.

If this becomes a problem, replace the stock spark plug wires with a set of Magnecor or similar quality wires. This will allow running a larger plug gap without a concern for insulating the higher voltage needed to jump the gap. Ducati Superbike Magnecor #2549 wires, for example, run $67.

For street bikes, you should use carbon core wires, preferably carbon wires with a spiral wrap center conductor. Straight, multi-stranded, unshielded wire conductors offer theoretical gains resistance-wise, but produce lots of electromagnetic interference (EMI). One major concern is with the computer found used on fuel injected bikes since the radiated EMI can interfere with the computer and corrupt sensor and internal signals which can affect engine performance and reliability. This concern also extends to the use of non-resistor type spark plugs.

For older Super Sport bikes, Dynacoil replacement coils are also available and a recommended upgrade.

76 Posts
Thanks for the info Strega & Belter. I have to assume my larger gap was deliberate & will keep it that way. The search function on the site is not well or I am not good with it.
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