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[dvst8r]

Is it better to have a "bigger" spark, or a "longer" spark? On one of the dirt bike forums I goto here and there, they were having a debate over a "spark plug mod" On an XR250/400 in the plug wire there is a spring and a resistor. Lots of people claim that it is an improvement to pull that spring and resistor and replace with a ss rod.

Now this would give the effect of a "bigger" spark, but slightly less duration?

Which would be better to have?

[walter]

it's the best to have the bigger ignition in the right time...

[dvst8r]

It is a fixed timing circuit, we are talking a more intense spark or a longer lasting spark, both would start at the same time.

[brine04]

both

[dvst8r]

both

Thanks captain obvious... 

[98vtec]

i would opt for a bigger spark.  IMO, a longer spark is going to contribute to wasted energy.  After the flame front ignition, there is really nothing else to combust.  The flame front travels down the bore and toward the cylinder walls, away from the plug.

[DmC]

Duration is what you want. The more duration you have the less chance you have of missfires. CDI sysytems commonly used on dirtbikes are used because they are small and the power of the intial spark helps keep plugs from fouling on a two stroke. 

[Foowee]

bigger spark over a short duration + low cylinder pressure = plasma tunnel and no burn(lots of people think they're experiencing breakup)... 

There is a point where you need a big short spark to get your burn going.  I'm fairly certain it's well above the power most people on here will make...

[98vtec]

why does it need to be any longer than what it typically is?  Ignition timing tuning anyone?

[Joseph Davis]

Sometimes the burn can be prolonged if the burn isn't propigated quickly in the first place.

[dvst8r]

So what I am gathering from this is that on an aircooled 19hp 250cc dirtbike, that the "resistor" mod is pointless.

[Joseph Davis]

Maybe yes, maybe no.  Well, the resistor trick yes but modifying the ignition on those might not be pointless.

I forget his name, but he posted 15-20 times on ex-HMT it'll come to me, dude was from India and big in modifying the late 70s early 80s Yamaha RD350's.  He was custom winding auto transformers to modify the stock ignition system or some aftermarket CDI??  Claimed a decent gain in power on the dyno.

[kgx]

EFI101 had a couple of good discussions on the subject a few years back:

http://efi101.com/forum/viewtopic.php?t=390

http://efi101.com/forum/viewtopic.php?t=1978

[HiProfile]

A longer spark will allow the mixture a better chance to fire, since it allows more time for ionizing the gap. You only want more powerful spark if you can't jump the gap. However it's best to reduce the gap size if you have that many molecules between the strap & electrode.

If they can't get it up with a massive 19hp gas-guzzler, they should spend more time with the mixture & plugs. I'll be taking the stance that they can claim all they want, however ass-dynos are about as reliable & consistant as a chinese space craft. They may be able to test it on a small bike dyno, but the difference will probably be w/in standard deviation - ie not measurable.

[DmC]

I have a really good ass dyno.

[widebody93]

BULLSHIT I'm stalking you on here

[Conceptz-X]

Also depends on combustion chamber design, quench, plug location, rod ratio...  all factors to how the fuel and air are mixed at combustion

if you have a lean spot at the plug from low cyl turbulence, a short duration spark may result in a misfire, no matter how intense the spark is...

[dvst8r]

In the end the best upgrade was the iridium plug. The starts are so much better, and it runs smoother.

[Conceptz-X]

Reduced gap resistance in the iridium means lower firing voltage and longer duration