Question For Spark Plug Gurus

Eight Champion resistor spark plugs were purchased yesterday. Due to my own curosity I checked the resistance of each plug from the point where connection to the plug wire is made to the side of the center electrode. Six of them were in the 50K ohm range. Two were in the 190K ohm range. Repeated checking always gave the same results. I assumed two bad plugs were bad. For the heck of it I tried them out on a small push-type lawnmower. I removed the mower's plug to make it really easy to pull the starter rope. I attached each plug to the wire and slowly pulled the rope. The plugs fired. Next I tested the resistance of the plugs again by placing the probe on the side of the electrode (not where spark occurred). Now the two 190K plugs have a resistance of 60K ohms. I installed them in the car and the car runs perfectly. What's going on here?

 

Don't overthink it, more resistance just means the spark will not be as intense. Use NGK next time

 

Unfortunately I don't have a lot of time right now to cover this, but I'll post what I can now.

measuring the resistance of the plug with a hand held DC meter operating off a 9v battery isn't going to tell you much, other then low voltage DC resistance. The voltage traveling thru the plug avg 25kV depending on system etc.. the peak current is ~5A primary side so secondary would be ~0.001 or 1mA. the pulse shape and frequency (rpm) regulate the ignition system to a high frequency HVDC pulse circuit akin to radar systems. What this means is that a scope is better suited to measuring the plug then a basic fluke meter.

That all said, good plugs should not measure above 5kohms in working conditions, the ignition coil wold not be able to generate the voltage needed to jump the gap and would short before the plug. your numbers may be high due to nano-meter oxidation on the electrode interfering with the probes contact and also due to DVOM not being able to generate the kV of voltage that the ignition system delivers, hence the use of a scope on a running engine.

 

SMG2 I find some of your posts hard to deal with. I appreciate you are an experienced engine builder and only trying to assist and I dont want to appear rude but when you describe ignition and other systems it is largely techno-babble and pretty much completely incorrect. I apologise for being critical but I just like to see information which is basically correct and not one single point in the above is right.

To OP the reason you saw the difference is likely to be the resistor element inside the plug is contacted at each end by a spring and probably had slight oxidation on the contact inside, which burned off when the plug was used for a while. But Ferraripilot is spot on on both points.

 

Don't worry I don't take it personally, but would you mind pointing out exactly what is wrong? what techno-babble? I'll gladly correct anything that's wrong.

 

Both of you say that the reading difference is likely caused by some oxidation. There is a difference in your proposed locations but both are plausible.

SMG2 adds to that with other correct information. A DC multimeter isn't a good tool to check a plug and the results are suspect. The 25KV+ on the secondary is correct and I suspect that the 5A peak on the primary is correct too (but I have never measured it).

The only thing I believe he said wrong is the peak secondary current because how a coil works is pretty complicated to analyze. The coil stores energy (like a battery) and the spark is driven by the breakdown of the static magnetic field, releasing the stored energy in the coil. As a result the secondary current is not directly related to the peak primary current but is related to the field strength, coil resistance, number of windings, core material, etc. As a swag: the arc across the gap has a resistance of approximately zero (yes, it is an excellent conductor). If the coil generates 25KV (it likely makes much more at T0) and the impedance is really 50K ohm across the frequency range (also not likely) then the peak secondary current would be 1/2 amp, limited by the internal resistor. Unfortunately there are too many unknowns and assumptions to be 100% accurate. At the moment of ignition (T0) the peak current may well spike over 50A but it is only that for a tiny fraction of a second.

So, why have resistor plugs? It slows the leading edge of the waveform and limits the high frequency components of that pulse. That reduces the noise that is generated (electrical interference) and it spreads the spark over a longer period of time improving the burn in the cylinder.

 

OK.

The first point about resistance, there is no difference in the resistance regardless of voltage unless we are discussing some special device such as a transorb element which we are not. So measuring with a meter is quite sufficient.

You cant relate primary and secondary current in that way. The factor across the coil which is constant is energy (minus losses). At the point of spark the primary current is zero because the points (or other controlling device) have opened (CDI ignition works differently). The primary current is applied over a relatively long period and the energy is stored in the coil during that time. When the field collapses the induced voltage is high (of course) but the current is also high once the spark has fired . All the stored energy is dissipated over a very short period.

"the pulse shape and frequency (rpm) regulate the ignition system to a high frequency HVDC pulse circuit akin to radar systems. What this means is that a scope is better suited to measuring the plug then a basic fluke meter. "
I cant really comment on that as I have no idea what you are on about!

"good plugs should not measure above 5kohms in working conditions, the ignition coil wold not be able to generate the voltage needed to jump the gap and would short before the plug".
At the point of spark there is infinite resistance in the circuit caused by the spark gap, so there is no voltage drop across the 5K resistor until the time that the spark actually happens. So the spark would be initiated fine but true to say that the energy would be lower because some of it would be dissipated in the resistor.

"your numbers may be high due to nano-meter oxidation on the electrode interfering with the probes contact ".
If the probes are applied properly this would not be an issue especially on a new part.

 

Fixed!!!!

 

Ignition systems are not DC circuits, though it utilizes a DC battery it is more of an AC circuit, but in reality it's pulsed DC. Pulsed DC ciruits are used in a variety of industries, notably RADAR, hence the reference to it. Most people will get that RADAR is high voltage dangerous stuff, and I think that is mostly due to how it's perceived.

the nano-meter oxidation is what occurs because we are dealing with plasma, the fourth state of matter. The spark plug operates by transferring electrons across the gap in the ionized gas and the plasma stream is the result. used plugs will require abrasive removal of deposits to get a clean contact, new un-used plugs will not. Also to note, this is the wear that occurs to plugs, the electrode deposits itself electrons at a time to the plasma stream to create that 'spark' or kernal.

onto the resistance, high voltage pulsed DC. the current will have transients and travel on the 'skin' aka skin effect. low voltage DC measurement with DVOM will not be anywhere near the same as when in use. The coil in an inductor ignition is always charged to 10~12v DC it's a static magnetic field, when that field is collapsed that BEMF will induce a current multiplied by it's ratio to the primary, a coil is a step-up transformer and will not work with DC but time variant fields only, AC or Pulsed. CDI is indeed different in that a capacitor is charged and then discharged, no BEMF field.

The current traveling to the plug is not 'in' the wires but 'on' the wires. the wires can be viewed as RF guides.

While I do indeed build and engineer engines, I also spend a fair amount of time on high voltage systems and plasma research. I understand that a lot of it may indeed go right over the top of many, I also hope that some may take something novel from it and see that there are many connections in technology and uses of them across industries.

Pulsed DC is used in,
automotive ignition
radar
radio
television
weapons
lasers
telephone
the interwebs...

 

The secondary current does vary quite a bit and you're right in that I'm not talking peak current as that gets into ionization of the gasses and impedance of the line distributed reactances etc..

And yes transformers are beyond complex, I've spent more time then I ever thought I would on them. between theory and design, two good books I do like a refer back to is "Principles of Transformer design" by Alfred Still and "Electronic Transformers and Circuits" by Ruben Lee, an old 1955 internal Westinghouse book.

Resistance is critical in the ignition system, esp in inductor coils where a balanced line will have the best result.

 

LAWL made my day. thanks! Gangster

 

I dont believe Skin Effect has any bearing on this despite what manufacturers of expensive ignition leads might say. This primarily happens in high frequency AC circuits where the magnetic field is constantly reversing. In ignition we have low frequency pulsed DC. Although the magnetic field is switching on and off, which might cause some eddy currents, its not reversing and the frequency is low.
RF is not a factor here other than the radiated RF interference, it does not form a part of the spark energy.

 

SMG : My 328 had iridium plugs in it, (NGK) so I replaced them with the same recently along with new extenders. Is there any benefit to iridium, in your ( expert to me) opinion?

Thank you.


I see this as germane to the the thread as some of my extenders had electrical tape around them and I had recently developed a slight miss. It went away with new plugs and new extenders, though this is the first time I have used iridium plugs.

( I had to buy 2 special 18mm sockets to make it easier to change them.)

I'll post about them in my sticky " 308 cross reference thread" later this week.

Thank you.

 

This is a debated subject, heavily debated and I've seen some serious vehemence thrown about over it. the coils magnetic field is switched on/off rapidly and being a transformer it generates transients, these transients have been measured into the several hundred Mhz range. the frequency domain that has been measured on vehicles is usually always within 0-100Mhz with the bulk being 70Mhz and up.

The trigger events for a ferrari V8 is 4k to 36k per revolution, now with multiple coils it will be split, lets say between 2, two coils triggering in close proximity to each other will have some interesting effects, transients are one. in mil-spec stuff the coils would have to be shielded or arranged in such a way to avoid inducing magnetic coupling or interference. these are all losses, less energy to the plug.

 

Long life, that's about it. it's an extremely dense material and has poor conductive properties. It's become the OEM plug for a many mfg's once they started to underwrite the maintenance on them.

I prefer and will use silver when I can for the toys and the DD will get copper. copper is getting harder to get though. silver has gotten pretty pricey, Bosch is on National back order for the silver plugs found in most all the M14 sized Ferrari plug motors.

The extenders are very prone to burn thru, as resistance builds up on the plug the spark energy looks for the next best route to ground. older engines don't like the new plugs as the ignition system is different and the combustion chamber and efficiency is also very different from modern stds. in other words the lower temps of the plasma will not keep the plug as clean as it could be, the fuel additives leaving deposits, etc..

 

OK, as long as we're picking nits I'll jump in. I hope I can clarify this subject a tiny bit for the great unwashed and, at the same time, avoid offending the gurus.

First I appreciate your efforts to explain what can be a rather complex subject in a simple way so non-scientific Ferrari owners can gain a better understanding of how their ignition works (or doesn't work). I would argue that a coil doesn't store energy "like a battery". It also doesn't store energy like I do by eating another jelly doughnut. I think the idea of "storage" is the part that's misleading, or at least potentially misleading for some Ferrari owners.

In other words, the battery analogy is not such a good one when it comes to coils, in my opinion. I guess that pointing out that a capacitor acts something like a high voltage battery might be OK. It's certainly not the whole truth about capacitors, but it's a starting point. I reckon most folks have a pretty good idea of how a battery works and a slightly more vague idea about capacitors, but most would say that both devices can accept energy input and (somehow) store it.

But when it comes to coils, I would think that folks would understand more about how a coil works if you said they change low voltage to high voltage much like a transformer does. That is to say a coil is much more of a voltage converter than an energy storage device.

 

No offense at all, actually I rather enjoy a good discussion on technical matters. I also tend to think even the non-technical people do to, though it may get a bit deep but can be fascinating -I hope.

You're of course 100% right that it's a transformer, and that is one of the 'black' arts in physics, oh sure EE's will claim it's simple or that it's not hard to calculate and design, but we all know it's deeply mysterious and utterly complex.

now condensers aka capacitors, oh don't get me started. them are some black magic

real nit picky, we can not store energy, we simply create and convert it with losses for the most part, lest we get into meta-materials and negative index of refraction.

 

Yes indeed this is the RFI that is such a problem. But this is just an unwanted by-product. The energy we are actually interested in is just a bunch of electrons moving down a wire. A DC pulse, nothing more. Nothing to do with Radar or the wires being RF guides or anything else.

 

Yes it can be misleading. But its also an important point that in a conventional ignition system all of the spark energy is stored in the coil as magnetic energy prior to discharge each time. So it is a short-term storage device. In a capacitor discharge system it is just a voltage converter.

 

In a pulsed DC transmission line the magnetic field is moving, as a matter of fact electricity does not flow nor does it travel in the wires at any Hz or in DC circuits. The water flow analogy is flawed and has caused more problems then it aimed to solve with understanding electricity. But I digress.

The unwanted RFI is a direct result of the type of transmission and transients from the pulsed circuit due to the nature of the collapsing magnetic field and frequency. look at the construction of the wire, early days a solid wire was employed and caused massive RFI, now a spiral core is employed, why? the windings are very close to perpendicular to the moving magnetic field, that means the spark energy is not traveling in the wires but in the magnetic field surrounding them. A solid wire becomes an antennae instead of a wave guide, the spiral core supplies two functions, wave guide and impedance.

 

I agree that they work on completely different principals. And batteries can be thought of as very big capacitors; with an electro/chemical component thrown in for good measure. I was really trying make the point that the coil is stored energy and isn't used in the way that people think of transformers (step-up/step-down).

Andyww, you only get radiated RF energy because there are very high frequency components to the leading edge of that waveform. That HF pulse is what initiates the spark on the plug so you can't just ignore it.

A good example of this is when I worked at LLNL we had a huge capacitor array that we used to fire a laser. We would charge the capacitors in parallel then switch them to series BUT they were all separated by a spark gap. Then you pulse one end of the chain causing breakdown of the first gap. The pulse travels along the chain breaking down the gaps in succession and soon all of the capacitors are in series and connect to the laser. As you can guess, that releases a HUGE amount of energy with a truly impressive slew rate.

 

We can not store electricity, a battery is a chemical cell that converts electron exchange via a chemical process into electrical potential. a magnetic field also does not store energy, a static magnetic field requires energy to produce, it is also in conjugate pairing to the electric field. a moving magnetic field will induce a moving electric field due to the conjugate pairing when a conductive director (wire) is in the path. I'll skip over the angles, rotations and relations to charge potential.

A coil is also employed in CDI but instead of a BEMF a voltage potential is used to generate an MMF. the CDI will produce a higher output voltage but shorter pulse duration to the inductance pulse using the BEMF. Both systems are voltage convertors in that sense but are really current transformers. If we simply tried to step up voltage potential using say a cockcroft-walton generator we'd never get the needed current with the voltage available in an automotive circuit, the losses are far to great. since all that is really needed is a ~25Kv breakdown and then only a couple hundred volts to sustain, the duration and current becomes the design factor.

Once again, there is no storage as that would violate the laws of physics, and it's very simple. Electricity is the movement of charge that is defined by amperes, if there is no movement there is no electricity. voltage potential is just that potential to do work but no work being done, this means that while you can have massive potential between two points you can also have zero current flow and thus zero work. It can seem confusing but in order to say there is electricity that means it is in use, thus it can't be stored. Conversely saying energy is stored is highly ambiguous and leads to the wrong assumptions for many people.

So in the case of an ignition coil the collapsing magnetic field induces a magnetomotive force (MMF) in the secondary windings stepping up the field potential and that induces a greater EMF in the wire. There is also a lot of other things occurring with saturation and leakages, rise times, pulse shapes and line impedance but that will get very deep for this venue. There was no stored energy at any point but a transformation of forces to generate the needed charge, it is completely dependent upon a delta of time, no time variance no energy.

 

Had I known you played with this the last time we met in Campbell we'd of had some more fun. Having played with both Marx and Cockcroft networks they are FUN and can generate some SERIOUS voltage. I've been working on the calcs for another type of generator that is, trying to think of how to explain without getting in trouble here.. electrostatic field charge that is impacted by an accelerated electron flow via high potentials. As with anything it requires massive voltage gradients to get kick started. and that is always the hard part, along with pulse shape, overshoot and slope. PFN's are fascinating and FUN. I'm a Nerd I know

 

There must be energy storage otherwise you would be producing energy from nothing which definitely violates the laws of physics. Energy is fed into the coil by the primary winding over a relatively long period of time and then discharged out of the coil via the secondary in a very short period of time.
At the time the points open there is no energy being added to the coil but the is energy coming out of it so it must be stored energy.
Its not stored as electrical energy of course. I didnt say it was.

 

You would still get a spark if the voltage were constant. It does not have to be a pulse. If you get a CRT TV and remove the final anode cap and switch it on (not really of course this is only theoretical!) and brought up a grounded screwdriver the spark would arc when its close enough. Sure, once the arc is made you can open the gap up and it will continue to spark but no pulse is needed to initiate the arc, just sufficient potential. The use of resistance wires and plugs is specifically to reduce the RF energy by slowing down the leading edge of the pulse.