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FORUM GODFATHER.....R.I.P. PAT
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Dual Plug conversions were popular with the BMW airhead flat twins. More complete combustion.
 

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In laymens terms the second plug is used to insure the mixture in that cycle has a better chance of being ignited. With the lean mixtures ( especially at cruise ) it is possible that the mixture in the area of one of the spark plugs is too lean to fire ( or sometimes fire but "die out" because mixture ratio won't promote a flame front ). The other plug likely has a better chance of igniting the mixture. They also promote a better quality of burn. Two flame fronts can burn the mixture quicker and that can eliminate detonation. Not all of this is for emissions, although you can imagine what unburned fuel does for emissions, but the two plugs can make more peak power. Two plugs per cylinder were originally considered a "band aid" for a poorly designed combustion chamber. With the emissions and fuel economy pressures on engines now, the two plug design is being utilized from the start of engine design instead of being a "band aid".
 

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Because three won't fit. :confused: Sorry, I couldn't resist.
 

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FORUM GODFATHER.....R.I.P. PAT
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Dual Plug conversions were popular with the BMW airhead flat twins. More complete combustion.
I did the conversion on my R90S. There was a time during the conversion to unleaded fuel when premium was not available. Dual plugging allowed the use of regular because two flame fronts shortened the time to complete combustion requiring retarding the spark timing significantly.
 

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FORUM GODFATHER.....R.I.P. PAT
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you didn't have Amoco stations in your area ?
I regularly rode out of my area and even Amoco did not convert all their stations from leaded premium to unleaded premium immediately. Believe me, finding premium was impossible in many locations for quite a while. People bought bottles of octane booster to get them through the transition. Those with unhardened valve seats pretty much lost their engines. Shops did a big business in dual plugging and hardened valve seat replacements for a while.
 

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Discussion Starter #9
band aid for not having high enough octane fuel

Two flame fronts can burn the mixture quicker and that can eliminate detonation. Two plugs per cylinder were originally considered a "band aid" ... ...
it may not completely eliminate detonation but, it helps. (not quicker) helps by igniting all fuel at the same time. so as there will be no small deposits of unburnt fuel on the outside edge of the flame, which can ignite on their own due to heat & pressure and cause detonation. its a "band aid" for not having a high enough octane fuel.
 

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FORUM GODFATHER.....R.I.P. PAT
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Igniting all the fuel at at the same time is the definition of detonation. Two flame fronts allow retarding the timing and reach all the points in the combustion chamber before a distant point would self ignite from heat and pressure. They also burn the very lean mixtures in modern engines more completely. It's not a band aid but a very efficient combustion chamber design.

Edit: I later realized I need to say the above is mostly about symmetrical dual plugging. Suzuki just added a plug at the periphery of the combustion chamber. It is a band aid to meet emissions regulations and I was disappointed the plugging was not done symmetrically. I guess that would be difficult in a four valve combustion chamber.
 

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In laymens terms the second plug is used to insure the mixture in that cycle has a better chance of being ignited. With the lean mixtures ( especially at cruise ) it is possible that the mixture in the area of one of the spark plugs is too lean to fire ( or sometimes fire but "die out" because mixture ratio won't promote a flame front ). The other plug likely has a better chance of igniting the mixture.
My 1976 CVCC Honda (Civic) engine got around the ignition of a very lean mixture by having a separate pre-ignition chamber (in which the spark plug fired), with it's own small intake valve and separate "barrel" in the carburetor. The flame front would then travel into the main chamber to burn the leaner mixture there. Other than unrelated problems with the engine and car, it worked very well, and gave efficient and clean operation.

With engines going to four valves, the "squish area" around the combustion chamber became smaller, decreasing turbulence of the air-fuel mixture in the chamber, resulting in uneven mixing - and the occasional mis-fire. Further modifications to combustion chamber geometry helped. So does the addition of a second spark plug.

The much heralded Hemi-Head combustion chamber was actually quite poor in this regard (squish area and resultant turbulence). The hemispherical combustion chamber in my 1965 47 HP 1290cc 403 Peugeot worked quite well. I could out-accelerate Mercedes 190 diesels and some earlier VW beetles.

I remember reading the whole of Harry Ricardo's The High-Speed Internal Combustion Engine when I was at University (It was old then, but relevant as a source of the history and technology of engines). That and David Vizard's The Theory and Practice of Cylinder Head Modification, a few years of the English magazine Cars and Car Conversions, Gordon Jennings' articles in Cycle Magazine and way more of the same crooked my mind back in the early seventies. All good reading, but I was never the same since.

Maybe why I ride a VStrom.

Marc
 

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FORUM GODFATHER.....R.I.P. PAT
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The CVCC engine was so well designed, it met future emissions standards without a catalytic converter. Detroit auto manufacturers were claiming the emissions standards Congress was calling for in the near future could not be met. The CVCC and the Mazda Wankel available as those talks were going on already met them.
 

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FORUM GODFATHER.....R.I.P. PAT
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:confused:Amoco Premium never had lead in it from day one, long before the requirement for unleaded
I didn't know that. Still there were places in my travels that had no Amoco stations and no premium of any kind. In 1976, before the lead free era, I had to put regular in my CB500 on a Colorado trip. The altitude saved me.
 

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AMOCO premium ( cannot remember about their regular grade ) was indeed lead free. Called white gas by some, it was good to burn in Coleman lanterns and stoves.......but of course that is another topic
 

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it may not completely eliminate detonation but, it helps. (not quicker) helps by igniting all fuel at the same time. so as there will be no small deposits of unburnt fuel on the outside edge of the flame, which can ignite on their own due to heat & pressure and cause detonation. its a "band aid" for not having a high enough octane fuel.
Twin spark ignitions do indeed BURN the mixture quicker because the combustion process starts in two areas. Not designed solely to eliminate detonation but to increase the "quality" of the combustion event. That means decreased bad emissions, better fuel economy, more power, and less detonation.

helps by igniting all fuel at the same time That is pretty much the exact definition of DETONATION! You want a smooth controlled burn spreading out from the initiation point. When pressures and heat allow fuel to ignite ahead of this flame front, you have detonation.

It seems there is a preoccupation with high octane fuel by some. Modern combustion chamber design, fuel delivery, spark control allow regular 87 Octane fuels to operate with full power in what were once considered HIGH compression ratios. 11.5 to 1 compression is common in newer car engines. Check out the new Ford 5.0 V8's which....run on regular fuel.
 

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because

Because it is cheaper to add a second plug than to redesign the cylinder heads. A current, well-designed cylinder head has sufficient squish, turbulence, and tight piston-head clearance that a single plug is usually sufficient except in very large engines (flame travel speed doesn't scale, unfortunately). Presumably Suzuki didn't want to spend the money to update the design.
 

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the 83½ Nissan King Crab truck I had, had 2plugs/cylinder
So did the '86 model, not that you had it ...
I assume the 2 plugs per cylinder in the nissan fired 1 after another, since it had an 8 point distributor and don't remember linking inside the cap.
 

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