Misc: Ignition Twin Plug Heads - Better or No Difference?

BigEd

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Graham recently posted Did You Know?: PtXXIX - Did You Know?: PtXXIX. In the article Ted Davis wrote:

"In the Vincent engine, nothing is gained by using a central plug, or two simultaneously ignited plugs one each side or central and either side. Nor, as I have discovered recently, using two-plug heads with a split second firing interval. This information should save a lot of wasted time and a lot of heads."

I wonder if people like David Dunfey and other people who have improved the performance of the Vincent engine might like to comment.
 

stu spalding

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I don't know about power but twin plugs certainly improve fuel consumption and, for me, give a sweeter feeling engine. dropping the degrees of advance from 38 to 26 BTDC has to be a good move. Cheers, Stu.
 

vibrac

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I tend to agree with Ted as far as power goes although with high compression ie methanol there may be an advantage however central plug is something that may work better. Certainly Ron Kemp had a 10mm plug firing between the two valve seats at the center of the head through a 1/4" hole on his racing flash.
Of course most of my heads have two plug holes but that's for a de-compressor to save on oil leaks, dropped lifter rollers, and all those knitting needles :D
 

davidd

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Glad to take a shot!.

I think that I have been consistent about saying that the twin plug conversion on its own does not cause any increase in power. I would note one side effect that some (like Stuart Hooper) have found extremely useful: better starting. That advantage alone might make twin plugs beneficial to the bike. This is in line with what Stu mentions: side effects like better fuel mileage and better starting are often very useful.

As for power gain, most of the power gained by changing to twin plugs comes from being able to use a much more efficient spark timing. This is because the earlier the spark occurs (say in the 30 degree area) the more the spark is slowing the piston just before TDC. If the spark can be moved to the teens or low twenties then almost all of the energy of combustion is used on the power stroke. That can be a noticeable power gain.

All of this has to happen together. The ports have to be efficient and the combustion chamber has to be small and efficient (usually with squish) to work well. Most modern tuners want twin plugs because they provide the platform for additional tuning.

Ted is most likely correct. I think a side by side comparison of dyno runs with no other changes will yield the same results.

David
 

Marcus Bowden

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I've been on twin plugs since 1990 after the rebuild in France with Herve Hamon where drive side main bearing was sleaved and Honda oil pump fitted. Riding a lot of miles with Herve L'Manach before rebuild then after there were three to four litres difference when filling our fuel tanks, his "D" had an eighteen-litre tank and mine was 15 litres. Before the rebuild, I was calling the fuel stops, after he was calling for fuel stops. Like Stu said the ignition retarded by 10 or more degrees certainly makes a difference, not so much negative work before TDC. But twenty years later and 125k miles one of my double ended coils failed so it was all too retarded and a front exhaust valve seat dropped it, recued by Christine next day removed head parceled up to Bob Dunn as he did one for me before by return post. in afternoon took rear head off and piston had a 20 mm hole in it. So split cases and timing side fly wheel removed and rear B/End out board roller track was well Brinelled the other two rows were OK as well as the drive side B/end. so then it was nearly nide months before it was returned from Bob D.
Is it wise to twin plug!? I have now got switches to isolate each coil so can be tested whilst in motion to check.!
The "B" on route to 2017 annual rally up north at 220miles ran out of fuel so stoped (as one tends too with no fuel) put in my two 7 litre containers of fuel so nearly full tank and reached rally and had just switched over onto reserve about a mile before campsite. Originally the two small panniers where to be fuel tanks but only had pop riveter on ship during manufacture, 12 litre each side so that would be a total of 39 so well over 500 mile range but I think I should have done it in 1992 when the panniers were made. It all goes through a tweny eight inch doorway so can filter trafic with ease, must have left Stumpy Lord a few miles back on the home run but he caught uo eventually before parting company at M5. nice weather though.

P1060993.jpg
 

Bill Cannon

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When my bike was a 998 I changed to twin plugs. With 10:1 compression I found the low speed behaviour much smoother.
I have noticed the same effect on lots of twin plugged airhead BMWs too.
 

stumpy lord

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I've been on twin plugs since 1990 after the rebuild in France with Herve Hamon where drive side main bearing was sleaved and Honda oil pump fitted. Riding a lot of miles with Herve L'Manach before rebuild then after there were three to four litres difference when filling our fuel tanks, his "D" had an eighteen-litre tank and mine was 15 litres. Before the rebuild, I was calling the fuel stops, after he was calling for fuel stops. Like Stu said the ignition retarded by 10 or more degrees certainly makes a difference, not so much negative work before TDC. But twenty years later and 125k miles one of my double ended coils failed so it was all too retarded and a front exhaust valve seat dropped it, recued by Christine next day removed head parceled up to Bob Dunn as he did one for me before by return post. in afternoon took rear head off and piston had a 20 mm hole in it. So split cases and timing side fly wheel removed and rear B/End out board roller track was well Brinelled the other two rows were OK as well as the drive side B/end. so then it was nearly nide months before it was returned from Bob D.
Is it wise to twin plug!? I have now got switches to isolate each coil so can be tested whilst in motion to check.!
The "B" on route to 2017 annual rally up north at 220miles ran out of fuel so stoped (as one tends too with no fuel) put in my two 7 litre containers of fuel so nearly full tank and reached rally and had just switched over onto reserve about a mile before campsite. Originally the two small panniers where to be fuel tanks but only had pop riveter on ship during manufacture, 12 litre each side so that would be a total of 39 so well over 500 mile range but I think I should have done it in 1992 when the panniers were made. It all goes through a tweny eight inch doorway so can filter trafic with ease, must have left Stumpy Lord a few miles back on the home run but he caught uo eventually before parting company at M5. nice weather though.

View attachment 32712
my twin was to wide
 
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