Spark plug gap for high-compression pistons? - Speedzilla Motorcycle Message Forums
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post #1 of 8 (permalink) Old 02-20-2007, 04:23 PM Thread Starter
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Spark plug gap for high-compression pistons?

Can anyone recommend what gap to use with HC pistons (I'm running JE 11:1 compression pistons)? I think the standard gap is too large. Thanks - B
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post #2 of 8 (permalink) Old 02-20-2007, 05:33 PM
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Flamefront and combustion

Originally Posted by BrianK
Can anyone recommend what gap to use with HC pistons (I'm running JE 11:1 compression pistons)? I think the standard gap is too large. Thanks - B
I'd also like to know....

BTW:Which plugs are you using?

00' 996 SPS
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post #3 of 8 (permalink) Old 02-20-2007, 05:35 PM Thread Starter
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post #4 of 8 (permalink) Old 02-20-2007, 06:06 PM
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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.

Ignition Amplifiers

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.
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post #5 of 8 (permalink) Old 02-20-2007, 06:13 PM
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+1 on the larger gap.
Dyna coils is another option. CA Cycleworks sells them.
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post #6 of 8 (permalink) Old 02-20-2007, 06:49 PM
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Another item that increases the need for a stronger spark is pressure in the combustion chaimber. With any given gap, increased pressure places a higher demand on the sparks ability to bridge this gap...i.e. supercharger, turbo, nitrous, high compression. In a lot of cars, guys running blowers and big nitrous systems usually close the gap a bit to aid in preventing blowing out the flame so to speak, unless some good high output ignition is used to assist. One thing I'm curious about though is your 11:1 compression ratio?? That doesn't seem that high to me. Most Ducs these days are coming with 12:1 or better. Don't see where this will be a big issue for you.

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post #7 of 8 (permalink) Old 02-20-2007, 06:58 PM
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And you wont probably even notice the misfire, at least I didnt, until I actually got a really fine air/fuel meter hooked up. There will be a bunch of things determining your optimum gap. And even with the same engine setup air/fuel ratio will affect how easy the spark jumps. There is no easy answer.
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post #8 of 8 (permalink) Old 02-20-2007, 07:06 PM Thread Starter
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I have Dyna coils already, but since installing the HC pistons it won't catch. I'm hoping it's the gap, which is easy to fix.

Not sure about 12:1 Ducs - mine's a 97 900 SS and stock CR was 9.2:1.
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