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where do the coil wires go on dual plug heads? #577134 12/19/14 10:25 pm
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kevin roberts Online Confused OP
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i had my T120 cylinder head drilled for dual plugs many years ago, and have run a markIII Boyer on them with excellent results.

i just read this on the Boyer website:

Quote
The Micro Power Coils
. . . With most dual output coils it is good practice to have only one output feeding a compressed cylinder, but with 35,000 volts it is possible to have both outputs feeding a compressed cylinder at one time.


i've been running both plug wires from a single dual lead coil to the same cylinders: left coil, left spark plugs . . .right coil, right spark plugs. the bike has run extremely well this way, but i'm wondering if i fail to understand something.

what are the recommended destinations for the four plug wires leaving two dual plug coils in a two cylinder motor?





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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577169 12/20/14 9:37 am
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Kevin, with your dual plugs, are you running less advance with the timing? If so do you know about what its set at ?

Last edited by konon; 12/20/14 9:38 am.

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Re: where do the coil wires go on dual plug heads? [Re: NickL] #577255 12/20/14 7:52 pm
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Originally Posted by NickL
Each cylinder would have one lead from each dual coil attached. That way each coil fires one compression stroke at a time. This will improve your sparks that's for sure.

Nick


this is what i read, now. originally, i connected the system leftie/rightie because i thought that if one plug would fire easier than the other, then all the spark energy from the coil would be diverted to the easy plug, and the compression plug wouldn't fire at all.

is that not the case? i'm not a rocket scientist when it comes to ignition.



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Re: where do the coil wires go on dual plug heads? [Re: konon] #577262 12/20/14 8:34 pm
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Originally Posted by konon
Kevin, with your dual plugs, are you running less advance with the timing? If so do you know about what its set at ?


nope, i put them in because at the time the best gasoline was 89 octane unleaded premium. i'd been trying all sorts of things to keep it from knocking-- MTBE made the pistons orange, Pep Boys octane booster made the plugs (and me) black and sticky, extra base gaskets did weird things to the rocker arm ratio.

when i put the two-plug head on, i left the timing at 38 BTDC, right on the rotor marks, and it ran great, no spark knock. it did what i was looking for then, and i haven't changed it since.

but i'm making a lot of changes to the machine, and looking much more closely at fuel and spark settings is in the plan.


every day you do not take a chance is a day of your life that you will never get back.
Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577265 12/20/14 9:09 pm
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and speaking of dual plug ignition ... It's not always known, but when fitting dual plugs in a single cylinder bike like our Goldies, you Must use two separate coils with the primaries connected in series vs fitting a dual output coil. Bear in mind a dual output coil isn't two coils in one case, but there's a spark plug lead connection to either end of the single HV windings. Because of this the voltage output to each plug is halved if they are both needed to fire at the same time under compression.
BTW, I've ended up tuning with the dual sparks by only retarding the ignition ~2 degrees, vs the 10 degree retard so often 'heard'. hmmm

FWIW .. With my road Goldie with dual plug ignitions I use the SRM/Boyer systems with two Boyer coils. Interestingly the total loss Boyer 'crank triggered' system on our dirt tracker uses a different Boyer box that is triggered with a modern design 'reluctor' sensing a steel tab on the primary belt pulley vs those delicate magnetic pickups in the typical Boyer set up.
For my latest GS project bike I have a yet untried Power Dyno alternator/ignition unit set up made in GER to fire dual plugs. We'll see.


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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577280 12/20/14 10:59 pm
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With a dual-lead coil, the current has to go through both plugs to complete the circuit. If one plug fouls, that can still happen. The current will just travel down the insulator around the centre electrode to ground, instead of having to jump the plug gap to reach ground.

If the coil has to fire two plugs, both on compression, you'll need a helluva voltage (about twice what it normally takes to fire a single plug). That might not be good for the coil. Coil voltage only rises up enough until it can jump the plug gaps (in that case, very high).

If one plug is on the exhaust stroke, it takes very little voltage to jump that gap, and most of the voltage is used to fire the other plug on compression.

Re: where do the coil wires go on dual plug heads? [Re: Dave - NV] #577295 12/21/14 1:33 am
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Originally Posted by dave - NV
triggered with a modern design 'reluctor' sensing a steel tab on the primary belt pulley vs those delicate magnetic pickups in the typical Boyer set up.

You mean just like the Lucas Rita? What an idea!


Stepping on others doesn't make you stand tall.

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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577311 12/21/14 4:46 am
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Quote


i've been running both plug wires from a single dual lead coil to the same cylinders: left coil, left spark plugs . . .right coil, right spark plugs. the bike has run extremely well this way, but i'm wondering if i fail to understand something.




You're putting a bit of a strain on the coils. It might shorten their life.


Amateur Loctite enthusiast.
Re: where do the coil wires go on dual plug heads? [Re: DavidP] #577312 12/21/14 4:50 am
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Originally Posted by DavidP
Originally Posted by dave - NV
triggered with a modern design 'reluctor' sensing a steel tab on the primary belt pulley vs those delicate magnetic pickups in the typical Boyer set up.

You mean just like the Lucas Rita? What an idea!


My crank triggered Boyer Micro Power had a fairly delicate looking magnetic pickup.


Amateur Loctite enthusiast.
Re: where do the coil wires go on dual plug heads? [Re: triton thrasher] #577378 12/21/14 2:51 pm
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kevin roberts Online Confused OP
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i'm thinking about all this.


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Re: where do the coil wires go on dual plug heads? [Re: konon] #577442 12/21/14 9:58 pm
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Originally Posted by konon
Kevin, with your dual plugs, are you running less advance with the timing? If so do you know about what its set at ?


The timing is set at a factory figure that is usually determined on the dyno. As Nick L mentions, any variation to the head will lead to the need to put the bike on the dyno and determine where the new timing number is.

The use of dual plugs is often for practical reasons like Kevin's avoidance of pinking. Some use it because the starting is more reliable. Dual plugs can allow the timing to move closer to TDC simply because there is a likelihood that it will speed up the combustion process. If your head is really inefficient, the combustion process will take quite a while. If it is efficient it will be quite quick. What you are shooting for is to have the highest mean effective pressure at 12 degrees ATDC.

If the pressure peaks at TDC, the piston and connecting rod will "stiff arm" the pressure and there will be a loss of power. If the pressure peaks BTDC, it will slow the momentum of the flywheel and thus reduce power. Both of these early power peaking possibilities will put additional strain on the components. If you go to the dyno, you can time the bike for peak power by moving the ignition a small amount for each run until the power drops. The power will drop if you go too far because the piston will already be on its way down when the highest pressure occurs. You can then go back to peak power and that is where your new timing will be.

If your timing is at 38 degrees it takes a total of 50 degrees to get to the "sweet spot", which is a long time. Some engines with deep combustion chambers take even longer. Additionally, if you are firing your intake charge at say 40 degrees rather than 20 degrees, you are starting the combustion process short of your maximum compression ratio, which occurs at TDC. This will lower power also. All this to say, the dyno is your friend!

David

Re: where do the coil wires go on dual plug heads? [Re: David Dunfey] #577457 12/21/14 11:24 pm
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okay. dave in NV pointed out that the secondary windings inside a dual lead coil are actually two ends of the same winding. this is an image of something like it i've found after some looking:

[Linked Image]

ignore the double switching device.

i can understand the idea that two spark gaps on a compression stroke have more trouble ionizing the gap than two plugs on an exhaust stroke, given that the total voltage available in the coil is fixed.

what i'm still not grasping is why both plugs on the same coil fire, if they're separated into one cylinder on compression and another on exhaust. if there is a big imbalance between the voltages necessary to spark them, then why doesn't all the spark energy get diverted to the cylinder on the exhaust stroke?

if that plug sparks at a lower voltage, why doesn't it simply leak down all the energy out of the joint windings, and allow the plug on the compression stroke to misfire all the time? don't misfiring gasoline-fouled plugs on a single tower coil do this by leaking all the electricity down the ceramic and out? why doesn't this happen when the wires are connected between cylinders?

Last edited by kevin; 12/21/14 11:30 pm.

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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577461 12/21/14 11:41 pm
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well, in addition to all your suggestions that i'm straining the coils (they're old harley 6V things, and seem to be able to take it), i'm glimmering that i might be straining the engine in general by NOT retarding the spark, if the cylinder pressure is peaking before TDC due to better flame travel. this is the culprit of my confusion:

[Linked Image]

i run a standard NGK B8ES in the 14mm holes, and then something else that i can't remember at the moment in the 12mm holes in the center. the idea behind this was to start the mixture burning at both ends. the pistons are the morgo 9.5:1 (i think), so the crowns aren't awfully high, but the combustion space is still the nasty old grapefruit rind-shape at TDC.

the 12mm plugs have a too-long reach, and they're held up on brass spacer pieces to keep the threads out of the combustion chanmber.

Last edited by kevin; 12/21/14 11:42 pm.

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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577504 12/22/14 9:40 am
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Kevin,who did the work for the dual plugs? hopefully the two added plugs in the center miss each other. grin
I'm thinking about doing a 650 dual plug conversion myself. But I'm unsure exactly where to drill the pilot hole...Might just have to practice on a junk head...
I had a talk recently with Franz and Grubb, a well know Triumph shop in California. I was told they don't believe a dual plug conversion is worth the trouble for an engine with stock 9-1 pistons. It's just a conversation piece that adds complication. An easier approach is to use MAP tight quench lower dome pistons that give the same compression as higher dome stock pistons if you are rebuilding the engine...
Their opinion,not mine...I would think if you only have low octane fuel available it's a good idea to dual plug.
Hate to mention Harleys,apples and oranges, but I will...I dual plugged a iron head Sportster and a big twin Shovelhead. Both engines had higher compression and hotter cams.
Making on the street comparisons between the normal plug only and dual plugs,the engine ran the same....But the dual plugs did help eliminate detonation.
And I originally wired the engine with one coil per cylinder...Not a good ideal...


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577522 12/22/14 12:07 pm
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Originally Posted by NickL
You have drawn single ended coils with the normal grounded end of the secondary, you have a single secondary which is connected to 2 plugs, one at each end. To jump a gap at atmospheric pressure takes little energy so the plug which is not under pressure will arc across and as an arc has no resistance will complete circuit to ground so the large energy will ionize the plug at the pressurized end.


nick, is this a better image of what you're saying is in there? i think in pictures.

[Linked Image]

i understand verbally the idea that the exhaust plug will fire with a weak spark, and the compression plug will fire with a hotter spark, but i don't see what pete was saying about the current having to go through both plugs to complete the circuit to ground. still seems to me that one plug could be ignored. obviously, this doesn't happen, because the engines run this way. i'm still trying to understand why the north half of the electrons don't all hurry backwards through the low-resistance windings and misfire.


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Re: where do the coil wires go on dual plug heads? [Re: Hillbilly bike] #577531 12/22/14 12:48 pm
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Originally Posted by Hillbilly bike
Kevin,who did the work for the dual plugs? hopefully the two added plugs in the center miss each other. grin


it was done by Big D in texas. the two 12mm plugs go in at a steeper angle than the stock plugs and just miss each other on top. in fact, the rubber spark plug boots are in firm contact. but because all the plugs fire at once, i've never understood that to be a problem.

as far as i know, franz and grubb might be right. as i understand it, the whole idea of moving the plug to the middle or adding one on the other side is to shorten the distance the flame front has to travel to get to the dead-end space down on the other side of the piston dome. if you've got really good turbulence from squish or from intake porting, then i guess you can accomplish the same thing, and a lower-dome piston would stay out of the way and let it happen.

but the domes raise the compression ratio, and if the extra plugs can make it burn okay with 10.5 to 1, then all other things being equal, you'll be faster than you would have been with 9 to 1.


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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577538 12/22/14 1:52 pm
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I'm interested in doing dual plugs,and can do in my drill press...But the cost of the tools probably exceeds the cost of a experienced shop doing it.
There used to be a guy here on Brit Bike that did dual plug conversions...Anyone know his name????


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: where do the coil wires go on dual plug heads? [Re: Hillbilly bike] #577567 12/22/14 4:02 pm
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if you drill them in the center of the head, the only thing to be careful about is making sure you stay clear of the valve seats. when i had Big D do mine, i stupidly didn't tell them that i had just had MAP install 3/32-inch oversize intake valves, and they nicked the valve with their bit:

[Linked Image]

they didn't say anything, and just installed a different valve with a different head shape and different spring pack. i didn't know until i next took the head off and discovered that their odd valve was tapping the edge of the valve pocket on the morgo piston. i took it off and had the pockets milled a bit deeper and wider, and solved that problem. it was my fault for not letting them know the head wasn't standard.

but their flycutting was exquisitely done. look at how they milled through the top fins on the head:
[Linked Image]

[Linked Image]

if i had told them about the oversize valves, the work would have been perfect. even so, the head has worked well for many years.


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Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577570 12/22/14 4:24 pm
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Kevin .. I've not seen or heard of a 'dual output' coil as your drawing shows. Certainly a different concept than the typical dual output coil. Do you have more info on it?


Hillbilly ... Harold with Johnson Cams in WA has added the dual 12 mm plug hole for me on umm.. 5 Gold Star heads. He has the shop machine tools, the experience and the talent to do a nice job. As the 2nd 12mm plug in my GS heads are vertical into the backside of the chamber due to the limitations of the location due to the rocker box. In this case a long reach 'extended tip' 12mm plug fits down better into 'pocket' cut into the chamber.

But ... 'I've been told' that moving the plug hole location and changing to a little 10 mm plug to allow squeezing it closer to the valve seats is as good as a dual ignition set up in our olde inefficient hemi head combustion chambers. This is certainly not a 'new idea', but I've not tried it. John Healy, what's your opinion on this??

Most of us have heard of The Era's tuning wizard/rider Dick Mann. He had already changed to total loss battery ignition vs the power loss spinning a mag. Here's 'A story' telling of a illegal mod at that time to the ignition of his Gold Star pro dirt tracker. He had located a 2nd central 10mm plug directly between the valve seats. The plug fired into a small cavity and then through a slot between the seats into the chamber. After awhile he was found out and had to disconnect the secondary ignition.
What's interesting about this idea is many years later the Honda auto factory 'invented' somewhat the same concept to ward off detonation sparking their 'lean burn' high mileage engines into a cavity connected to the combustion chamber.


Dave - NV
Re: where do the coil wires go on dual plug heads? [Re: Dave - NV] #577587 12/22/14 5:25 pm
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Ya mean the CVCC engines?

Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577599 12/22/14 6:10 pm
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Setting up "squish" so it actually works is more than putting in a set of new pistons. This is especially true when you have a head that has had the head gasket surfaced skimmed. There is a narrow window where having a squish area actually works. The target recommended by a lot of "tuners" for a Triumph is .032". If you are racing on a short track you can go as little as .025" and the pistons will not hit the head, but at Daytona you need the full .032" or there will be contact. The squish is easy to measure by laying 4 or 5 "u' looped pieces of solid solder around the outside edge of the piston and rotate the engine over with the head on just once. You can then remove the solder and "mike" the area that was squished.

For our 500's we always got semi-finished pistons from Aries. The valve pockets were cut, but the rest of the top and underneath the dome were not. This allowed us to accurately cut the squish area on the head and match the piston to it. To get a squish band on a 500 you have to use the early squish head.

Part of setting up the crush, and getting everything square, included shaving material from the base of the cylinder. Then we could use various thickness copper base gaskets to vary the squish.

[Linked Image]

The head would look like this: The valves are skimmed to allow them to clear each other on overlap. As it is, there is just .060" between the valve margins as they cross each other at TDC. The early 500 head has a shallower valve angle than the later head and is a lot more work to get right than the later head.
[Linked Image]

When searching for the right timing for the dual plug I got to use Daytona where I found 25 on both the 750 twin and 500 gave the best lap times.

All this is interesting to some of us nuts, but it has little to do with reality... IMHIxperience if you are having problems with pinging I would look into it. Twin plugging helps a lot. If the bike runs well as it is, leave it alone.


Last edited by John Healy; 12/22/14 6:34 pm.

Re: where do the coil wires go on dual plug heads? [Re: kevin roberts] #577620 12/22/14 7:47 pm
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As others have said, that wire on your schematic the leads from the centre of the secondary winding back to the primary shouldn't be there. It doesn't exist on every dual lead coil I've checked.

The secondary winding is completely insulated from the primary and on its own. One coil lead is positive and the other is negative. The current flows through both plugs and across the head between the plugs to complete the circuit.
If one plug is fouled, or even if you joined its plug lead directly to the head, the other plug will still fire (if that's not fouled, too).

Re: where do the coil wires go on dual plug heads? [Re: Pete R - R.I.P.] #577625 12/22/14 8:17 pm
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catching up . . .

Originally Posted by NickL
You are still drawing a ground connection on the secondary of the coils.....there isn't one.
. . .
I thought most blokes used 10mm plugs for the second set these days.


i've never taken a dual-plug coil apart, so i was guessing at the windings. so the voltage in the secondary in the coil is completely induced by the adjacent primary windings.

Originally Posted by Pete R
. . . One coil lead is positive and the other is negative. The current flows through both plugs and across the head between the plugs to complete the circuit.
If one plug is fouled, or even if you joined its plug lead directly to the head, the other plug will still fire (if that's not fouled, too).


^^^ okay. now i think i understand. i didn't realize the polarities were different. this means one plug sparks from the side electrode to the center, and the other from the center electrode to the side, correct? so neither can complete the circuit to ground unless current flows through the other.

my center plugs are NGK D8EA, 12mm. i have a picture of them showing the spacer, but transferring photos from a telephone to the net is defeating me at the moment.


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Re: where do the coil wires go on dual plug heads? [Re: NickL] #577630 12/22/14 8:53 pm
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Originally Posted by NickL
We won't go into the realms of preferred electron emissive flow with heated anodes etc.
Suffice it to say that dual ended coils work very well.


agh

yes, please, spare me. i already confessed i'm no rocket scientist on ignitions. i have enough trouble figuring out the easy stuff.

Originally Posted by NickL

The advantage of the 10mm plugs is that they don't seem to induce the cracking between valve seats that larger ones do.


i've heard about that when the plug hole is drilled right at the top. there isn't much metal to spare there.

back when i had this done MAP said they were experimenting with duals using 14mm plugs. i don't know whether they were ever successful at it.


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Re: where do the coil wires go on dual plug heads? [Re: John Healy] #577631 12/22/14 8:58 pm
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Originally Posted by John Healy

When searching for the right timing for the dual plug I got to use Daytona where I found 25 on both the 750 twin and 500 gave the best lap times.


that is not a lot of advance compared to the stock figure, john. i didn't realize how significant the improvement in burning was.


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