ignition wires -- resistance

[mtiso]

ok...
I just got my frigging Alfa repair manual.... and no reference on the resistance max/min value on the ignition wires...

Anyone knows how much they can be?
1988 Alfa Spider.....

Thanks, Mike

[papajam]

I think it's something like 1000 - 2000 ohms per foot.

[tubut]

See http://www.mightyap.com/Data/OTL0798.pdf

[papajam]

Silica leaching? Silicone/polymer conductors? Oh, myyyy!! Guess I'm still living in the pre-auto computer, zero ohm, solid core plugwire days whose RFI emissions were so high they screwed up radio reception for a hundred meters in all directions!

[mtiso]

he papajam....
you need resistive cables..... to get a better spark.

.... in order to get the very high voltage (without inducing a huge radio wave killing everyone with a pacemaker) you need a high resistance to generate a high voltage (Ohm's law).. thus you decrease the engine power if you'd add a low impendance (like a copper wire) to a "modern" ignition system.....

[mtiso]

ok here the results...
(seem a little off to me)

6000
6200
7000
7500

the main cable was 1200 Ohms.....

should I replace the ingnition wires?
The plugs lookes kind a black and fouled.. not to bad.. only one plug had the brownish center I expected............

I will get a new rotor, distributor cap (shows deteriation on the contacts) and new bosh plugs......

[papajam]

Those results don't seem too bad. But since your changing the plugs, cap and rotor, may as well change the wires too - especially if they're the 15 year old originals.

I knew this discussion would come up sooner or later. This is what I was taught.
High resistance ignition wires are used to reduce/eliminate RFI to the radio and onboard electronics and to a lesser degree, premature wear of secondary ignition components. Period. The resistance in the secondary ignition circuit, including resistor sparkplugs, has nothing to do with how "hot" the spark at the plug is.
To back up a bit, when the low voltage coil primary field collapses when the primary circuit is interrupted either mechanically (points) or electronically (optical, Hall effect), the collapsing primary magnetic field induces a high voltage from the coil secondary. This high voltage then travels to the distributor cap, thru the rotor and finally to the sparkplug thru the plugwire. For discussion, let's say that the voltage required to jump the gap at the sparkplug is 10,000 volts (although pre-emission engines with smaller plug gaps and richer mixtures need only half that). After the plug fires at 10,000 volts, there's still 9,999 something volts left over. So where does it go? Back to the distributor. But since the rotor is no longer lined up with plugwire tower, it has no place to go so it reverses direction and goes back to the plug. The voltage is too low to jump the gap at the plug so it reverses direction again, and again and again until it finally dissapates. RFI is the result of all this voltage bouncing back and forth.
Enter resistance plugs and wires. This added resistance results in much faster dissapation of this 'leftover' voltage virtually eliminating RFI. Although the added resistance forces the coil to produce a higher voltage to overcome this resistance, the voltage required to jump the plug gap doesn't change.
This brings up a common misconception thanks to clever marketing about high voltage (or high performance) coils. A high voltage coil does NOT increase the voltage at the sparkplug. It increases AVAILABLE voltage to the plug. A coil will produce only enough voltage to jump the plug gap and no more. But the coil may also not be able to produce enough voltage under cetain conditions. If, because of higher cylinder pressures under acceleration, wide plug gaps, lean mixtures, high compression or fouled plugs to name a few, the existing coil can't produce the required voltage to jump the plug gap, a misfire will occur. Under these conditions, a high performance coil may produce the required voltage to jump the plug gap preventing a misfire. So it's not that a high performance coil increases performance but rather it can help prevent performance loss.

[mtiso]

Hi Jim,
cool that we are starting a more technical discussion here...
there are a couple of things that I alsways wanted to know...

first off all I thought that modern high voltage transformers NEED the additional resistance on the spark plug wire... otherwise the voltage would not reach high enough values

secondly. as far as I understand this.... before the spark the transformer generates a (for arguments sake) 15kV voltage pulse. This voltage pulse travels along the spark plug wire to the spark plug where the gap and environmental varibale determine the spark voltage.. lets say 10kV. As soon as the spark voltage is reached a spark occours. The duration is depending on how long the rotor closes the contact (and how much energy there is to keep the current up to create this amount of voltage).
Lets say that the spark would stay until it is interupted by the moving rotor:
a) low impendance cable (copper wire)
b) high impendance cable
on a.. there would not be any voltage bouncing forward and backward (the 5kV "leftover" voltage") since the resistance is so low that there is (practically speaking) no voltage difference beween the input and output of the wire. The only voltage leftover is based on the capacity of that cable...... which is very minimal... HOWEVER there is a current bounce that induces a high energy pulse(es) into the system........
Additionally, the low impendance calbe causes an excessive current flow (limited only by the transformers impendance)... espscially because we are talking about pulses that will cuase most of the RF noise induced by the ignition transformer. ..... consider that the transformer will be shorted to ground every time the spark voltage is reached... all the leftover voltage would be abosorbed and converted into heat (and RF noise)....

b) since it is a high impendance cable.. there will be an instand voltage shift as soon as the voltage reaches the spark voltage.....
the 15kV form the transformer will be connected through the rotor.. reach the plug... spark voltage occours... current starts flowing.... the voltage now divides between the source and the wire, substantially limiting the current and load on the ingition transformer.. when the rotor passes.. the left over voltage in the cable has only a very limited energy amount (based on the cables capacity) and will be converted to "heat" very fast

Oh yeah.. I really hate the sales pitch that one needs a high performance ingition system to produce higher voltage sparks
however the spark itslef could very well be different (looking at a square angle current wave form) ..the performance ignition system might provide a higher energy level that under certain circumstances increases the spark duration.... ????


Cheers, Mike

[papajam]

Starting a technical discussion? HA!! I thought I was finished 'cause that's about all I know (or more accurately, remember)! Please keep in mind that I'm going by what I was taught at university before electronic ignitions were the norm. So your thoughts on modern electronic ignition systems could be true. Seems I should do some reading to get a little more current before responding to your respose, eh?
Found a neat little write-up on how to pick the right coil for your car. Although it's written with the drag racer in mind, the principles should be the same. Interesting stuff.

[mtiso]

Hi Jim,
interesting article.. was a good read
I would like to know how todays systems work.... since a normal Eurospec 4 cyl 4valve engine cranks to 6500 RPM.. looking into smaller displaced ones (especially Honda) they crank up to 8000 RPM....
Looking into more high tech ignition systems... I remeber Audi uses (used) a individual ignition coil per cylinder (was kind a build in the spark plug) on their 5 cyl "S" cars (was back in 93') and cranked close to 7000 RPM's.....

[Bill]

Gentlemen,

Not to get into this discussion, because I am NOT knowledgeable about this subject, but because I am interested.

Based on what I understood from your discussion, am I correct in assuming a '78 Spider with a stock twin point Marrelli and Bosch coil with internal ballist would be best served by using 'high resistance' plug wires? I am using Champion plugs set at .030"

Keep this thread going because it is fasinating!!

Thanks for your input.

[alfa_corse]

Mitso-

If your alfa is hard-starting, and suffers this worse in wintertime/moisture/rain, it's very likely the ignition wires. Changing to new wires makes a world of difference. Also, be sure and use wires of the proper resistance along with the stock resistors that go on the ends of the plug wires (they suppress noise to the ignition system). I've been running the Bosch Platinum IV plugs and like them over the Lodge or other brands.

-Russ

[tubut]

Quote:

If your alfa is hard-starting, and suffers this worse in wintertime/moisture/rain, it's very likely the ignition wires.

[alfa_corse]

Yep; definitely do the cap/rotor if you have not eliminated them as the problem also. Just wanted to mention that the wires are so often overlooked; and they can test good with the Ohm meter but still function poorly when high voltage is applied. The weakest point in the Bosch system is the 'weak' spark; so good wires/distributor/plugs/battery&alternator are very important for good operation

[gotglasses]

Hi Guys ! My experience with four Alfas and several Fiats is somewhat different. I found that Alfa Ignition in general is pretty good, the FIATS always marginal. The Bosch wires are the best - durable and well insulated so they should not cause arcing problems unless they are damaged / burned. Substiting "modern" silicone wires is asking for trouble. Better to have old ones that work. The Bosch wires I have are durable metal wire core with strange resistor ends that provide a good contact. I hope that these are still available. Alfas are not normal cars - their engines get a lot of attention and owners are constantly fiddling with them . The Ignition wires, esp the carbon core type, do not take kindly to handling.

After replacing the cap and rotor be sure to replace that black plastic clamshell distributor cover that came on the car, you may have to cut up the rubber wire grommet to put it on new wires , then tape over it. Unfortunately that clamshell is rarely present because some mechanics throw it away since it is a knuckle buster to work around. The clamshell/distributor cover will be a first line of defense against condensation and moisture and will end your soggy morning problems. My 79 X1/9 simply would not run more than a week if that shell , or the black plastic cover that comes on original Bosch caps ( My Audi / Volks) and marked "do not dispose", was not replaced by the mechanic.

My Quad was missing the cover and had been giving some wet weather trouble to the previous owner. I was very pleased to get a complete cover at a wrecking yard where I bought spare electronics/relays/ computers off a Spider headed to the crusher.