I bought plug wires (cap and rotor too) for my '95 F150, 300 I6. I got Motorcraft and after installing them saw an extra wire. It looked like an alternative coil wire, but with a different connector. Anyway, having that laying around I decided to test it's resistance as representative of all the new wires. It was 5,000 ohms and 14 inches long (about 4300 ohms per foot). Then I tested the old wires that were used 20,000 miles ago when I bought the truck. They tested out to around 1000 ohms per foot (They are "Triple A Silicon"). What gives? The Chilton manual says max 5000 ohms per foot for the '87-'88 (I don't have the newer manual) and another book says the resistance is meant to be there and vehicles are made to run with the resistance. Truck seems to run better, but runs at least as good as before.
1) Did I replace a high performance wire set with a stock OEM set?

2) Did I get a faulty set?

3) Does it run better with the higher resistance OEM wires because it is designed to run with them?

4) Other (please explain)

At this time, since it runs fine, I am inclined to leave well enough alone, but if the OEM wires are so close to the spec maximum resistance, how long will they last?


INLINE SIX POWER!
300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!
"Drive a stick young man! There'll be plenty of time for automatics when you're old and unable."

 

Good questions Tall Paul. If Chilton says the resistance is meant to be there, I wonder if were hurting things by putting some of these high performance wires such as MSD with (if I remember correctly) 50-100 ohms/ft resistance. I'll sign up for email notification on this thread and hope someone can help us out.
Tony

 

The resistance is there, as it is in the spark plugs, to prevent radio interference. If you had non-resistor plugs and copper wire ignition cables, like many pre-1960's trucks, there would be static you could hear on the radio that would vary with the engine rpm. Modern electronic ignition systems are designed to work with the resistance.

Jim

 

My manual says that the resistance shall not exceed 5000 ohms per foot. That means that anything under that is fine.

 

I posted my problem with the SalemBoys Automotive
(http://www.salemboysauto.com/default.htm) and got this response:

"NO, all is just fine. Higher resistance wires cause a hotter spark from the coil to the plugs because of the higher resistance. The coil sends just enough spark to jump the gap, if we fool it in believing the gap is bigger, it sends a bigger spark."

 

>"NO, all is just fine. Higher resistance wires cause a
>hotter spark from the coil to the plugs because of the
>higher resistance. The coil sends just enough spark to jump
>the gap, if we fool it in believing the gap is bigger, it
>sends a bigger spark."

Wow, is that ever wrong thinking!

Look at it this way, if more resistance is better for the spark, what's the limit? Millions of ohms that you'd get by replacing the plug wires with string or boiled spaghetti?

Put resistors in the line cord of a toaster or light bulb and see if they burn hotter. They won't. Guaranteed.

The formula for determining how much power is delivered through a wire to a load, in this case a spark plug is P(power) = E(voltage) divided by R(resistance).

If you increase R(resistance) the power goes DOWN. If you increase E(voltage) the power goes UP. If you increase the resistance of the plug wires, the voltage at the COIL end of the wire may go up, but the power delivered to the SPARK PLUG end of the wire will be LESS.
We really don't care about the voltage at the COIL end of the wire, what we DO care about is what's being delivered to the spark plug end of the wire.

You need some resistance to prevent interference to the radio, and on late model vehicles, to prevent the same interference from causing possible problems to the on-board sensors and computers like the ECU.
Any more resistance than that is contrary to performance.

If you don't have enough spark, get a better coil or ignition system, using higher resistance plug wires for that reason is just plain wrong.

 

I am not getting in to this discussion but would like to clear up an equation.

Power(watts)=Voltage(volts)multiplied by Current(amperes).

Voltage(volts)=Current(amperes)multiplied by Resistance(ohms).

P=EI

E=IR

I don't think we can change OHM's LAW.

 

>I am not getting in to this discussion but would like to
>clear up an equation.
>Power(watts)=Voltage(volts)multiplied by Current(amperes).
>
>Voltage(volts)=Current(amperes)multiplied by
>Resistance(ohms).
>
>P=EI
>
>E=IR
>
>I don't think we can change OHM's LAW.

Book 'em, Danno!

The equation I cited was the relationship of Power to voltage and resistance, not Ohm's law. Power = E squared divided by R. The squared symbol didn't copy-n-paste into this forum very well!

Nevertheless, even without the voltage being squared, the general relationship I cited between power, voltage and resistance is correct. Higher resistance plug wires *might* result in more coil output voltage, but always a weaker spark and less sparking power at the plugs, not more.

Thanks.