My plug wires have no conductivity

How high an impedance?

 

Time for some new ones.

 

What you are attempting to read is the wires RESISTANCE to DC current flow.

Impedance is a measurement term for AC (alternating current) and is meaningless without specifying a frequency or range of frequencies.

Unfortunately, both express measurement units in ohms, hence the confustion.

Some plug wires have quite high resistance but since current is relatively small, doesn't alter voltage that much. Initially, resistance wire were used for plug to reduce radio interference, back in the days when people listened to music from the dash rather than the tailpipes.

 

Well that's pretty cool! Question is, is that red rust? Shouldn't wires be something more conductive than steel? It's not surprising it still ran, that high voltage can still make it across some fairly bad wires. Just not enough to run well. On a related note...saw some wires for the 355 on Ebay...has anyone tried these?

 

Maybe temperature? Any higher towards the back?

 

Good day,

Not to nit-pick this too much, but impedance and resistance can be used interchangeably when the voltage is DC... It is true that Impedance is a function of the frequency, however DC is just AC with a frequency of zero... Hence the same units and why they can be used interchangeably in this case.

On a secondary note, the spark plug voltage is really not DC, as it is constantly being turned on and off...which makes it (relatively) a low frequency AC signal. So, impedance would be a correct term for the spark plug wire.

Cheers,

Sam

 

Gene,

The OEM wires come in sets of four........so it's possible to replace "one half" of them if they only had a problem with one side..

I always buy both sets and replace at one time.

 

^^ The sets I carry also include the coil wires. It's a sealed pack of 10 wires -- 8 plugs + 2 coils.

 

That's a good idea!

 

Let's pick this nit just a little further: The spark is made of up of two parts, a capacitive component and an inductive component. The capacitive component lasts for only a few microseconds! The inductive component lasts for many hundreds of microseconds. It is the inductive component that makes the nice fat spark we all like and it is this component that generates enough heat to vaporize the charge for cold starts. A warmed-up, well tuned engine really runs on the capacitive component. Okay, with all that said, we have gotten to the frequency bit. Because the capacitive component is a really short duration spike, it has a very broad spectrum. That is, low frequency out to very high frequency. In fact, the frequencies are so high that they can be received by AM radios as static - a million Hertz and beyond!! (Those of us with grey hair can remember having to add noise supression to our cars. Its also why fiberglass cars have shielded ignitions.) So the point is, the periodisity of the spikes is realtively low frequency but the frequencies contained in the spikes are very high and broadband.

Rick / WD4KIB
'79 308GTB

 

So is the ignition system then AC, DC, or both?

Its funny what a guy can forget in 30 some years, but working with the points and forcing it to behave has caused a re-learning. I was reading how the condensor suddenly absorbs energy as the points break, and how it builds up a higher voltage to in some cases well above 300 volts, and then backfeeds the energy in reverse back into the coil. This sudden spike in energy in the coils outer windings, saturates the inner windings and "induces" the voltage, rising to whatever voltage is needed to cross the gap at the plug. I'm sure one of you can better translate what I just described, I just find it facinating.

I too was having some minor running dificulties when I first got the car running, little pops and sputters, but especially a breakup above 5000 rpm. And again, what you forget. I remember some 20 years ago fighting the same crap with another points car, and no matter what I did it would miss. Of course the testbook answer was to dump the antiquated points and jump to electronic, but darnit, I wanted to make it work. Never did though.

So here I am fighting this again, and along comes Scott (Smg), and reminds me that points ignitions dont generally like resistor plugs. They work "okay" if the system is at 110%, but problems develop as the system breaks down. Swapped in some BP6ES plugs, and the majority of my running troubles vanished. But later, when I was tearing into the car to put in belts and adjust the valves, I happened to notice the end of a plug extension that looked melted. The plastic or whatever it is, was all melted near the upper tip, and down about the middle of its length it had a hole burned through the side. No others appeared faulty, but I swapped them all out and gave all of them a good look over. Then, because so many of you have suggested checking the wires for resistance, I did that and low and behold found three that were bad. Two were high, one was dead. Yet the car ran fairly well. Just goes to show how messed up things can get and still appear to be alright. One would assume a higher energy electronic ignition system would easily mask far greater problems in the secondary side. Probably not a bad idea to go through the entire secondary side on some regular basis.

Rick. What does WD4KIB mean?

Paul

77 308GTB 21181
77 308GTB 21645

 

Although it is for a very small period, the ignition spark is generated by a DC voltage of several hundreds Volt and the current flowing is a mainly a DC current.

About resistance, try to measure the resistance of your sparkplug and the result will be that you meassure no resistance at all.
But after having received a very high voltage on its tips, in the order of some 20.000 Volt, a conductive plasma is formed being the actual spark.
The voltage goes immediately down from 20.000 Volt to below 600 volt for the time needed to discharge the energy stored in the ignition coil, being 1/2*L*I*I Joules.
L being the inductivity of the primary winding of the coil, and I the current flowing through the primary winding at the moment that current flow is interupted.
The time needed to discharge the energy stored in the ignition coil is the time that the plug is producing a spark, which normally takes 1 to 2 msec.
So if you cannot measure a resistance, it does not prevent a current from flowing.

But to minimize losses, and to keep as much energy left for the plug, the resistance in the cables should be low.
On the other hand, a DC voltage spike of very short duration and with a very fast "rise time" like the one that is generated before the plug starts conducting, is composed of a very broad spectrum of AC frequencies and generates therefore much noise in the radio frequency band ( this probably sounds confusing).
To suppress this noise, a resistance can be added. This resistance will cause that the 20.000 volt spike has a slower rising time and because of that will act as an effective noise suppressor.

You can use spark plug caps with an inbuild resistance, or alternatively plug wires with a resistive conductor.
Those resistive cables tend to increase their resistance over time, and should be replaced every so many years.

 

So this is why HP wires, and the wires on the early Ferrari's are solid core wires?

 

Let's nitpick even further. A spark plug voltage is not AC, but technically speaking, pulsating DC. The input signal to the spark coil is a pulsating DC voltage fed from the distributor, so the collapsing field on the coil secondary is also DC, not AC as would be the case in a power transformer in which the input side is fed an AC signal. The voltage on the secondary does not technically swing in the negative direction, therefore, it is only pulsating DC, not AC.

An antique vibrator power supply operates in a similar manner, but the DC voltage is constantly being switched back and forth to the transformer primary coil, thereby simulating an AC input. An automotive spark coil is only fed pulsating DC from ONE direction, thereby producing only a momentary high voltage DC output.

Er,.....isn't that correct? Isn't transformer technology fun to discuss?

Sincerely, Horsefly (averaging .707 times your peak unfiltered output voltage. Assuming of course that I'm using a choke input filtered power supply as opposed to a capacitive input filter.)

 

But if the condensor absorbs current, and then pushes it backwards up the coil negative wire into the coil, wouldnt that be considered alternating current? Isnt the concept of alternating current occur when the current flow is fluctuating in opposite directions?

 

A condensor/capacitor can only discharge its current in one direction. Unless you have a pair of capacitors wired back-to-back feeding opposite sides of a transformer coil winding, one would think that you would still only have DC current.
If a capacitor absorbs a +12 voltage, it can only discharge a +12 voltage. Some method of applying that +12 voltage into the OPPOSITE side of the coil would seem to be the only way of creating a reverse current flow. Correct me if I'm wrong, but a distributor only "makes and breaks" the flow of +12 volts to one side of the coil winding. There is no reverse switching. I thought that the condenser was primarily there to prevent arcing of the points in a point type ignition system.

This all warrants further discussions. Possibly 10 or 12 pages worth!

 

I'm pretty sure that the rotor does not change phase as a function of timing advance, so the rotor tips have to be wide enough to be lined up over a contact no matter how much advance the distributor calls for. Keep in mind that as the engine speed rises, the little weights in the distributor spin faster and advance the spark. If the tip of the rotor was not wide enough, it would end up away from the contact when the spark fired and you would get an arc, or perhaps not even an arc, in the distributor.

I believe the complaint about RESISTANCE versus IMPEDANCE early in the thread was based on the fact that when one measures plug wires with a multi-tester ("ohm meter"), you are only measuring simple DC resistance, and you cannot possibly get a measure for what the wires are doing with high voltage, or with the "AC" like signal created by high speed switching by the points.

Birdman

 

Correct.