Iridium Spark Plugs

[Semper Gumby]

I'm just guessing but I thought the reason for resitor plugs/caps was to cut down on Radio Frequency Interference.  So you go with either a resitor plug or a resitor cap and only use both if you need them.

How would you know if you need them (One or both)?

My 1969 BSA Firebird is a case in point.  WHile I don't have an AM radio on the bike I DO have a GArmin GPS.  As bike RPM increases I noticed that the GPS would stop recieving Satillites.  In fact at 5200 rpms it would stop recieving them completely and go blind.  I changed the plugs from B7ES to BR7ES and WALA the GPS is happy again.  If I needed more RF supression then it would have been obvious in the gPS's satilitte reception and I would have then added the caps.

I wanted to put on the Iridium plugs because they are fine wire plugs that last a really LONG time.  For some reason the fine wire plugs just work better.  The bike starts easier in extreme conditions (Like 18'F) for a long time without much effort from me.  I was afraid that with both the resitor cap and the resitor plug I would loose some spark.  (No evidence yet but perhaps I will check this out somehow with a testor).

So in my book its 1) GO with the resistor plug (preferably Iridium fine wire) first and then if you need more resistance 2) add the resistor caps last.

My 2 cents.  (Not what it used to be)

[Barry]

Some resistance is needed for radio suppression. The reason it works is complex, essentially it reduces the harmonics caused by sharp spikes in the voltage waveform.  

The effect of resistance on the spark energy is also complex. Some will say that 10K ohms resistance is insignificant compared with the infinite resistance of the spark plug gap, therefore a spark will be initiated just as easily as with no resistance in the plug or lead.  This may be true but once the spark is initiated the plasma in the spark gap has a resistance very much lower than infinity and simple ohms law tells us that if a current is passed through two resistors in series then energy is dissipated in proportion to the resistor values.  In other words resistors placed in the plug or lead will dissipate energy which would otherwise be dissipated in the spark.  Whether this has any practical effect on the engine running is the real question. Using iridium plugs may offset any tendency to a weaker spark because a spark is more easily emitted from a sharp edge or point. Same principle as a lightning conductor.

This is a compromise between the resistance needed to provide efficient radio suppression without too much detriment to the spark.

What is certain is that points ignition is less tolerant of excessive resistance so personally I would use the minimum required for radio suppression. BMW knew what they were doing when they designed the points system to use 1K ohm caps.

The short answer is that Snowbum says 1k ohms for points and 5K ohms for electronic ignition which fits nicely with a scientific paper I read which in summary said 5K ohms is enough for radio interference suppression and this would reduce the spark energy by 7 %.  The paper also said that 6k ohms was typically used in Resistor type plugs although some were as high as 10K ohms. So add a 5K ohm plug cap and you have 15K ohms or 21% loss of spark energy. The engine may still run fine but with what safety margin ?

[Semper Gumby]

OK Now I'm going to go out in the shed (tomorrow when its warmer) pull a plug and measure the resistance of the NGK BR7EIX.  Inquiring minds need to know.  

[Motu]

Resistance doesn't effect the KV required to jump the plug gap....but it does reduce the burn time.

[Barry]

Resistance doesn't effect the KV required to jump the plug gap....but it does reduce the burn time.  

You condensed most of my last post into one sentence. I said less spark energy but yes that means a shorter spark burn time.

[Semper Gumby]

NGK BR8EIX (That I use in the BSA's) 4680 ohms or about 5k (Using fuzzy logic).  OAT 46'F.

I bet the restance of the BPR7eix is similar.  I wonder if it changes with temperature.  

[Barry]

Approx 5K ohms seems to be the optimum resistance value and it's good that they are not using a higher value resistor. It may make them the plug of choice if you must have a resistor plug. I'll try and find out what value other manufacturers are using.

 Here is a quote from the NGK website:-

"NGK "R" or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking.

NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.

They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer."

[razz2234]

OK....now that I've read all of the replies to this ongoing question...and am thoroughly confused as to what the right answer to the original question was...
SO...it's OK??? to run a resistor Iridium plug on my 81R65... I do use a GPS and understand that it will stop the RF interference. BUT...I've also read that running a resistor plug will mess up/ruin my starting system because it was made for non-resistor plugs.
A resistor plug runs hotter...correct? So the Iridium plug will dissipate the heat faster being a fine wire?
Sorry for the confusion...a definitive answer would be appreciated.. I'm not an electrician and am easily confused...probably from the era I grew up in.

[Barry]

Electronic ignition bikes must have some some resistance in the lead or plug otherwise the ignition can be damaged. Points bikes should have 1K ohm resistor plug caps but many people run ok with 5K ohms caps.
You should not run resistor plugs and resistor caps together as some energy is dissipated in the resistors and it will weaken the spark especially so on the points bikes. You are more likely to get away with it on bikes with electronic ignition.

Resistors have no effect  on running temperature that I know of.

[Bob_Roller]

If you want to install resistor type spark plugs, you need to have ignition wires that are non-resistor type .

On our bikes, the cap that fits onto the spark plug, has the 5K resistor in it .

You can find 0 ohm caps by NGK, and then install them on your existing spark plug wires .

[Semper Gumby]

<snip>
You can find 0 ohm caps by NGK, and then install them on your existing spark plug wires .

...just make sure you use the resistor plugs if you use the 0 ohm caps.

I chose the BR7EIX plugs because that was the only way to get a fine wire plug.  They don't come in a non-resistor plug that I know of.  I just didn't want too much resistance in the total equation (resistor plugs and resistor caps).