BANDO vs. OEM coil resistance

[gary2gs]

I was comparing the resistance measurement between a Bando hi-pref. coil for a 150cc scooter and the OEM coil from a ZNEN Powersports 150cc scooter. Let me first say both coils worked fine. The Bando MAY have been slightly better.

For my own knowledge I was checking the resistance of each coil.
The Bando: primary winding is 0.2 ohm. Secondary winding to the end of the spark-plug wire is 3.35k ohms. The spark-plug end cap is 1.5 ohm.
The OEM coil: primary winding is 0.4 ohm. Secondary winding to the end of the spark-plug wire is 2.96k ohms. The spark-plug end cap is 4.63k ohms.

The primary and secondary measurements seem to correspond. But why the 4.63k in the OEM spark-plug end cap. I know a resistor blocks current flow, but how does that impact the actual ignition spark. Perhaps that resistor is only for radio interference? Would no resistance shorten the spark plug life? Will someone explain this to me

Thanks, Gary

[gary2gs]

Somebody must know something about that 4.63k resistance

["Big Guy"]

Spark plug wires should be between 4.6K and 7.4K. The coil wire should be 1.2K to 2.8K.

Why? I don't know, don't ask me, but you asked anyway didn't you?

["Big Guy"]

I guess if you think about it, radio frequency interference (RFI) suppression is probably the most likely reason.

RFI occurs when high voltage passes through the plug wires. Creating a controlled amount of resistance in the wire, suppressing RFI would prevent onboard electronics from picking up false signals that could cause problems.

Yes, the additional internal resistance may in theory create internal heat, and over time, I guess this could cause the plug to age prematurely, causing resistance to increase, and, as resistance goes up, so does the chance for misfire and need for replacement.

It could be RFI, or at least RFI would make be the most logical train of thought, but the best thing to do would be to ask a trained mechanic I guess. We're probably just babbling here...

[gary2gs]

Most likely for RFI.
Now that I think about it, 4.63k may not impact the high voltage output of the coil to any noticeable degree.

[earlwb]

They use a resistor or resistance wire to suppress the rather wicked Electromatic pulse that comes off the high voltage spark so that it doesn't generate as much radio frequency interference. On non-suppressed spark systems, you can pick up a popping like sound on AM radios quite easily. A suppressed RFI setup should reduce that considerably on down towards nil.

Some manufacturers use a carbon resistance wire instead of copper wire in the high voltage spark plug cable. If they use copper wire then they have to put in a resistor someplace else, thus you can measure some resistance in the spark plug boot as they put it in there someplace.

Since we are dealing with high voltages and almost no current, there really isn't a lot of wattage in the high voltage side, so the resistor doesn't need to deal with a lot of power or heat buildup from the voltages and currents involved.

The Bando coil has more windings on the secondary high voltage side to generate a higher spark voltage, so it would read a little higher in coil resistance.

Performance-wire, you won't really see a big notceable difference with the higher spark voltages. But the effects are more subtile. A higher voltage can still fire a plug in a engine with higher compression and inside cylinder temperatures. The higher voltage can still fire a plug that might be dirty or partially fouled where the lower voltage wouldn't. The higher voltage would improve higher RPm running with fewer misses or misfires. Unbeknownest to people, spark plugs tend to miss a lot when the engine is running at higher speeds. A really good spark plug and coil and wire can give you up to 100 RPM or so more top end performance. Also improved sparking gives you a little better gasoline mileage per gallon too as you have less missfires so the engine burns fuel more efficiently.

[earlwb]

I forgot to mention that the Bando coil shows a lower primary coil resistance as compared to the OEM coil, as they intend it to run at a higher input voltage, which is usually what the racing coils provide. Something like up to 350 volts in on the primary coil so they can generate 50-60,000 volts or so on the secondary coil.
The Regular OEM types of CDI's output something like 90v to 120v or so to fire the coil with.

One final thought is that both types of coils do not do well if the spark plug boot comes loose or the wire comes loose screwed into the boot or coil. This effectively causes the spark plug gap to increase, which causes the spark peak voltage to increase spectacularly in order to jump the larger gap. That extra high voltage causes the insulation to break down resulting in early failure of the ignition coil unit. The higher voltages can actually cause the secondary coil windings to short out. They only use really thin copper wire coated with a thin layer of simple varnish as insulation (so they can really pack in the turns or windings tight).
So extended running with the spark plug boot not plugged in down tight on the spark plug will cause the coil to go bad on you faster.

Many years agpo in high school autombile shop, we had a ignition coil test jig setup to demonstrate how a coil, points and a distributor worked using a hand crank to operate the unit. Anyway we used to have to replace the coil quite often on the test rig as everyone would try to see how long of a spark the could get going and it would obviously start damaging the ignition coil pretty quickly. So this happens to everybody's coils, not just the cheapie Chinese ones. But due to questionable Chinese quality on the coils one should exercise more care though.