Redeasysport

I still can't see it unless the plug is isolated from ground.There is no potential difference between the 2 straps if they are at ground.

Nathan Plemons

Refer to my example and consider the "negative" terminal of the battery as simply a zero point. It is not absolute zero, it is simply a point of reference. Potential can be lower than or higher than this point. If the potential is higher you have a potential difference, and the POTENTIAL for current flow. Likewise if the potential is flow you still have a potential difference and the POTENTIAL for current flow, simply in the other direction.

Try to visualize it in terms of temperature. Say you have an object at -10 degrees, one at zero degrees, and one at 10 degrees. Heat will always transfer from a higher temperature to a lower temperature. So, you can transfer heat from the 10 degree point to the zero point, call this positive if you will because you're transferring heat to the zero point. The zero point is warmer than the -10 point, so it can transfer heat to it. In this case heat would be flowing out of the zero point and could thus be considered negative.

The biggest point that I inteded to clarify without arguement is that ZERO doesn't mean aboslute, it simply means your point of reference.

Nathan Plemons

Absolutely the two ground straps are at the same potential, but the electrodes are not. Draw it on paper word for word, it might help you visualize it a lot better. Assuming the conditions between the electrodes in the spark plugs is the same you will see the same MAGNITUDE of potential difference, but you will see reverse polarity if you keep the current flowing the same direction. If you reverse the current you'll swap the polarities.

corveteer

this is an 87 L-98 with original wiring in distributor.
please note i do not have the original coil so don't know
coil primary lead colors from the factory. also note that some
coils can be purchased with a choice in primay lead colors.
reversing the primary wires will allow you to go to either polarity.
lead colors available are red // white or red // yellow.
it is possible to buy the wrong colors and then have
opposite of intended polarity. i thought i read once ---some time
ago that spark polarity should be positive (+)
thanks for your efforts here

JAKE

For the 86, and probably the 87 as well, the Corvette Shop Manual cautions to make sure the coil you choose has the correct color wires. The color of one of the coil wires and the color of one of the the pick-up coil wires have to agree.

Here's what it says: "The color of the pick-up coil connector has to be yellow if one of the ignition coil leads is yellow. If the ignition coil has a white lead any pick-up coil connector color except yellow is OK".

So yellow has to go with yellow and white can go with any color except yellow.

When I asked about this years ago, I was told it's a polarity issue.

Also, if you inadvertently cross the two coil wires when you plug them into the distributor cap an intermittent no start condition will result.

Jake

rocco16

You are normally pretty straight up on your explanations, Nathan, but I have to call you on this one.
For that to occur, the head would have to be both + and - at the same time. A coil fires to ground (in this case, the head), the coil is not a ground. The head is grounded, period. It cannot fire back to the coil.
If you'd stuck a finger in each of those two spark plug wires and cranked the engine, you would have gotten shocked from both terminals....they were both firing to ground. I guarantee it.
If your explanation was true, everytime a plug fired, four of the eight plugs screwed into the engine would fire.
Larry
code5coupe

Nathan Plemons

Hey I'm just going off the wiring diagram in the book. When I get home I'll scan it so you can see how you interpret it.

The picture show the secondary terminals of the coil connected to a spark plug. If they were both positive this would mean that your coil has TWO positive terminals and no ground on the secondary, which isn't exactly going to work.

Edit - nevermind, who would figure this info is quite readily available online?

http://www.procarcare.com/icarumba/r...a_ignition.asp

Nathan Plemons

Of course after all of this I still don't think we've answered the original question. You may know what the polarity is, but no idea which is "better" if it makes any difference at all.

Tom Piper

Interestingly, my 2002 Ford Ranger 3.0 V6 with a three coil "waste spark" system has two types of spark plugs (three of each) in it from the factory.
So, it seems, Ford determined that different plugs worked better depending on the polarity.

On another note, every "waste spark" system I ever saw has the secondary of the ignition coils electrically isolated from the primary of the coils (except for the common connection at the spark plug threads, that is really not needed, and if it didn't exist they would still work properly. To prove this to yourself, take the two plugs, for one coil, in a waste spark system and remove them from the engine and connect their threaded bases together with a wire. Then crank the engine and watch the sparks with the spark plugs in the air).
So, looking at it using the engine block as a reference, the secondary high voltage is really AC not DC -- the secondary of any inductive discharge ignition system (not capacitive discharge) is a dampened oscillation of several dying pulses that makes up the spark duration.
In most single coil (I'm not sure about most multi-coil) non-waste-spark systems, the primary and secondary of the coil are not electrically isolated, which makes the dampened ringing oscillating DC.

In other words, the normal single coil system has one polarity (DC) across the plugs for the entire spark duration and that spark duration is a dampened ringing signal.
And, the "waste spark" system has a signal that changes polarity across the plug (AC) for every alternation of that dampened ringing signal. The only difference is one plug in the series "waste spark" system startes firing with a positive alternation when the other plug is negative. The next alternation of that dampened ringing spark duration the plugs exchange polarity -- this continues for the entire spark duration.


Tom Piper

Nico

I am afraid that is not true. e.g. The 2CV Citroen is a two cylinder boxer waste spark system. The secundary of the spark plug is connected to the two plugs, WITHOUT ANY GROUND CONNECTION. So the spark current runs from one sparkplug via the engine to the other sparkplug and back to the other side of the coil. The plugs are connected in series. That is done to avoid a distribution, the only good reason to use a waste spark ignition system. So the plugs get reverse polarity.
PLEASE GUYS, the spark is mainly AC, not DC. So polarity is NOT an issue. You can not transform DC and it is not a single pulse. If you transform a DC pulse you end up with a long tail current with the same energy as the pulse, you do not want that in an electronic system. The capacitor and inductance of the coil do resonate and the spark actually flies a few time VV very rapidly. That is the reason why many early capacitive discharge systems did not really work well. The Thyristor used did prevent the current reversal and the spark was a one-time one. Worked less despite the fact that the energy was high.
Further more, I fully agree with the clear explanation of Nathan

Nico

I am sorry Larry, Nathan is exactly right and did explain it well. It is a current matter wich can flow only in a full circuit. If yo touch a plug without touching anything with your other hand you will be struck also because the high voltage will try to charge your body and the current to charge you notice!!. So that is no prove. When one coil fires, only the two plugs connected in series at that coil will fire

silver & red CE

For spark to occur, the voltage must build until the gas (air/fuel together) is ionized and broken over. For the gas to ionize, a very large voltage is need (tens of kVs), but the polarity of the leads doesn't matter. The electrode is isolated from ground, so you can take the electrode really negative or really positive, the only difference is in the direction of the current flow. Ground can source or sink current.

silver & red CE

only one of the plugs fire, the other plug is in a cylinder full of exhuast gas, very ionized, current flows freely (in either direction) with a very low voltage

silver & red CE

Doesn't really matter. Either polarity is fine, you're looking to see the event in time and no matter the polarity, the signal is easy to recognise. The scope doesn't care, it's protected for a max input voltage and the polarity doesn't matter with respect to protection.

The only important point is to ground the scope to the engine block and connect the probe's tip (not ground ring) to the spark plug wire. That way, the scope's protection will work and you won't burn out the scope's ground path by dumping a bunch of current through the ground leads. If you connect the ground of one channel to the spark plug wire and a second probe's ground to the engine block, it's ugly.

LT4BUD

Simple answer is negative polarity would be best polarity to have on the sparkplug tip.......will take less voltage to fire the plug reliably.....in a nutshell there IS a significant difference between positive voltage and negative voltage......

silver & red CE

I'm interested why you say this. I did some work on ignitions systems for Motorola AIEG in the mid 90's. We had a spark chamber with a plug and coil and we never noticed any difference (we did look, we had to design the IC for either polarity for single plug systems and dual waste style).

LT4BUD

I made my statement because the spark electrode system is asymetrical.......an example of asymetrical would be if one electrode was a needle point and the other was a flat surface......symetrical would be like the two spheres you sometimes see in science fiction flicks, that is both electrodes are the same shape...a spark plug is more asymetrical and thus there will be a polarity sensitivety......beyond this explanation it goes beyond KISS

I tried a quick internet search to reinforce my statement...didn't come up with much. Check this out

http://www.corollaperformance.com/Te...parkplugs.html

An excerpt from this article:

"While under operation (at a design operating temperature of between min. 900 degrees F and max. 1600 degrees F), the center electrode gets hot. Anyone who can still remember the tube transistor will remember that a hot anode can radiate more electrons more easily which is exactly what happens here, and the breakdown voltage of the gap (the voltage needed to bridge the gap) is reduced. Note that for this to happen efficiently, the center electrode should have negative polarity"

There are many parameters that influence the functioning of a spark plug.....



BTW I quess this is the Snapon tool that started this interesting discussion

http://buy1.snapon.com/products/diag...00&dir=catalog

Tom Piper

It's been a long time ago, but I seem to remember seeing a simple way to check for spark polarity on a spark plug by inserting the graphite tip of a lead pencil in the spark while it is arching. Something about seeing which way the orange flame went could tell you the polarity.

Does anyone else remember seeing this, or was that a previous life?


Tom Piper

silver & red CE

Interesting, makes sense. We never had combustion in the lab, didn't have the heating effect, might explain why we didn't see the effect. I wonder what the difference in arc-over voltage is?

edit: now that I'm thinking about this, I wonder which gets hotter, the insulated tip (that has the spark plug wire attached) or the grounded tang. The tang is thermally/electrically connect to the cylinder head and has coolant flow, the insulated tip is thermally/electrically isolated in ceramic.

Tom Piper

Vacuum tubes had/have indirectly heated cathodes (cathodes are negative) and directly heated cathodes. And, in most cases, the cathodes had a layer applied (don't remember what it was) to them to cause more electron emissions.

Tom Piper