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[Dennis C]

I normally keep well out of threads like this so this may well be my first and last post on the subject.

It would appear that Tim NZ is pretty much correct, Tribonnie states that the Hitchcocks scramber with twin plugs runs at 8 deg retarded compared with a single plug, this fits in with my experience, so if one plug stops sparking for whatever reason then the engine is running 8 deg retarded, which will cause it to overheat.

The makers of double ended coils have no real reason not to let them generate double the voltage and in my experience they do just that, I have in the past built many twin plug engines and never had a problem with a double ended coil set up, indeed the link posted by Thack states that a single coil generates eg 12Kv and a double ended coil generates 12Kv + 12Kv = 24Kv.

I seriously doubt that the problem lies with the coil, a twin plug set up is more efficient at burning all the fuel in the cylinder and at a faster burn rate, therefore it is more likely that the ignition is still too far advanced and or running lean.

In conclusion I rather feel that perhaps some of what has been posted before has been misunderstood, before jumping back in guys read through it again and try to understand the other posters point of view, the written word won't always come across in the same way as a face to face discussion.

[Thack]

Dennis C: thank you for pointing out something that I hadn't considered: that Tim was referring to the fact that a twin-spark system requires less ignition advance than a single-spark system.



In other words, if one spark dies, you need more advance on the other to keep the engine happy.



It never occurred to me that this is what Tim was talking about when he said that the engine is now running retarded, and I am happy to stand corrected on that part of my post. Please accept my apologies, Tim, for arguing with a different point than the one you actually made!

[Thack]

I thought I'd take a look on Wolfram Alpha for some more information on spark plug firing voltages. It gives a formula into which I've plugged some numbers.



If we assume a plug gap of 0.8mm, the breakdown voltage at 1 atmosphere is just over 6kV. Now, this is for air - I don't know how much effect the presence of fuel vapour has, although apparently it reduces the required voltage.



Anyway, if we assume air for now, then with a closed throttle the air pressure in the cylinder after compression might be as low as half an atmosphere. This requires a voltage of 3.5kV. With a wide open throttle we might see a compression pressure of, what, 7 atmospheres? That would give us 32kV spark voltage.



I don't trust those numbers in absolute terms because I don't know what effect the fuel has on the breakdown voltage. However, I think we can be confident that large throttle openings require a big increase in spark voltage to the plugs. We can also be confident that even at 1 atmosphere in the cylinder (i.e. what a "wasted spark" plug is likely to see) we need several kV (6kV or so) to jump the gap. It will be a bit different as it isn't air, of course, but spent exhaust gas.



For what it's worth, at college we measured the spark voltage on a Honda CB500 at idle, and it was a shade over 6kV, so I guess we're in the right ballpark.



Finally, I want to repeat my sincere apology to Tim for my dreadful gaff of completely failing to understand the point he was making with respect to ignition timing.

[Norm]

Trust me Thack, Tim knows his stuff

[Thack]

I agree, Norm, although we all have knowledge which we can contribute. I've learned plenty from various other members.



The problems arise when it becomes a "willy-waving" contest, and we stop listening to each other. I'm as guilty as anyone of that. It seems that men, in particular, like the idea of being the "group guru" whose knowledge should never be challenged. There's an individual like that on the American forum, too.



It isn't helped when Tim says things like "the previous comments... were seriously misinformed", which is very rude and disrespectful, as well as being wrong. But it doesn't matter about being wrong: we are all wrong at times, including me. What does matter is being rude and disrespectful, which we should all try really hard to avoid.



I've noticed something else about all such forums: a lot of people speak with complete conviction, even though they don't know what they are talking about and spout rubbish. (This does NOT include Tim, I hasten to add!) For instance, I've read all sorts of nonsense about the ECU/EFI system in our bikes, all spoken like it was fact. Talking like this makes it hard to know whether you can rely on what you are reading. Sadly, I don't think there is any way we can get round this problem.



I'm going to make a particular effort to not "react" when I feel goaded - something I all too easily forget!

[Norm]

After all he is a Kiwi lol

[Dennis C]

O Hell, back again, I should know better by now.

I don't think it was Tim's intention to be rude, sometimes we are told things or read things which ARE "misinformed" but we take them as gospel and ever forth believe it true.

Way back in time there was a spark enhancer called I believe a Spitfire it is possibly still availiable, anyway this thing despite the rubbish written in the adverts for it did just one thing, it created a second spark gap partway along the HT lead the gap was normally about 1.5 times the normal plug gap therefore the spark now has two gaps to jump one in the HT lead and one in the spark plug but in real terms the spark now has to jump 2.5 times it's normal distance using just a standard coil or in some cases a magneto, Guess what these things actually did work, totally unnecassary, but they still produced a spark at the plug, so possibly just possibly you don't need double the voltage to spark two plugs in series, can it be that two plugs in series will work in just the same way as the Spifire spark enhancer did?.

As a young foolish lad I made my own version of the Spitfire by cutting the HT lead and glueing the two ends into a length of clear plastic tube, the bike obviously now went at least 20 MPH faster but riding at night put on quite a good show and became a talking point with my mates who soon copied it.

[Thack]

Hi Dennis. Yes, you've made an important point: the reality is that most coils will produce far more voltage than is actually needed to jump the plug, which is how those "spark enhancers" can work at all.



That's why I think you would very likely get away with a wasted-spark coil in a twin-spark system - there's probably enough headroom for it to work. I also think it is POSSIBLE that it won't because you've got *two* pressurised plugs, not just one. The reality is, the only way to know is to try it. It will all depend on the particular coil, the particular spark plugs, the particular compression pressure, etc.



Chris's symptoms are consistent with insufficient spark voltage at large throttle openings, but as Tim says, they are also consistent with a fuelling fault.



Those spark enhancers are interesting, although they don't enhance the spark, exactly. Rather, by "holding back" the HT current until they spark internally, they provide a spark at the plug with a MUCH faster rise time, which is a great way of firing a fouled plug. So on those vehicles which suffer fouled plugs (much rarer these days) they can get the engine to fire up and run when it wouldn't normally. Indeed, one well-known trick amongst the old hands to get a misfiring or dead cylinder firing, is to pull the plug cap off the plug by about half an inch until the cylinder kicks in, and then replace it. A kind of Spitfire-on-demand.

The downside of these spark enhancers is that you lose some of the spark energy, which can be a problem when running at weak mixtures.

[Dennis C]

"Those spark enhancers are interesting, although they don't enhance the spark, exactly. Rather, by "holding back" the HT current until they spark internally, they provide a spark at the plug with a MUCH faster rise time, which is a great way of firing a fouled plug. So on those vehicles which suffer fouled plugs (much rarer these days) they can get the engine to fire up and run when it wouldn't normally. Indeed, one well-known trick amongst the old hands to get a misfiring or dead cylinder firing, is to pull the plug cap off the plug by about half an inch until the cylinder kicks in, and then replace it. A kind of Spitfire-on-demand. The downside of these spark enhancers is that you lose some of the spark energy, which can be a problem when running at weak mixtures."

All the above I agree with, but consider this. If you look at the HT output of a coil on an oscilloscope without load you will see a voltage which rises to a peak then falls away at a similar rate, this peak will be very high voltage, now put a spark plug on the output and look again you will see a voltage rise until the voltage jumps the spark gap then the voltage will almost instantly dissapear, lets say for the fun of it that the coil pruduces 12kv and the spark jumps at 6kv the first test will show the 12kv peak and the second only 6kv therefore you have a 6kv spark. now put the extra gap into the equasion and look again is it now possible that 9kv is needed to jump the bigger gap?, when this 9kv is reached then the voltage will jump both the extra gap and the plug which now means the spark at the plug is now 9kv not 6kv. Interesting don't you think, perhaps there is more to the universe than we understand.

[Thack]

Dennis, that makes sense all apart from this bit, which I disagree with:



When this 9kv is reached then the voltage will jump both the extra gap and the plug which now means the spark at the plug is now 9kv not 6kv



First of all, take a look at this:






Online version: Ignition curves



I took this trace from a Honda CB500R last week. The trace we are interested in is the red one, which uses the right hand axis. It was taken at tickover. As you can see, the ignition voltage rises to a shade over 6kV before the plug ionises, at which point it immediately falls to just 500V or so, which is maintained for the duration of the spark (which looks to be about 1.5ms).



I think I'm making two points, really. Firstly, if we had two spark gaps in the circuit, the energy stored in the coil would be shared between them, so the spark energy at the plug will be less. This is undesirable, of course.



My second point is the bit where I disagree with your statement above. In your two-gap scenario, why would all the 9kV be developed across the spark plug? Why not the Spitfire? I think what would happen is that the circuit would begin to conduct when the ignition voltage reaches the combined ionisation voltage of the two gaps. If we take the Honda as an example, and imagine the Spitfire has a similar gap as a plug, we'd see the HT voltage rise to 12kV (6kV * 2), at which point both gaps would ionise and current begin to flow. The voltage would then immediately drop to 1kV (500V * 2) for the duration of the spark.



Energy = volts * amps * time, so if the voltage required is increased, either the current or time (or both) must fall (conservation of energy and all that). So the "sharing" of the spark energy will show up as either a shorter spark duration, or less spark current, or both. As I say, it's not really something we want. I think that's why we don't see Spitfire-alikes fitted automatically.



So, what do you think is wrong with that argument, Dennis?