Q - spark plug wires

[tosoutherncars]

Hi all,

I'm trying to get to the bottom of a very rough idle, and a hard miss throughout the rev range. (Details to follow in another post on that topic.)

Among many other things, I am suspicious of my spark plug wires. I have several sets, all of dubious heritage.

What is the acceptable resistance for plug wires? (If it makes a difference, I'm using a standard ballasted coil, optical pickup in the distributor, and Fireball ignition mgmt.)

 

[Woodie]

Duncan, I used 8mm wires (blue streak ??) got em from the parts place behind east side marios in Bells corners...don't know resistance, but they have been great in mine using sport coil and pertronix

 

[JerryB]

Stock type carbon core plug wires have about 3000 to 7000 ohms per foot. Spiral core resistance wire has about half that and the performance grades of each of those has about half as much more and even much less depending on the mfgr.

You want to use a digital VOM and let the wires sit in the hot sun a few hours befor checking or just off a hot engine. The key here is not so much the resistive value but consistency or readings per foot foprm wire to wire. Measure the entire length and do the math per foot and as you are measuring, wiggle the wire at each terminal end to see if anything changes. Most problems with resitive wire is burn back of the core at the terminal end or people raunching off the boot by the wire.

Usually a clue to a failing resistive wire is missing under heavy load. Try putting an induction timing light at the spark plug end of each wire and shining the light at a dark object to see dfferences in spark patterns.

 

[Dave Russell]

Hi all,

I'm trying to get to the bottom of a very rough idle, and a hard miss throughout the rev range. (Details to follow in another post on that topic.)

Among many other things, I am suspicious of my spark plug wires. I have several sets, all of dubious heritage.

What is the acceptable resistance for plug wires? (If it makes a difference, I'm using a standard ballasted coil, optical pickup in the distributor, and Fireball ignition mgmt.)

Hi Duncan, Jerry has good advice.
Depending on the wire design, resistance could be anything between a few ohms to 15,000 ohms. Typical might be 9,000 ohms. (9K ohms) Check your wires & see if they measure in this range.

To put this 9k ohms of plug wire resistance into perspective, the distributor rotor to cap gap is likely to be around 60k ohms when ionized. Additionally, the plug gap itself may be somewhere around 160k ohms or more when ionized, & the cap center carbon to rotor around 11k ohms, for 240k ohms total resistance.

The resistor plug wires at 9k, represent about 4% maximum increase of total circuit resistance. Since the average coil puts out enough voltage to handle two or three times this 240k resistance, I doubt if there will be much gain from decreasing the plug wire resistance, unless there is already something marginal in the ignition system.

If estimated plug current is 50 milliamps. 240k * .050 A = 12 kv to fire the plug with 9K ohm plug wires.

With zero resistance plug wires, 231k * .050 A = 11.5 kv to fire the plug.

The numbers I have used are subject to many variables & they should be taken as an approximation.

Vendors make a big deal out of high performance, very low resistance plug wires but they have very little affect on plug firing. However, it does make folks feel better.

Any good shop with an ignition scope could quickly tell you if the problem is ignition related.
D