Resistor Plugs

[Kestas]

Someone told me that the resistor element in a resistor spark plug can get used up or change during the duration of service, and that this is one reason these plugs need to be changed... kinda like the carbon element in ignition wires. Can anyone verify this?

 

[Quest]

nope. Most resistor plugs are of copper alloy electrode, and they (the copper alloy electrodes) erode away over time, causing difficulty to fire (which requires higher HT voltage in order to jump gap).

The resistor element inside is a form of fused carbon compound under high heat, so it will not electrically deteriorate over time. On the other hand: the carbon element on spark plug wires aren't the culprit when it comes to failures; instead: it's the breakdown in insulation properties of the sheath (skin/sleeve) of the silicone or rubber jacket that causes all kinds of trouble (mainly).

Remember: you are exerting over 6KV+ of high tension through these cables in order to spark a spark plug under all elements, and that's a serious punishment to all the possible insulation materials such as silicone, rubber, etc.

Q.

 

[mechtech2]

Most resistor plugs are NOT copper alloy electrodes. Why say this??

It is almost unanimous that passenger cars use resistor plugs. They use a wide variety of types and construction.
But The internal resistor should be checked, even with new spark plugs. They're not going to wear, but can crack from overtorquing, and can be bad even when new.

The normal wear and increased resistance is usually in the carbon core spark plug wires. Umpteen jillion sparks will break down the carbon particles, and resistance goes up with the age of the wires.

 

[PandaBear]

I remember the spec of most wires are 10 Ohms. If it exceed that it is time to replace it.

 

[Quest]

http://www.aa1car.com/library/2002/ic120214.htm

 

[ewetho]

If you measure a spark plug from the center electrode to where the spark plug wire/coil attaches to the plug you should get 5000-6000 Ohms (Champion normally are 8000-9000 Ohms after being run) and spark plug wires are normally 5000-6000 Ohms per foot of length other than High Performance after market wire like MSD which could be 150 Ohms per foot or lower.

 

[gtx510]

Originally Posted By: PandaBear
I remember the spec of most wires are 10 Ohms. If it exceed that it is time to replace it.

MSD's racey-race wires are ~80 ohms/foot.
OEM wires are more like 1k ohms/foot.

 

[BrianWC]

Turbo'd SAABs have used resistor plugs in conjuction with a direct ignition cassette (coil over plug) for near 20 years now. Most SAAB owners think resistor plugs are some magical voodoo that only NGK makes correctly.

 

[OVERKILL]

Originally Posted By: PandaBear
I remember the spec of most wires are 10 Ohms. If it exceed that it is time to replace it.


OEM carbon-core plug wires are 2-7K Ohms per foot on average.

MSD Super Conductor wires are 50 Ohms/foot
Morosso Ultra 40's are, as the name implies, 40 Ohms/foot
Accel 300+ Race wires are 150 Ohms/foot

My Ford Motorsport wires Ohm out the same as the MSD's and their construction is basically identical. I own both.

Resistor plugs are common. My Yamaha trike for example requires a resistor plug. Many older cars DO take a copper-alloy resistor plug as mentioned earlier in this thread; most new cars take a platinum plug for longevity reasons.

 

[gtx510]

http://www.sparkplugs.com/sparkplug411.asp?kw=Resistor&mfid=0

http://www.sparkplugs.com/charglossary.asp?kw=Resistor+plugs+for+race+engines

I think my '72 Datsun 510 originally used non-resistor plugs (and a points ignition), but now it's got a Pertronix and resistor plugs.

 

[Kestas]

Originally Posted By: from above link
... Some high out put systems specify the use of resistor plugs, while others will fry the internal plug resistor turning its 5k ohms into 60k ohms...

This is the point on which I was looking for information. Is the resistive carbon element in a resistor spark plug prone to degradation? I ask this because I often clean and regap my plugs to extend their use. I don't want to do this if the resistor changes value. This could tax the ignition system of my cars.

 

[gtx510]

ohm test old and new plugs?

 

[Kestas]

I'll start doing that.

 

[Lazy JW]

You may be surprised at how much variation exists in brand new plugs.

The open-circuit resistance of the plug gap is actually several mega-ohms so a few kilo-ohms difference inside the plug is pretty insignificant.

It does make for an interesting study however. Enjoy,
Joe

 

[andyd]

Isnt the main reason for resistor plugs to reduce radio interference?

 

[Lazy JW]

Originally Posted By: andyd
Isnt the main reason for resistor plugs to reduce radio interference?


Correct.

 

[BrianWC]

I thought that stray spark energy could also fry the ECU and other components???

 

[tom slick]

the ECU is sheilded from EMF but the radio picks it up in the antenna.

 

[BrianWC]

Hmm, is that a blanket statement? Kinda depressing if so.... Glad to finally hear that, though

 

[BrianWC]

Okay, I remember what the danger is in SAABs, it's not the ECU that the resistor plugs are supposed to shield. It's the Direct Ignition Cassette, which is basically a coil pack (that also senses knock through an ionization current thru the plugs).