plug gap with Dyna and Accel coils/wires?

Hello,
I was just wondering what the consensus is regarding plug gaps in modded ignitions. I have a 76 KZ900 with Dyan S, Accel coil and wires. Book says .028" to .032" stock. Would it be OK to widen it to say .035"+ with that hot of a spark? Or is that pushing it?
Thanks

I think that's about what the forum member "Otakar" run on his bike, but I believe he's got the 2.2 ohm gray Dyna coils with some custom made sparkplug wires.

His plug gap may be as high as 0.045" or more.

Thanks, Ive bumped them open to about .035 and may go higher based on some test runs. Didnt have time to take it out afterwards though...

I have never seen data showing why that would improve performance. The ignition is a flyback converter design where a fixed amount of energy is stored during the "switch closed" duration and released across the spark gap when the switch opens. Increasing the spark gap will increase the voltage level at which the spark occurs but will not change the total energy delivered to the spark once it fires. FYI, opening up the gap does increase the voltage stress on the secondary winding since the spark occurs at a higher voltage level. On old (stock) coils it is probably not a good idea. I assume newer coils could handle it, I just don't see how it would increase performance.

As always, I wonder, if any increase in power or economy could be obtained, why would the bike makers not be shipping them with wider gapped plugs to take advantage of it?

bountyhunter wrote: As always, I wonder, if any increase in power or economy could be obtained, why would the bike makers not be shipping them with wider gapped plugs to take advantage of it?

I hear you, but I think with all aftermarket parts, its a cost decision. I mean, you could ask yourself why wouldnt every 350 chevy motor come factory with all those aftermarket go fast parts that are dynoed to make power. The original manufacturer has to view cost/value/performance/functionality/etc of everything it puts on the machine in order to sell it to the masses.

What Bounty hunter says about the stress being increased on the parts is true. But ignition parts can almost be measured in decades, rather than years, so if you cut the life in half, from 30 to 15 years, what are the chances you'll notice?

However, if the parts can handle it, the motor will seem more lively with a larger gap. I don't know if you would notice any difference from .032 to .035, though. You'd have to really bump it up to .040 and I think Otto bumps it up more than that considerably.

On a well designed, well tuned motor, (stock engine), research has shown that spark energy won't alter performance by much. If changing spark energy changes the running by a noticeable amount, then usually that means something else is wrong. The flame front, starting at the spark, should move faster than the swirl of raw charge moving through the spark, so that means after the initial arc, the spark is just arcing through exhausted gas, so makes no difference.

Increasing the size of the gap, though, increases the size of the initial flame front, so this will increase rate that the mixture burns.

Notice, in either case, the spark duration won't matter much, and with flyback ignitions, greater spark duration requires greater spark energy (all else being equal). So the spark energy, in a stock motor (designed to run good at all RPMs), won't really affect the performance.

John De Armond had a great article on it somewhere.

hal0tw0 wrote:

bountyhunter wrote: As always, I wonder, if any increase in power or economy could be obtained, why would the bike makers not be shipping them with wider gapped plugs to take advantage of it?

I hear you, but I think with all aftermarket parts, its a cost decision. I mean, you could ask yourself why wouldnt every 350 chevy motor come factory with all those aftermarket go fast parts that are dynoed to make power. The original manufacturer has to view cost/value/performance/functionality/etc of everything it puts on the machine in order to sell it to the masses.

No, actually, Kawi has been making a pretty good electronic ignition for about 30 years that is pretty comparable in performance to aftermarket types. I still would ask, if widening the gap made an improvement, why don't they do it? If there is any actual dyno data showing a repeatable improvement, I would be interested in seeing it.

MFolks wrote: I think that's about what the forum member "Otakar" run on his bike, but I believe he's got the 2.2 ohm gray Dyna coils with some custom made sparkplug wires.

His plug gap may be as high as 0.045" or more.

Yes I run considerably more. Here is a thread that has a discussion about it.

kzrider.com/forum/4-electrical/351334-wh...plug-gap-do-you-run-

loudhvx wrote:
Increasing the size of the gap, though, increases the size of the initial flame front, so this will increase rate that the mixture burns.

I am not sure I get that. In reality, you want a slow burning flame to avoid pinging. Higher octane fuels actually burn slower preventing the flame front from "jumping ahead" or jumping around which causes pinging.

I have not seen any data indicating that a taller spark makes anything burn better. If there is data, I would be interested in reading about it.

otakar wrote:

MFolks wrote: I think that's about what the forum member "Otakar" run on his bike, but I believe he's got the 2.2 ohm gray Dyna coils with some custom made sparkplug wires.

His plug gap may be as high as 0.045" or more.

Yes I run considerably more. Here is a thread that has a discussion about it.

kzrider.com/forum/4-electrical/351334-wh...plug-gap-do-you-run-

What exactly is it supposed to improve and has it ever been measured?

Well, really, the same could be said about making the gap smaller. If the gap just needs to be some tiny amount, why make it as big as it is?

It's probably just a tradeoff and they (probably not Kaw, whoever makes plugs or ignitions etc. maybe even Kettering himself) arrived at the gaps that come stock for reliability, fuel economy, (emmisions?) etc.

I don't have any dyno data, but I'm pretty sure i didn't imagine the difference. But a good study on it would be nice to see.

bountyhunter wrote:

loudhvx wrote:
Increasing the size of the gap, though, increases the size of the initial flame front, so this will increase rate that the mixture burns.

I am not sure I get that. In reality, you want a slow burning flame to avoid pinging. Higher octane fuels actually burn slower preventing the flame front from "jumping ahead" or jumping around which causes pinging.

I have not seen any data indicating that a taller spark makes anything burn better. If there is data, I would be interested in reading about it.

It's the same idea behind using two plugs firing at the same time in the same cylinder. I don't know about buring "better" but a larger flame front will spread faster, similarly at two fronts will burn the charge faster.

And yes, this means you can run with less advance on the ignition. This is advantageous since you won't start building pressure from combustion until much later... so it won't be trying to push the piston back down while it is coming upward on the compression stroke. This frees up some crank horsepower for moving the bike forward, rather than trying to compress the burning gases. So if you keep the timing advanced to where it's on the verge of knocking, opening up the gap may push it over the edge and cause knock. Hopefully there is some margin there, and the rider can recognize detonation when he hears it. I think twin plug heads use less advance (at least the ones where both plugs fire at the same time).

At least this is the thoery as I understood it.