Join Date: Jul 2001
Location: San Diego, California
Spark Plug Gap – 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.
So, the larger the plug gap you can run (without misfires) the better.
Unfortunately, the greater the plug gap, the higher the voltage requirement to jump the gap. The difficulty lies in that, any added gap creates more strain on the other ignition parts. Coils, for example, may not have enough stored energy to cross the gap, creating a misfire. Old plug wire insulation can break down at the higher voltages required to fire a larger gap.
The stock Ducati coils are good enough to fire a 0.044-inch plug gap. Any gap larger than this will likely result in more misfires (there’s always a few) and subsequent power loss. The correct new plug gap is specified at about 0.020-inch. But, remember, as a plug wears, the gap opens up further.
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, 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 larger gap. The NGK dash 9 series starts at a 0.9mm (0.035-inch) gap, and is used for that reason.
If you run iridium or platinum electrode plugs, start with the 0.035-inch gap that they are shipped with. Don't run them at smaller gaps or you'll loose throttle response.
If you have a older bike, you may arc over the plug wires before you can reach the optimum 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 jacket will start to break down after a couple of years which is why most good aftermarket wire is insulated with silicone.
If this is the case, 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 916 Magnecor #2549 wires, for example, run $67.
Running a larger gap is the main benefit of installing an ignition amplifier, such as the one sold by Evoluzione for Ducati's.
The Evoluzione ignition amplifier increases the primary voltage to the stock Ducati coil from the existing battery voltage to either 16 volt or 18 volt (user selectable). A higher primary voltage means you get a higher secondary voltage applied to the wires and plugs. The higher the secondary voltage - the larger gap it will jump across. Evoluzione recommends that for best throttle response, you run a 0.060 inch plug gap.
One reservation that I have about ignition amplifiers as a group is that they could possibly cause overheating and eventual 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, but throttle response is the major benefit.