Resistor plugs and caps.

Thanks for this thread, I had no luck finding NGK BP8ES OR BP7ES and when I did the plug was either misrepresented (is actually an "R" plug) or was priced very high.

Grabbed 4 non-resistor NGK 8381 LZFH Spark Plug caps just in case they stop production. Then purchased the Iridium R plugs from NGK. I think this is good in a forward looking way.

Does anyone know, and care to share, why the mix of resistor caps plus resistor plugs is a no-no?

Also, what is the resistance of a resistor plug? (maybe it varies with brand, type, etc?)
To test, it would probably need to be done at coil voltage?

robs ss said:

Does anyone know, and care to share, why the mix of resistor caps plus resistor plugs is a no-no?

Also, what is the resistance of a resistor plug? (maybe it varies with brand, type, etc?)
To test, it would probably need to be done at coil voltage?

Click to expand...

Generally - 5k ohms for a cap or 5k ohms for a plug. However, some cars use 10k ohms which it part of the confusion in this thread.

*IF* you have a proper plug, proper plug gap, good wiring, and a decent battery, the math says 0, 5k, and 10k ohms will work just fine. 10k ohms starts getting iffy as several little things add up.

In an earlier post, I mentioned that NKG does not make plugs with the proper gap for a Lucas motorcycle coil. If you simply install NKG plugs without setting the gap you're on the edge - do that with 5k and you're more on the edge - do that with 10k and you're probably over the edge. Also, Norton specified a larger gap range than other Vintage British Motorcycles. 0.028 is too much, and especially too much with resistors and high compression.

The resistor, no matter where installed does reduce slightly the potential spark voltage. That is generally negligible since the available potential is greater than generally required. The resistors job to stop side effects of the spark jumping the gap - a filter of sorts. 5k ohms is best for that with Lucas motorcycle coils. With Lucas and other car coils, 10k is often used but the potential spark voltage is higher.

BTW, for the automotive electronics gurus, this is written to try to explain to the masses - if you want to bitch at me - a PM would be appreciated!

No bitching, but I could add a few things for discussion. A coil ignition system is somewhat analagous to an LRC network, at least until the spark begins at which time it becomes an LR network with a virtually shorted capacitor. Having nearly zero resistance in the circuit can destabilize it and cause the spark to extinguish more quickly which may start to matter at extremes like high dynamic compression, starting or improper mix. Extended spark duration is a good thing. A non-functional benefit of the duration is less radio interference because the dv/dt (rate of voltage change) is much less. Also, overly large plug gaps are hard on the coils since the greater the gap, the higher the voltage needed to develop in the system to bridge it (maybe only a concern for 50-year-old originals.)

robs ss said:

Does anyone know, and care to share, why the mix of resistor caps plus resistor plugs is a no-no?

Also, what is the resistance of a resistor plug? (maybe it varies with brand, type, etc?)
To test, it would probably need to be done at coil voltage?

Click to expand...

You can directly measure what you have by taking the HT lead off the capacitor and the plug out of the head, them use an Ohm meter to measure resistance plug center electrode to the capacitor end of HT lead. Typically, NGK caps are 5k. Lucas/Champion branded hard bakelite caps are typically 10k. Sometimes the resistence is stamped/imprinted on the cap itself but can be hard to see without having strong light at an angle etc.

As I mentioned previously, my modern air cooled Bonne has resistor plugs and caps, 10k total and uses a dual output modern type coil pack. Pretty sure any modern bike will be similar, as the ECU and EI circuits etc can be much more sensitive to radio interference from unsurppressed ignition setups.

robs ss said:

Does anyone know, and care to share, why the mix of resistor caps plus resistor plugs is a no-no?

Also, what is the resistance of a resistor plug? (maybe it varies with brand, type, etc?)
To test, it would probably need to be done at coil voltage?

Click to expand...

So... it appears the answer is NO

robs ss said:

Does anyone know, and care to share, why the mix of resistor caps plus resistor plugs is a no-no?

Also, what is the resistance of a resistor plug? (maybe it varies with brand, type, etc?)
To test, it would probably need to be done at coil voltage?

Click to expand...

robs ss said:

So... it appears the answer is NO

Click to expand...

I thought I answered fully.

Mixing a resistor cap and resistor plug has the potential to lower the voltage enough to stop a spark from forming at the electrodes.

The resistance of a resistor plug is generally 5k ohms.

Measuring the resistance has nothing to do with the coil voltage. An ohmmeter between center electrode and the contact where the cap attaches is how you measure. There is a simple resistor inside the plug (or cap).

Regarding why it is considered a no-no to run resistor caps & plugs together, just seems to be from midsts of time when point and lower output coils were the norm. We have many examples of folks reporting good running with 10k Ohms suppression (either a 10k cap with non-resistor plugs or 5k cap and 5k plug). So its not a huge issue, assuming your coils are good etc.

Yeah - I have modern dual output coils and EI.
So it appears the "either this or that" is to be ignored.
You just have to ensure the combined cap/plug resistance is within the capacity of your ignition system - and good systems will handle at least 10 ohms.
Cheers

robs ss said:

Yeah - I have modern dual output coils and EI.
So it appears the "either this or that" is to be ignored.
You just have to ensure the combined cap/plug resistance is within the capacity of your ignition system - and good systems will handle at least 10 ohms.
Cheers

Click to expand...

Correct.

The resistance is a portion of the things that determine what spark you get. Of course, the coil capability is a BIG part. The voltage to the coil is very important as well (bad battery, poor connections can have a large effect). The relative humidity, compression ratio, air fuel mixture, advance, and many other things contribute as well.

All things being good even with 50-year-old Lucas coils and a Combat engine you will have a decent spark with 0 to 10k ohms, but you better set your plug gap to .025" if you have more than 5k ohms.

Without specifications modern dual output coil does not automatically mean better than two old Lucas coils. Loosly, a coil has a primary and secondary winding. The ratio of the number of turns of those is what sets the potential voltage. Having one primary and one secondary split between two spark plugs may not be better. I like them, I just haven't tested them.

Keep in mind that if 12,000 volts is required to jump the gap, a coil that can provide 24,000 volts is little to no better than one that can provide 12,000 volts.

marshg246 said:

Keep in mind that if 12,000 volts is required to jump the gap, a coil that can provide 24,000 volts is little to no better than one that can provide 12,000 volts.

Click to expand...

As someone told me in grad school, a B is as good as an A if your objective is a B

And did i miss it, is there a consensus on plug gap for dual output coils with EI and 5k-10k overall plug resistance? Stock gap? or is the answer "it depends"

Good thread

robs ss said:

and good systems will handle at least 10 ohms.
Cheers

Click to expand...

If you mean 10,000 ohm, you’re not very precise, are you!

TomU said:

And did i miss it, is there a consensus on plug gap for dual output coils with EI and 5k-10k overall plug resistance? Stock gap? or is the answer "it depends"

Good thread

Click to expand...

Yep, you're lucky wasn't a rattle snake! Read the paragraphs in my post before the one you quoted in my post To hell with consensus, I have spoken It's a joke, don't attack!

TomU said:

As someone told me in grad school, a B is as good as an A if your objective is a B

And did i miss it, is there a consensus on plug gap for dual output coils with EI and 5k-10k overall plug resistance? Stock gap? or is the answer "it depends"

Good thread

Click to expand...

I would set the plug gap based on what typically gets provided as normal spec for the coil(s). It is the coil output that determines the max limits of gap size afterall. If coil is used on more modern bikes at a typical wider plug gap than what our Norton manuals spec, go with the more modern gap range.

Tornado said:

I would set the plug gap based on what typically gets provided as normal spec for the coil(s). It is the coil output that determines the max limits of gap size afterall. If coil is used on more modern bikes at a typical wider plug gap than what our Norton manuals spec, go with the more modern gap range.

Click to expand...

Absolutely, if the coil you buy provides the information. I'm testing a dual-fire coil for a Harley. I believe it is the same one cNw uses but I'm not positive. The ignition is Tri-Spark's new twin unit (Tri-0006). The primary coil measures 4 ohms and the secondary 6k ohms. Unfortunately, that doesn't really say much about the potential output voltage. It's the ratio of the primary turns to the secondary turns assuming that same effect of the metal they are wound on. If you went by the ohms, two Lucas coils would have much more potential!

Not scientific at all, but the spark across a 0.025" gap on NKG BR8ES coils on that bike and the other setup the same but with Lucas coils looks the same. But, like I said in a earlier post, once the needed voltage is reached to jump the gap, more voltage potential makes little to no difference. There is also the possibility that the duration of the spark if different, but I don' have a way to measure that or the spark voltage right now.

marshg246 said:

Absolutely, if the coil you buy provides the information. I'm testing a dual-fire coil for a Harley. I believe it is the same on cNw uses but I'm not positive. The ignition is Tri-Spark's new twin unit (Tri-0006). The primary coil measures 4 ohms and the secondary 6k ohms. Unfortunately, that doesn't really say much about the potential output voltage. It's the ratio of the primary turns to the secondary turns assuming that same effect of the metal they are wound on. If you went by the ohms, two Lucas coils would have much more potential!

Not scientific at all, but the spark across a 0.025" gap on NKG BR8ES coils on that bike and the other setup the same but with Lucas coils looks the same. But, like I said in a earlier post, once the needed voltage is reached to jump the gap, more voltage potential makes little to no difference. There is also the possibility that the duration of the spark if different, but I don' have a way to measure that or the spark voltage right now.

Click to expand...

Could one of these testers be used to compare coils for maximum plug gap?

Tornado said:

Could one of these testers be used to compare coils for maximum plug gap?

Click to expand...

Just read about it - sounds bad for a EI on a bike. The write-up in not plain English but you're testing from the coil to ground with the engine running. A Norton runs OK that way but a dual fire may not give a true indication.

Here's one that meassure the three things needed, but expensive: https://www.ebay.com/itm/1255972989...044&customid=3ed37048ae271e00be061478d63f85d0

marshg246 said:

Just read about it - sounds bad for a EI on a bike. The write-up in not plain English but you're testing from the coil to ground with the engine running. A Norton runs OK that way but a dual fire may not give a true indication.

Here's one that meassure the three things needed, but expensive: https://www.ebay.com/itm/1255972989...044&customid=3ed37048ae271e00be061478d63f85d0

Click to expand...

I think you can test a coil in isolation just be applying 12v to primary, and checking spark jump when toggling the 12 v on/off with manual switch. Thats essentially what the points or EI are doing.

I've seen folks triggering EI to trigger a spark by just waving a magnet near the EI stator plate (even with points cover in place) and watching the plug electrode. Just an easy way to confirm spark without needing to kick engine over.

As an aside to the resistance of plugs & caps - I had been concerned with unwanted resistance in HT leads.
I make my own and have thought that the crimped connection for the coil may be prone to increased resistance over time so, to combat this, I:
1. Crimp the connection
2, Put a drop of ACF50 onto the exposed copper wire
3. Bind the connection tightly with PE fishing braid (I use 80lb because it's handy)
4. Put some super-glue over the braid.

My rationale is the prevent corrosion and maintain healthy clamping pressure on the end fitting.
I also use ACF50 when screwing the cap onto the lead.
Seems to work - the set that I just changed had 0.7 ohms per side.
Cheers

Jdub said:

Thanks for this thread, I had no luck finding NGK BP8ES OR BP7ES and when I did the plug was either misrepresented (is actually an "R" plug) or was priced very high.

Grabbed 4 non-resistor NGK 8381 LZFH Spark Plug caps just in case they stop production. Then purchased the Iridium R plugs from NGK. I think this is good in a forward looking way.

Click to expand...

John,
The NGK BPR7ES are readily available at all the standard auto parts stores in my small town. I have been using them for quite some time.
Mike