Spark Plug and Ignition

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This is an article regarding Ignition and Spark plugs on Ducatis. It was originally posted at another BBS site by LJ (aka Shazzam). With his permission it is being archived and posted here. This is a good read if you have questions regarding what spark plug to use!

Contents

Ignition System Types

There are basically two kinds of ignitions; inductive and capacitive discharge (CD).

An inductive discharge ignition charges a coil (inductor) that is switched using either breaker points or more commonly a transistorized ignitor module. A recent innovation is to replace the single coil with a coil adjacent to each spark plug. The benefit of coil-on-plugs is that they don't have lengthy ignition wires.

CD ignitions use a transformer to raise the voltage and charge a high voltage capacitor. When the capacitor is triggered (switched), it releases its energy more quickly (needed at high RPMs) and at a higher voltage than an inductive system. You typically get double the spark voltage and five times the spark energy. Another advantage is that the spark energy is independent of battery voltage in a CDI system.

Ducati Ignition Systems

A good ignition system needs to be able to ignite the combustion chamber fuel-air mixture with as few misfires as possible (there’s always a few.) When you reduce the number of missed combustion events, you improve overall power, fuel economy, and particularly, throttle response. Generally, missed ignition events are most likely to occur during throttle transitions when the fuel-air mixtures are changing rapidly. This is usually described as “poor throttle response”, the inability to respond rapidly to throttle inputs.

You can reduce these misfires if you can increase the size of the plug gap and still reliably maintain the intensity and duration of the electrical arc across the gap. A highly-compressed fuel-air mixture is difficult to ignite, so a larger gap increases the opportunity for fuel molecules to enter the gap and a longer duration, more intense spark allows for a wider window of time for, and likelihood of ignition.

But, increasing the plug gap places higher demands on the sparkplug wires and ignition coils because it takes a higher voltage to fire a larger gap. The coils need to robust enough to supply an adequate spark kernel and the plug wires need to be able to transmit this electrical energy at the higher voltages needed without failure of the conductor or insulation.

Plug Wires

The stock spark plug wires are constructed with a stranded core covered with an EPDM insulating jacket. Like most wires, the conductor itself will last a long time, but the insulation will start to degrade after a couple of years from engine heat which is why most quality after-market wire is insulated with silicone rubber.

If a spark plug wire with old insulation is too close to adjacent conducting components, the wire will arc across the insulation at a voltage lower than that needed to fire the plug gap and a misfire occurs. This is more likely to occur in damp weather since moisture in the air is a good electrical conductor.

So the first step in upgrading your ignition system is new spark plug wires.

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 performance gains, but unfortunately produce lots of electromagnetic interference (EMI).

EMI can interfere with the computer used on fuel injected bikes and corrupt internal signals which can affect engine operation. This concern also extends to non-resistor plugs. Theoretically, straight-wire components can provide more power, but the reality is that the EMI produced can and does corrupt computer sensor signals and computer operation itself. It also raises hell with the radio in the car next to you.

When the time comes, usually after a couple of years, I recommend replacing the stock sparkplug wires with a set of Magnecore wires. Magnecore #2549 wires for a Ducati superbike cost $67, but other after-market suppliers also offer quality EMI suppression wires.

Coils

Ducatis use an inductive ignition where coils are charged with 12 volts and then apply a high voltage to the sparkplugs when signaled by the igniter. The typical firing voltage is 8–10,000 volts, with higher firing voltages needed for a high compression engine. Once the spark jumps the gap, it only requires several hundred volts to maintain the electric arc while the coil discharges.

The main advantage in using an inductive system is that you get a relatively high current, long duration spark (2,000 microseconds) that works very well for low speed, lean mixtures and partial throttle applications. The main disadvantage is that because it takes a finite amount of time to charge a coil, and at very high rpms there isn’t enough time to fully charge the coil between firings. Fortunately, Ducati street bikes don’t rev high enough for this to be a concern.

Stock coils on early Ducatis can be improved upon by installing after-market coils such as those made by Dyna, Accel, or Nology. Their primary advantage is that due to the larger physical size of the coils, the duration of the spark is longer. An intrinsic limitation of after-market coils with more windings is that they take longer to charge and discharge, so they have less and less time to cycle as rpm increases. Therefore, their available secondary voltage drops (faster than small coils) as rpm increases.

Fortunately, the newer stock Ducati coils are quite good given that they’ll fire a 0.044 inch plug gap even though the factory recommended gap for standard plugs is around 0.024 inch. Consequently, there’s no demand for after-market coils for superbikes.

Ignition Amplifiers

Another way to fire a larger plug gap is to increase the operating voltage of the stock coil.

One product, the Evoluzione ignition amplifier increases the primary voltage to the stock Ducati coils from 12 volts to either 16 volts or 18 volts (user selectable). The way a coil works is that if you put in 12 volts (primary) and get out, say, 12,000 volts (secondary), then if you put in 18 volts you will get out 18,000 volts. Again, this requires coils and wires in good condition.

Evoluzione recommends for best results that you run a 0.060-inch (!) plug gap instead of a 0.024 stock gap on a superbike, the larger gap being the key to getting a stronger spark and increasing the overall likelihood of a combustion event.

One reservation that I have about ignition amplifiers is that they could cause overheating and premature failure of the stock coils or wires. This reliability consideration has to be balanced against improved performance. An independent test by Road Racing World magazine on a GSXR 1000 saw only about a 0.2 HP improvement. Again, the main benefit here is throttle response.

Spark Plug Gaps

So, if you’ve got good plug wires and coils (and maybe an ignition amplifier) then you can fire a larger plug gap.

If you use conventional sparkplugs, start with the recommended gap and try opening the gap up in 0.002 inch increments. You should note a progressively smoother throttle response if not more power. When the bike begins to lose power (misfires), go back 0.001 - 0.002 inch and this will be your optimum gap.

As a good rule-of-thumb, if you go more than 0.008 inch over the out-of-the-box gap you won’t maintain parallel surfaces between ground and the center electrodes. So if you reach that point, change to a plug that starts at a higher gap. The NGK dash 9 series starts at a 9 mm (0.035 inch gap), for example.

Platinum and Iridium Spark Plugs

The initial reason this type of plug was developed was to extend plug life due to US EPA-mandated exhaust emission system low maintenance requirements, not because they offered any improved performance over conventional electrodes. They incorporate electrodes made of harder materials that erode more slowly and consequently don’t need to regapped as often.

The big 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.

But remember, a smaller electrode will initiate an arc at a lower voltage.

Consequently, you should NOT run these plugs at the factory recommended gaps. If your coils and wires are weak then a smaller gap will reduce misfires, but should be applied only as an interim measure.

NGK and Denso pre-gap their Ducati application iridium plugs to 0.035 inch. This should be considered a MINIMUM gap for this kind of plug. Their smaller electrodes fire this larger gap at about the same voltage as a conventional plug fires the factory recommended gap - with one added benefit - a strong spark kernel across a larger plug gap = improved throttle response.

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. Try increasing the gap beyond 0.035 inch for further improvements. You should be able to go to 0.040 inch on a new superbike.

Said another way, platinum or iridium plugs in your Ducati will give you worse performance than a conventional plug unless you use a larger gap than is recommended for the steel electrode plug equivalent. I have seen repeated examples of poor Ducati running and throttle response problems cured by replacing platinum or iridium plugs that were gapped too small (i.e. at the 0.024 in. Ducati recommends for conventional plugs.).

LJ (Shazzam)

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