Home › Forum › Ask A Member › 2-stroke vs 4-stroke spark plugs;am I dreaming
- This topic has 17 replies, 7 voices, and was last updated 8 years, 9 months ago by
chris-p.
-
AuthorPosts
-
July 8, 2015 at 6:54 pm #1964
My memory is starting to have flash backs now and then “at my age” and I remember reading somewhere about spark plugs for 2-stroke motors being different than 4-stroke, particularly OMC outboards in the 50’s and 60’s. I may have read this in an outboard motor repair guide in the section about making sure the spark polarity is correct, you know the diagram with inserting a pencil between the block and the spark plug lead to see which way the sparks from the pencil fly.
My dim memory recalls something about the design of a spark plug for a 2-stroke has the side electrode ending half way across the center electrode. The theory was that electrons are more likely to jump off a sharp edge of the side electrode to the center electrode making a hotter spark in an oil environment.
4-stroke spark plugs have the side electrode extending past the center electrode for better wear with the transfer of electrons. Fouling is less of an issue so a sharp edge on the side electrode over the center electrode is less of a concern.
I recall somewhere also if you have a 4-stroke spark plug where the side electrode extends past the center electrode and will be using it in an old 2-storke outboard to cut the side electrode back to where it is over the middle of the center electrode.
Am I just dreaming all of this up? Did the old Champion J6J and J4C have the side electrode end over the middle of the center electrode?
July 8, 2015 at 7:39 pm #19677You aren’t dreaming or senile. Everything you said is true. The J6J is a two-stroke, and the J4C is a four-stroke. The J or C tells the tale. They do not make the J-suffix two stroke plugs any more.
July 9, 2015 at 12:13 am #19697I file the J6C or J4C plugs electrode back a bit, essentially making a J6J like the old days. Not sure if its in my head, but they seem to run better like that.
Now, on a side note, I have not read this pencil trick!? Can anyone elaborate on that?
July 9, 2015 at 1:30 am #19706That has been around for ages. You hold the pencil in the arc as you hold the wire away from the plug and you can tell whether the spark is of negative or positive polarity by the way the spark "splatters" (my word). Personally, I’m aware of it but have never tried it. As if it really makes any difference anyway. The old V-4 magnetos made two positive and two negative sparks per rev. The cylinders don’t seem to notice.
July 9, 2015 at 4:37 am #19716Over the years, I’ve heard a lot of woo-woo talk about spark polarity, but only as concerns + vs. – in a 4-cycle engine. The subject most often comes up when discussing double-ended coils, that fire a pair of plugs at once, such as in a 4-cyl motorcycle engine – obviously, the sparks will be of complementary polarities (one +, the other -).
But I’ve never heard any talk of one polarity or the other being more or less suitable to igniting a 2-stroke or 4-stroke engine. The discussion has always been limited to + vs. – in 4-stroke engines.
Seems like a lot of hooey to me, no matter how it’s applied – but I’m ready to learn, if there’s good data on it!
July 9, 2015 at 9:58 am #19723The theory is that the spark jumps easier from the center electrode to the side electrode of the plug. There is some validity to that theory, just how much is debatable. I just looked it up and the book said 15%. So why does it jump easier from center to side? Well first we have to accept the truth in which way electricity flows. The truth is that it flows from negative to positive. That is known as the Electron Theory of current flow and that electrons are of negative polarity. The other theory is that it flows from positive to negative, known as the Conventional Theory of current flow. Almost everybody believes the Conventional Theory, so that is pretty much accepted teaching even though not true. Now let’s think about the vacuum tubes used in old radios. It would be difficult if not impossible to understand how they work if you want to cling to the Conventional Theory. The flow of current through a vacuum tube is from cathode to plate (anode). That is made to happen by applying a high voltage between the two, negative to the cathode and positive to plate. But even that is not enough to make the (negative) electrons break loose from the cathode and travel through a vacuum to the plate. The cathode is heated by a filament which makes the electrons in its atoms excited and dance around and thus they can be attracted away by the plate. The hot filament is the glow you see within a tube and the reason the old radios had to "warm up" before it would work after you turn it on.
With that limited understanding, it is easy to see where the theory concerning spark plugs came from. They figured that the center electrode was the hotter of the two electrodes, and it’s negative electrons are excited and dancing around and easier to attract to the positive side terminal. So there ya go, believe it or don’t.
July 9, 2015 at 1:32 pm #19735What legendre said is also true of opposed twins. Their plugs are in series, and they fire simultaneously, with current flowing from center to side in one plug and side to center in the other. I’ve never heard of a correlation with one cylinder being more prone to fouling, etc.
John Van
July 9, 2015 at 2:55 pm #19744This whole current thing has been screwed up for centuries. I remember hearing in school (more memory flash backs?) that Benjamin Franklin assigned the terms positive and negative with regards to voltage and current flow and he got it backwards. Electrons, which is the actual current flow, go from negative to positive, the opposite to the convention we are all taught.
With tubes used in radios the object was to have a heating elements to “boil off electrons” as the old timers used to say. This cloud of electrons would then be manipulated by the anodes and cathodes to amplify, etc. That is the point of having the polarity assigned at a spark plug to boil off electrons.
The center electrode is hotter than the side electrode since the side electrode transfers heat directly to the head. The center electrode has a longer path to transfer the heat hence the length of the center electrode to the base of the spark plug defines the heat range.
The goal is to have the center electrode more negative than the side electrode so the electrons will flow from the center electrode to the side electrode. Given the center electrode is hotter than the side electrode the electrons are freer to “boil off” the center electrode increasing the ability to make a spark. In theory it makes sense.
However as pointed out many engines run with one cylinder’s center electrode negative and the other positive. My 1943 Harley works this way since the spark plugs are in series with the secondary of the ignition coil. One spark plug’s center electrode will have a positive potential and the other a negative potential.
The polarity may be more critical to prevent plug fouling in low tension ignition systems used on 2-stroke engines that use a heavy oil mixture. Certainly with modern high voltage ignition systems and lighter gas/oil mixtures on 2-strokes it doesn’t matter anymore.
July 9, 2015 at 4:57 pm #19755I’s say the whole point concerns ignition systems that are marginal at best. If the available spark voltage is barely adequate, then the 15% I previously mentioned could make the difference between firing and not firing. Nowadays, modern CD systems can throw a 3/4" spark with ease, so that piddling difference amounts to nothing and is all but forgotten, except by us old relics (me).
July 9, 2015 at 5:34 pm #19757And me (old relic also)
-
AuthorPosts
- You must be logged in to reply to this topic.
