spark plugs

26 replies to this topic
  • grayracer513

Posted October 04, 2007 - 12:57 PM


Starting with a larger gap (if you can jump it) will give you faster combustion. If you look at the time lag on a graph of flame propegation, the spark kernel growth is very slow up until about 0.100". At that point the graph just shoots almost straight up. Ultimately you will have to adjust your spark curves to fully take advantage.

there is a lot of tech in ignition, and still a lot of unknowns. What is known is that a larger initial spark, will burn the fuel/air faster, and when it needs to burn quick at 12000RPM, that makes a difference.

A larger gap, assuming you can fire it, does not produce any changes in combustion speed at all. It does produce more certain ignition, however, simply by exposing the air fuel mixture to a larger spark that is more likely to come into contact with an ignitable fuel molecule. If the spark ignites 1 or 1000 molecules at the same time, it matters little to the speed at which the flame front spreads, because the distribution of fuel in the air in the combustion chamber dictates that the flame must spread from one to another, and all that happens is that two or three molecules end up trying to ignite the same neighboring molecule. But at the same time, one can easily understand how that enhances the certainty of getting the burn started, and that can be very important. Any engine that misfires because of poor spark exposure can benefit from this, but what's important to understand is that in order for such things to improve on an engine's performance, the engine must at least occasionally misfire.

What can increase the flame spread a little is a multi-spark ignition in which a series of sparks are produced over 10 degrees or so of crank revolution, but this is only true if there is a great deal of motion in the air fuel mix in the immediate vicinity of the plug. A sort of reversed serial drive by shooting.

When you speak of looking at a graph, I'll guess that what you're seeing is a graph of the spark rise time and peak spark voltage as detected by an ignition oscilloscope. That's looking at the electrical energy in the secondary wiring, and not anything to do with the flame condition in the chamber. The reason a wide gap causes a display of a higher voltage is because more voltage must be built up before the current is able to cross the larger, higher resistance gap. If you want to see a real spike on a scope, disconnect one of the plug wires and look at that. By contrast, if you short that same wire, almost no voltage will display because none is required to start the flow of current in the wire.

A good secondary scope pattern would show two things: low voltage required to jump the gap, and a short "rise time". Rise time is the time beginning with the cut off of current in the coil primary, and the resulting collapse of the magnetic field built around it, and the point at which the voltage peaked and the spark actually happened. The shorter this time can be made, the better, because if the secondary voltage induced in the coil by the collapse of the primary field takes too long, energy building up at the plug electrode has an increased opportunity to run to a short across the insulator (fouling). If the build up is very quick, the spark charge develops its own kind of inertia, and when it gets to the end of the electrode, it just flies off, even if there actually is another path it could take if it turned the corner at the end.

And that's one of the reasons for CDI's right there. They use the fact that a capacitor will dump whatever is stored in it extremely quickly, much like a low voltage spark, and this becomes the source of the coil primary voltage. That gives the coil an increased primary current (for a stronger magnetic field) that will collapse faster (for a more rapidly induced secondary voltage and a shorter rise time), and a stronger, higher energy spark that will cross larger gaps under higher compression loads with less input energy.

  • Wes Woodin

Posted December 22, 2007 - 11:44 AM


Gray, with a multiple spark discharge ignition system you're not going to see a difference with precious metal plugs over standard plugs are you?

  • grayracer513

Posted December 22, 2007 - 12:18 PM


From a strictly measured electrical standpoint, as with an oscilloscope, yes, you would. As a practical matter, probably not.

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  • Wes Woodin

Posted December 22, 2007 - 12:21 PM


From a strictly measured electrical standpoint, as with an oscilloscope, yes, you would. As a practical matter, probably not.

I know the JD makes ignition systems. Do you have any experience with these, or would it be better going with a programable system like a vortex.

  • grayracer513

Posted December 22, 2007 - 05:00 PM


I'm not familiar with the JD ignitions, or what they offer as a performance enhancement. Programmables allow you alter the advance curve to change the power delivery characteristics a little. I don't know what you're looking for when you're looking at after market ignitions, though. The stock setup is not lacking in spark energy or timing accuracy in any way, so an MSD box really won't do much for you. An exception might be if you found yourself in a long distance race the included a large enough drop in altitude that the bike was running very lean, and misfiring because of it. MSD's are useful in such cases.

Otherwise, I'd guess you were looking to tweak the curve of an '08 to be more like an '06, or something. A programmable would be what you'd want for that.

  • smith3973

Posted December 23, 2007 - 05:44 PM


i think there is some advantage in the iridium plugs because they don't wear out, i haven't had to replace my iridium plug since i installed it, and i fouled mine all the time. But it is your choice.

  • grayracer513

Posted December 23, 2007 - 07:15 PM


On one level, you're right; precious metal plugs, iridium included, will last longer. OTOH, my son's '06 still has the OEM CR8E in it after a year and a half. I suppose I should change it pretty soon, huh?

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