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MG MGA - Positive Earth Not Good For A Spark
|I have just read through an article in the the latest edition of the MGCC 'Safety Fast' magazine. Titled 'The Kettering Ignition System' it addresses the conventional system devised by Charle Franklin Kettering, an American, in 1908. Although the article is aimed primarily at MG T-types, the principles apply to all vehicles from the early 1900s to about 1990.|
I was particularly drawn to the following statement: "The polarity of the voltage presented to the plug by the coil is also important. Ideally it should be negative. If the voltage is positive a plug will require around 30% more voltage to produce a spark."
|Steve, Convert to neg earth and fit an alternator? Plenty of oomph for a spark then! Works for me .... AB|
|Steve, it's not so much the polarity of the battery with regard to earth as the polarity of the HT coil. That is why it is important to reverse the old CB/SW coils when you do a polarity change. If you have a + - coil, its connection is obvious. Lindsay.|
Negative earth has been my set up for 12 years. Alternator went in about 3 years ago.
I do not have expert knowledge on the statement, hence the invitation for comment.
|Steve, I have read in more than one place that what Mr Kettering says is true. There is a piece about it in this article from the Sprite and midget Club. http://www.mgcars.org.uk/midgetspriteclub/MascNew/TechTips/PosNeg.html|
|Steve. What the claim by Mr. Kettering means is that the positive ground system is superior to the negative ground system. Since the flow of current is from the coil, through the distributor cap, through the spark plug lead, through the spark plug, to ground, the only way to have negative voltage at the plug is to have a positive ground system. This is one of several reasons advanced in favor of the positive ground system. US made vehicles, at least up until some time in the early 50's, were positive ground systems. My first car, a 1953 Mercury Coupe, had a positive ground, six volt, electrical system. |
But, it is worth noting that a system which conferred significant advantage back in the very early days of automotive history, may well lose some of its advantages due to subsequent advances in technology. If there is, today, any form of inferiority associated with the use of a negative ground system, such problems are not commonly known.
|A negative voltage at the center electrode of the spark plug takes advantage of the cathodic nature of a heated electrode essentially "boiling electrons" off of the conductive material, making it easier to get current flow to produce the spark. That said, it doesn't matter what the car's overall polarity is, all that is necessary for this cathodic action to take place is to have the center electrode of the spark plug negative in relation to ground electrode of the spark plug. This condition can be produced just by hooking the coil up such that it provides a negative pulse to the spark plug center electrode. And yes, I know, someone is going to say that in a negative ground vehicle, the battery voltage is going to subtract from the voltage produced by the coil if the car is negative ground, thus making a positive ground vehicle more efficient at producing a spark and this is true, but - let me ask, what percentage increase in efficiency is 12 volts going to make to a coil voltage of 20,000 volts? The answer to this is 0.06% - not enough to write home about. |
I have an article written by Dave Edgar regarding coil polarity and how to determine if it is correct, that I will send to anyone interested. Just e-mail me at the above address. Cheers - Dave
|The plug should always be presented with a negative HT pulse regardless of battery orientation. The polarity of HT or secondary side of the coil is independant to the battery voltage although influenced by the connection of the low tension or primary side of the coil. You can change the polarity of your spark by simply reversing the terminals (sw/cb or +/-) on your coil. There are devices around that you can use to test that your plug is receiving the correct negative pulse.|
|Don't want to cause upset Les but I think you are incorrect. The direction of the spark jump at the plug will be determined by the polarity of the coil primary windings. Thiese being insulated from earth are swapped when the coil power and dissy connecions are reversed.|
|Robert (Bob) Midget Turbo|
|Here is a simple test to make sure your spark polarity is correct. http://www.dansmc.com/polarity.jpg|
|What is the difference between the centre electrode and the ground electrode? They are both lumps of conductive metal. I can't see that the electrons care which way they jump!|
|It' a question of which electrode of the plug is the hottest Art. Electrons, which carry a -ve charge, are liberated from a hot surface more easily, which, in the case of a spark plug, is the centre electrode. So the plug is offered the -ve side of the coil's HT winding. It is the same principle that operates inside a thermionic valve or "tube" to you Americans. The cathode, or -ve electrode has a low voltage heater inside it to liberate the electrons so that they can be drawn towards the positively charged anode.|
|Lindsay (Logie Baird) Sampford|
I am glad I asked for the comments. It's interesting how one can draw the wrong conclusions from an article on a subject about which one has only limited knowledge.
|Hi Steve - I read the same article but got the opposite conclusion - the penultimate paragraph says that " if you have reversed the polarity of your battery (from + to - earth) you will need to reverse the connection to you coil to produce a negative spark. Even then it will be marginally less efficient thanit was with a +ve earth battery". The answer if you have swapped to -ve earth is to go with electronic igntion it seems. Thanks to Linsay for the explanation - studied thermionic valves for A level many years ago - hadn't thought of the plug electrode in the same way before - cheers Cam|
|Cam - "" if you have reversed the polarity of your battery (from + to - earth) you will need to reverse the connection to you coil to produce a negative spark. Even then it will be marginally less efficient than it was with a +ve earth battery""|
That is what I stated in my post, "I know, someone is going to say that in a negative ground vehicle, the battery voltage is going to subtract from the voltage produced by the coil if the car is negative ground, thus making a positive ground vehicle more efficient at producing a spark and this is true, but - let me ask, what percentage increase in efficiency is 12 volts going to make to a coil voltage of 20,000 volts? The answer to this is 0.06%". The operative word in the statement you quoted is 'marginally'. At 0.06%, I would add, very marginal. Cheers - Dave
|I am no electronics wizz but reading the original thread, with a neg earth system requiring a 30% greater voltage to jump the spark gap. Could it be that this could be an advantage and not a minus. It, like a wider plug gap will cause the spark to be hotter when it does fire. Just my thoughts. Denis|
|D M HILL|
|Many modern cars use a "waste spark" ignition system with one coil firing two spark plugs simultaneously in the same "series" HT wire circuit. One plug will have negative spark while the other plug will have positive spark. They do run of course.|
Given enough voltage you can make the spark jump either direction equally well without any misfire. My MGA has been running a 40Kv Lucas Sport Coil for many years, and I seriously doubt there would be any noticeable difference if the spark polarity was reversed.
|Following on, I have been going through my car electrics and carbs to try and get that elusive increase in fuel economy that so many of you boast.|
I have been using a very basic coil for the last 6 years. No markings on it to say what voltage and ohms etc. However, chatting to my local supplier it seems to be one that is fitted to old style Minis, Ford escort, etc. So, having read all the blurb about sports coils (40,000 volts) giving improved combustion, and perhaps better economy etc, I have fitted a 'Flame Thrower' from Aldon to sit along side my Ignitor (pertronix) electronic ignition.
Used it to work today. Working fine. Not sure whether I have noticed any change in performance, but I will be monitoring my MPG over the coming weeks.
So my questions to the experts are: Should I really see any change in performance? If I do get a more thorough fuel burn, should I weaken off the fuel a tadge? My plug gap is currently set at 30 thou; is this about right?
|Steve, I set the gap on my NGK B6ES plugs to 35 thou with a Lucas Sport Coil (40,000 volts).|
|"""Many modern cars use a "waste spark" ignition system with one coil firing two spark plugs simultaneously in the same "series" HT wire circuit. One plug will have negative spark while the other plug will have positive spark. They do run of course.""""|
Not sure of that Barney.
A wasted spark is simply that we fire 2 plugs at the same time one of which is fired on another cylinder on the exhast stroke. It is still fired in a normal manner and as the name suggests is simply wasted. I use this system on my Midget which I have installed a programmable ignition system onto. Not sure of the supposed benefits of wasted spark system however?
|Robert (Bob) Midget Turbo|
|Steve many yeas ago (racing) I spent about 4 hours on a rolling road trying to determine the most beneficial plug gap for a lucas sports coil?? I eventually convinced myself that the ideal gap was 32 thou, although it was far from conclusive but enough to convince me.|
|Robert (Bob) Midget Turbo|
|Steve, I have a sports coil and 35 thou gaps. |
Surely if it starts and runs well, that's as much as the plugs and coil can do (relative to performance)? Otherwise it's just the reliability and the maintenance intervals, that are effected (i.e. preventing a spark too weak to fully ignite the mix)? Or am I missing something?
I doubt if you will see any improvement in performance, fuel consumption, etc. My experience is that coils either perform (adequate voltage for a satisfactory spark) or not (misfire, etc). If it feels to be running smoothly and "not sure whether I have noticed any change in performance", then that's probably as good as it's going to get! I believe there is a minimum voltage necessary for full performance and after that, no difference. A really big spark might give some benefits for starting under cold and/or arduous conditions but that's about it.
I've always felt there has been a load of hype about coils and huge voltages to be honest!!
|Barney is right. The coil on my J$tt# has 2 HT windings, each of which fires 1+4 and 2+3 plugs simultaneously (of course not all 4 together!) The coil HT side is not grounded, so the plugs are fired in series. So I guess if you pull one lead you lose 2 cylinders, not good for diagnosing dud cylinders.|
|I was questioning the comment that one plug was fired positive and the other one negative? I believe they are both fired the same!from as you describe 2 common coils in series.|
|Robert (Bob) Midget Turbo|
|Bob, the HT coil is not grounded like the traditional system which finds its earth via the capacitor, but what you need to imagine is an HT coil with two unearthed ends. The HT pulse leaves one end of the coil and travels via its connector to one of the pair of plugs, it jumps the plug gap, travels through the aluminium of the head to the body of the other plug,jumps that gap, then up through the centre electrode to the plug cap and back to the othe end of the coil. Simple!|
|Ah I see that! but what is the benefit?? This is becoming interesting. :-))|
|Robert (Bob) Midget Turbo|
|Don't really know. Later MGFs have two little coils up in the head. The coil itself is attached to one plug and a conventional plug lead comes out of the end of the coil and goes to the other plug, and this is repeated for the other pair of plugs. I suppose it cuts out the long plug leads that the earlier ones had, they go half way round the engine! They must get ever so hot where they are though.|
|It seems my casual "waste spark" comment about spark polarity not being very important has redirected this thread to something generally not relevant to MGA. However, since there seems to be significant interest, I will elaborate a bit while answering a couple of questions.|
Reason (advantage) for using the waste spark system is first to get rid of the mechanical distributor, and second to minimize the number of ignition coils needed to fire all spark plugs. Wiring the HT output side of the coil in series with two spark plugs allows it to power two plugs with one coil, so you don't need a separate coil for each plug. I have seen some 4-cyl engines with two coils lying flat on top, each symmetrically between two plugs with HT connectors on each end of the coil and no flexible HT wires. As Lindsay mentions, placing the coil over one spark plug and running an HT wire to the other plug achieves the same purpose. My 6-cylinder Chevy has three coils mounted on a common electronic trigger module with each coil having two HT wires running to spark plugs.
The "waste spark" bit comes in when two plugs fire at the same time, as is required with the series wired HT circuit and single coil. One spark will occur at end of the exhaust stroke while the other spark happens at normal time at end of compression stroke. Spark on exhaust stroke is "wasted" but causes no problem when there is no combustible fuel to be ignited at that time. The follow up bit of this process has to do with how little of the energy is actually wasted.
You may know that a healthy ignition spark will jump an open air gap of 1/4-inch or more, while it has problem enough jumping across a spark gap of 0.025" or so on a spark plug under compression. Reason is because low pressure air presents relatively less electrical resistance so the HT spark can easily jump across a larger gap. Air under higher pressure inside the engine on the compression stroke presents much higher electrical resistance, so more voltage is required to make spark jump a smaller gap.
You likely know from prior discussions that the actual voltage encountered is determined by the width of gap and pressure in the cylinder. When it's time for spark the voltage increases until it is sufficient for spark to jump the gap, at which time the sparks drains power out of the coil and voltage goes no higher. In round numbers (approximations), 20Kv may jump 0.250" in open air (80Kv per inch of gap), while the same 20Kv will jump 0.025" under compression (800Kv per inch of gap). The "wasted" spark jumps the gap on the exhaust stroke with very little pressure, so it may need as little as 2Kv to do that. In the end the coil needs to produce 22Kv to fire both plugs at once, using 20Kv on the compression plug and "wasting" only 2Kv on the exhaust plug.
On Bob's comment (or question), the two plugs absolutely must be wired in series with the HT circuit of the coil. If the two plugs were connected together on the same output terminal of the coil, the exhaust plug would fire around 2Kv and drain off the coil power before it could ever get up to the 20Kv needed to fire the plug under compression, and the engine would never run.
On Steve's question, the quality of engine running should not be affected one way or another by various the means of producing spark, as long as it doesn't misfire. If a high voltage coil makes any difference in power or fuel economy there must have been some defect or deficiency in the prior setup. That said, changing type of spark plugs can make a difference in running quality, for better or for worse, and higher voltage coil may be able to overcome some problems with incorrect or ailing spark plugs.
Interesting stuff, but it still remains that the two plugs fire in opposite polarity with no detriment to combustion, as long as there is enough voltage to make it happen. With technology available a hundred years earlier (lower HT voltage in the ignition coils) there may have been some concern about needing the last 15% to 30% of available coil voltage to make a reliable spark, so spark plug polarity may well have been a valid concern. Today you can have all the spark reliability you need by using a higher voltage coil, as long as the insulators don't allow the smoke to leak out of the HT circuits.
|No problems with smoke leaks these days when you can buy refill kits. http://www3.telus.net/bc_triumph_registry/smoke.htm|
|Interesting discussion; one thing not mentioned regarding Sport Coils or high voltage coils, at least as I understand it. Somebody else may wish to elaborate. |
The coil voltage is needed as Barney said, to jump the gap on the plug. A sport coil would be better for a high compression engine, as also stated, but if you have a higher voltage, you can also widen the plug gap without problems. Doing this, you may wish to re-time your engine, as the burn will be a tad slower, and you want optimum spark at TDC or slightly before.
|I must have one of those Lucas smoke refill kits, If one driving an "A" cant find humor in that all is lost.|
|I just thought I would mention that, following a week of use with the 'sports coil' (Aldon Flame Thrower), the car is starting even better than before (engine fired before it even seemed to turned over this morning) and there is a definite feel of improvement in performance. Seems more solid. Idle is totally stable. Went like a rocket accelerating from 60 - 80 mph this morning and hardly touched pedal.|
That car of yours just gets better and better!
|If it gets much better Steve, you'll come out one morning to find it has left without you!|
|Neil and Lindsay|
It depends on your starting point! Having only ever owned and driven one MGA with nothing to compare against, it is difficult at any one point in time to say "This is how an MGA should perform". Yes, it's improving all the time with my tweaking and upgrades, but I suspect all I am doing is bringing it up to your standards!
This thread was discussed between 07/07/2009 and 16/07/2009
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