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#642905 - 03/04/16 10:56 pm Dual plug head question  
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Gary Scallen Offline
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Wanting to try a dual plug head on my 70 650 Tiger. I have a new rotor as stator but want to stay with points. Would I want two twin tower 12 volt coils and cross one to the opposite plug or run two single tower 12 volt coils with twin plug leads both to the same cylinder. I was concerned that the power of the twin tower coil might be diminished firing two plugs at the same time, I am a bit confused I guess.

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#642910 - 03/04/16 11:31 pm Re: Dual plug head question [Re: Gary Scallen]  
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Use 2 x 6v coils with primary's in series on each set of points. It keeps it simple. That way each pair of coils fires one cylinder, only 1 condenser is required per pair.



#642919 - 03/05/16 4:19 am Re: Dual plug head question [Re: Gary Scallen]  
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The dual-tower-coil cross-over strategy is for wasted spark ... all 4 plugs are firing at the same time , but only 1/2 of each dual-coil is under compression .

With 2 sets of points you have an opportunity and challenge of
sparking each cylinder individually .
The choice between 4 -6volt or 2 -12volt dual-fire coils
comes down to mount-ability .
Where are you going to put 4 coils ? On a bandolier ?
.

#642924 - 03/05/16 5:41 am Re: Dual plug head question [Re: Gary Scallen]  
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Hi Gary,

Originally Posted By Gary Scallen
I was concerned that the power of the twin tower coil might be diminished firing two plugs at the same time,

"two single tower 12 volt coils with twin plug leads"? confused Do such things exist?

"Twin tower coils" work slightly differently from "single tower" ones, in that the HT circuit is entirely independent of the LT - the HT leaves one end of the coil, travels along the connected HT lead, across the connected plug, across the head to the other plug, across that second plug, along its connected HT lead and back to the other end of the coil. That's why the advice is to connect the HT leads from the same coil to different cylinders - only one cylinder will be on its compression stroke; however, with "twin tower" coils capable of, say, 30 KV, I'm not sure that's necessarily an issue?

However, if you do "cross one to the opposite plug" - connect one HT lead from each coil to a different cylinder - you will need to fire both coils whichever cylinder is on compression. So, as Nick's posted, you'll need '6V' "twin tower" coils connected in series, same as you would with electronic ignition; on that basis, is the additional complication of points, condensers, etc. worth it?

Hth.

Regards,

#642925 - 03/05/16 5:49 am Re: Dual plug head question [Re: quinten]  
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Hi Quinten,

Originally Posted By quinten
The dual-tower-coil cross-over strategy is for wasted spark

Yes and no.

"wasted spark" is a term applied to electronic ignitions that fire multiple coils together, irrespective of how many spark plugs a given cylinder has.

We're talking specifically of twin-plugs here. Only if Gary chooses to connect one HT lead from each coil to a different cylinder will there be a "wasted spark" here.

Originally Posted By quinten
The choice between 4 -6volt or 2 -12volt dual-fire coils comes down to mount-ability . Where are you going to put 4 coils ? On a bandolier ?

Why "4 -6volt" coils? confused There are '6V' "twin tower" coils, just as there are '12V' "twin tower" coils.

Hth.

Regards,

#642927 - 03/05/16 6:19 am Re: Dual plug head question [Re: Gary Scallen]  
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I have run a single 12V dual coil running dual plugs on a B44 single with no issues despite it being specified for one cylinder on compression and one non compression ie a wasted spark. The ones I have used are the Dyna coil, Generic Harley and 2 6V PVL coils, the Dyna coil was on the bottom limit of the specified ohms for the Boyer which overheated on hot days, the Harley was a big coil to find space for but worked fine otherwise and the 2 6V coils are easier to find space for and also no problems.

#642930 - 03/05/16 6:50 am Re: Dual plug head question [Re: Gary Scallen]  
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Can someone explain why you would have a twin outlet coil with a plug in separate cylinders. Since electricity takes the path of least resistance wont the low comp side get the lions share of the spark? Or does this place less strain on the coil given that two plugs in High compression will be harder to fire than one in low comp and the other in high?
Just my idle curiosity.


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#642932 - 03/05/16 7:17 am Re: Dual plug head question [Re: gavin eisler]  
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Originally Posted By gavin eisler
Can someone explain why you would have a twin outlet coil with a plug in separate cylinders. Since electricity takes the path of least resistance wont the low comp side get the lions share of the spark? Or does this place less strain on the coil given that two plugs in High compression will be harder to fire than one in low comp and the other in high?
Just my idle curiosity.


On a twin output coil, the two spark gaps are in series in the HT circuit, which goes from one coil HT terminal to the other.

Two spark gaps in series, both under compression, are hard for the spark to jump.


Amateur Loctite enthusiast.
#642933 - 03/05/16 7:19 am Re: Dual plug head question [Re: Stuart]  
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Originally Posted By Stuart
Hi Quinten,

"wasted spark" is a term applied to electronic ignitions that fire multiple coils together, irrespective of how many spark plugs a given cylinder has.



Regards,


You can have wasted spark with points.


Amateur Loctite enthusiast.
#642960 - 03/05/16 11:57 am Re: Dual plug head question [Re: Gary Scallen]  
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Good Morning Gary: These 2 bits of information came from the BMWMOA.ORG site. They are about a dozen years old and also in the articles they state that some of the information has changed with the times, but you might find some of it useful. http://www.rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf and http://www.gunsmoke.com/motorcycling/r100gs/dual_plug/index.html I hope this helps.
Frank Coleman

#642963 - 03/05/16 12:05 pm Re: Dual plug head question [Re: triton thrasher]  
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Hi,

Originally Posted By triton thrasher
You can have wasted spark with points.

Why?

Regards,

#642966 - 03/05/16 12:18 pm Re: Dual plug head question [Re: Stuart]  
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Originally Posted By Stuart
Hi,

Originally Posted By triton thrasher
You can have wasted spark with points.

Why?


Why not?

The wasted spark system was used on BMW and various other 4-stroke twins, also four-cylinder engines, as only one points set is required for a twin, two sets for a four, etc.




Last edited by L.A.B.; 03/05/16 12:26 pm.
#642968 - 03/05/16 12:44 pm Re: Dual plug head question [Re: Stuart]  
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Originally Posted By Stuart
Hi,

Originally Posted By triton thrasher
You can have wasted spark with points.

Why?

Regards,


Ask Citroen.


Amateur Loctite enthusiast.
#642988 - 03/05/16 6:12 pm Re: Dual plug head question [Re: triton thrasher]  
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Originally Posted By triton thrasher
Originally Posted By Stuart
Hi,

Originally Posted By triton thrasher
You can have wasted spark with points.

Why?

Regards,


Ask Citroen.


Wasted spark will not work with 2 sets of points , unless the points are wired
In series ... greatly complicating timing and reliability .
Yes it is possible but again , Why would anyone do that ?

... In the interest of keeping replies somewhat specific to the original question .
Is citroen making a dual plug head
for a 2 cylinder engine
With 1 individual trigger (points-set) timed to each cylinder ? ...


L.a.b's second pic. , show again here , shows the dual-tower set up .

Each dual-tower coil ,
Throught it own trigger
would drive one cylinder , with 2 plugs ... no wasted anything .

.

#642995 - 03/05/16 8:03 pm Re: Dual plug head question [Re: Gary Scallen]  
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Using double ended coils is not as effective as using 2 individual coils per set of points with this type of system. A double ended coil producing 2 sparks under compression will have reduced power per spark compared to individual coils unless the dual coil is specifically designed for the purpose. Look at any modern car or bike set up, many use double ended coils, none use a double ended coil firing 2 compressed cylinders simultaneously.

If you are staying with points, you can use 2 12v coils in parallel or 2 x 6v coils in series each side to keep it simple. 2 x 12v coils will use more power and shorten points life. The value of the condenser should be increased if that route is taken.

If it were me i'd use EI and 2 double ended coils cross wired but what do i know eh?



#643001 - 03/05/16 9:16 pm Re: Dual plug head question [Re: NickL]  
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Nickl: In my earlier posting with the RUBBER CHICKEN RACING thread, they discuss using an Accel Ignition Amplifier to help the points on page 25 section E. Earler in the post they also talk about retarding the timing because of faster flame spread.

#643002 - 03/05/16 9:27 pm Re: Dual plug head question [Re: Gary Scallen]  
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If you cannot retard the ignition without loosing power then the whole object of the twin plug thing is lost!

2 simple transistor switches triggered by the points is a huge plus for extending points life.
Just ensure the current across the points is about 250ma, that will keep them clean.



#643022 - 03/06/16 5:04 am Re: Dual plug head question [Re: NickL]  
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Quote:
Using double ended coils is not as effective as using 2 individual coils per set of points with this type of system. A double ended coil producing 2 sparks under compression will have reduced power per spark compared to individual coils unless the dual coil is specifically designed for the purpose. Look at any modern car or bike set up, many use double ended coils, none use a double ended coil firing 2 compressed cylinders simultaneously.


Good point nickl , your the man , ... I'm just a curious hacker
But Let me put this out there.

Dual fire coils , in a wasted spark arangememt ,
Used on modern cars ... are there to save money .
It not the same to say they are limited to this applicarion .

If one spark is wasted and the other is commpressed ... as is typical with dual towers .
It is the higher resistance gap that controls the spark of both plugs
The lower resistance gap cascades with the breakdown of the higher .
This is true whether the second plug in under the same compression or not .
( this is above my pay grade , if the above is wrong , dont be shy )

#643031 - 03/06/16 6:56 am Re: Dual plug head question [Re: triton thrasher]  
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Hi,

With apologies to Gary The O.P. ...

Originally Posted By L.A.B.
The wasted spark system was used on BMW and various other 4-stroke twins, also four-cylinder engines, as only one points set is required for a twin, two sets for a four, etc.

Originally Posted By triton thrasher
Ask Citroen.

I'm aware of this for single-plug cylinders, my CB750 has the arrangement. But Quinten posted:-
Originally Posted By quinten
The dual-tower-coil cross-over strategy is for wasted spark

Is or was there a motorcycle or car engine made in commercial numbers with twin-plug cylinders/heads, double-ended coils, wasted spark, triggered by points?

Regards,

#643037 - 03/06/16 7:56 am Re: Dual plug head question [Re: Stuart]  
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Originally Posted By Stuart

Is or was there a motorcycle or car engine made in commercial numbers with twin-plug cylinders/heads, double-ended coils, wasted spark, triggered by points?

Regards,


Most modern US car/truck engines have one one coil per cylinder...The new Dodge Hemi has dual plugs..And one single "double ended" coil per cylinder ...But no points obviously...

25 years ago I dual plugged a modified Harley Shovelhead. I converted the stock Harley single point wasted spark to a dual point ignition . Then used two Dyna dual output coils, one per cylinder...It was supposed to work but always fouled one plug per cylinder eventually....And we all know why.....


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#643095 - 03/06/16 4:05 pm Re: Dual plug head question [Re: Stuart]  
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Originally Posted By Stuart
Hi,

With apologies to Gary The O.P. ...

Originally Posted By L.A.B.
The wasted spark system was used on BMW and various other 4-stroke twins, also four-cylinder engines, as only one points set is required for a twin, two sets for a four, etc.

Originally Posted By triton thrasher
Ask Citroen.

I'm aware of this for single-plug cylinders, my CB750 has the arrangement. But Quinten posted:-
Originally Posted By quinten
The dual-tower-coil cross-over strategy is for wasted spark

Is or was there a motorcycle or car engine made in commercial numbers with twin-plug cylinders/heads, double-ended coils, wasted spark, triggered by points?

Regards,


Perhaps if you'd asked a better question then you might have received some better answers so I suggest you stop being so bloody pompous! (No apologies to anyone)

#643121 - 03/06/16 6:50 pm Re: Dual plug head question [Re: Gary Scallen]  
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An unloaded/uncompressed air gap will ionize very quickly and easily, the arc formed has very little resistance, this in effect closes the secondary circuit at one end and allows most of the stored energy in the coil to be expended on the 'business end' which is loaded. Some manufacturers made a double ended coil with a centre earth/ground connection giving 2 independent coils in a single housing but they were unusual. Many modern cars have moved to a single coil per cylinder as this keeps heating down, a major influence on coil performance.



#643128 - 03/06/16 7:55 pm Re: Dual plug head question [Re: NickL]  
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NickL, are you implying that some wasted spark ignitions systems may discharge more energy to the cylinder not on compression stroke?


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#643129 - 03/06/16 8:01 pm Re: Dual plug head question [Re: NickL]  
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Originally Posted By NickL
An unloaded/uncompressed air gap will ionize very quickly and easily, the arc formed has very little resistance, this in effect closes the secondary circuit at one end and allows most of the stored energy in the coil to be expended on the 'business end' which is loaded. Some manufacturers made a double ended coil with a centre earth/ground connection giving 2 independent coils in a single housing but they were unusual. Many modern cars have moved to a single coil per cylinder as this keeps heating down, a major influence on coil performance.


With all due respect ...

How can the wasted spark happen until the circut is complete ?
... Its either on or off .

Seems to me
The compression event spark , requiring higher voltage , determines
The timing
Of simultaneous sparks , of different voltages in each plug ?

#643131 - 03/06/16 8:26 pm Re: Dual plug head question [Re: Gary Scallen]  
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When the coil primary is switched off, the stored energy appears on the secondary side, the unloaded side forms an arc /ionises to ground/earth very quickly. This arc has little resistance, the stored energy is then expended across the other end of the secondary to earth. The closest comparison i can make is that of a spring.... The timing difference is very small, typically less than 4 u.secs. to achieve ionisation at the working end.
The system has it's pro's and con's..... spark polarity etc. but we won't go there eh? Discussing the physical properties of electron emission from heated anodes as to cathodes etc, it was all too bloody long ago. Magnetoman will give a far more eloquent and mathematically explicit answer i was just a mere 'technical grunt' in comparison.

Tony
The energy expended to form an arc at the unloaded end is very much smaller than that of a loaded one. Once the arc has formed it's resistance is tiny so in effect you now have a single ended coil, with an air gap at the correct end, that's the best way to put it.

Last edited by NickL; 03/06/16 8:32 pm.


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