how much resistance?

ok, i really want to get a better understanding of resistance in reference to plugs vs. wires vs. plug caps.

from my very basic understanding, the higher resistance a plug, wire or cap has the less emi and rfi is emitted. and i think it also means that less power is going to the plug, correct?

modern high output ignitions can react to the em and rf interferrence (along with the radio in your car and other electrical systems) so the resistant plugs/wires/caps are needed, correct?

on older bikes, especially ones with points, is all this resistance needed? what about with tci and pamco bikes?

and lastly, where is the resistance needed? at the wire, cap or plug? or all three? and how much?


thank you.

In addition to the RFI issue, all electronic ignitions also need some resistance in the plug wire circuit. 5 K is the generally accepted nominal value and you want it in either the plug or the cap but not both.

Points ignitions should also have some resistance for RFI. I think the only reason that it is thought to be not necessary is because when points were prevalent, the problem wasn't recognized.

thanks for the reply pete. could you expand on why electronic ignitions need the resistance? wouldn't you want the least amount of resistance so all the electricity goes to the plugs for maximum efficiency?

Well, things are a little bit more complicated with an electronic ignition. The resistance has no effect until the gap is bridged to draw current through the resistor for a drop in voltage, so the higher voltage that you get from an electronic ignition is still effective in jumping the gap.

The resistance of the plug gap itself is infinite until current starts to flow as a result of the high voltage jumping the gap, so the gap resistance is a product of the current in the arc and the voltage across the gap, which is reduced to just enough to sustain the arc after the gap is bridged. Without a resistor, the resistance of the gap would be further reduced due to the high current flowing across the gap. As the resistance is reduced, the amount of power (heat) in the arc is also reduced, and its the heat that we want to ignite the mixture. So, with no resistance, you might think you are getting a hotter spark because it is brighter, but that isn't necessarily true. The brighter arc is due to the higher current flow, but it is not necessarily hotter, and it's heat we want, not illumination.

Of course, if the gap was never bridged or if the gap was shorted, we would also not get any power (heat) so the purpose of the resistance in the circuit is to provide just the right amount of current flow for the optimum resistance of the arc for the optimum heat from the arc.

An analogy? Well, it's like a light bulb. Inefficient light bulbs give off a lot of heat but aren't very bright. Very efficient light bulbs give off a lot of light but run cool. We want the heat, not the light.

The 5k in the plug line is miniscule compared to the effective resistance encountered at the plug gap.
Wasn't till somewhere in the `60-`70s (I think) that consumer complaints about rfi affecting tv reception caused the FCC to start writing class a & b rules for radiated emmissions. The resistive plug wires/caps solved that.

Now comes the fun part...

Having that resistance in the line can actually give you a hotter spark. One trick we used to 'quick fix' a fouled 2-stroke plug (like during a race, no time to swap), was to pull the plug cap up a bit, forcing the spark to jump a little gap (maybe 1/8"). This would eliminate the voltage ramp-up (which would immediately shunt thru the fouling). The plug would receive an instant 'hi-voltage' jolt, and arc thru the fouling, starting the engine, allowing it to burn off. Then push cap back on, off you go. Only took a few seconds.

Ever seen that jc whitney gadget that plugs into your spark plug line, 'spark-thru-gap', 'fires-thru-oil' thing? Same principle. Delay the ramp-up, get hotter spark.

The delay is in microseconds, won't affect your timing...

ok, the analogy helped. i understand we want the heat in the spark and the resistance helps create the heat by making the arc slightly less efficient. another question(s) if you don't mind, how has it been determined that 5K is the needed resistance for these bikes and is that across the board for points, tci and your ignition? and when you say 5K is needed, can that be mixed and matched throughout the ignition system? for example, wires have x amount of resistance plus the caps have x amount and the plugs have x amount all adding up to 5K. does the coil resistance play a part in that also?

2many - neat trick for the 2 smokes. with that being said, at what point is too hot? why not keep the hottest spark you can get? too high operating temp?

sorry for all the questions. all the info on google so far seems fragmented and i like to understand how it specifically relates to our bikes. thank you very much for the lessons.

deadchef said:

ok, the analogy helped. i understand we want the heat in the spark and the resistance helps create the heat by making the arc slightly less efficient. another question(s) if you don't mind, how has it been determined that 5K is the needed resistance for these bikes and is that across the board for points, tci and your ignition? and when you say 5K is needed, can that be mixed and matched throughout the ignition system? for example, wires have x amount of resistance plus the caps have x amount and the plugs have x amount all adding up to 5K. does the coil resistance play a part in that also?

Click to expand...

Very good questions, but I think that the short answer is that 5K caps are available and most resistor plugs also have 5K built in. The ignition system on the XS650 uses a dual output coil, so both caps and both plugs are in series, so the total resistance affects both spark plugs. One could experiment with just one resistor plug cap to reduce the total resistance and that would have an effect on both plugs, as an example. You could use one resistor plug (BPR7ES) and one non resistor plug (BP7ES) as an example, but the difference might be too small to bother.

Ooooh, a brain that wants more input!
Pamcopete and I are saying the same thing, just sort of a theoretical/applied physics viewpoint.
Typical plug wires nowadays are carbon-core, 5k per foot. You're running about a foot, so 5k.
Hot spark is good, two plugs better. Why doncha try those iridium plugs?

Ok, about the 5k thing. From the FCC's point of view, that modification reduced emmissions (damped the hf/vhf component) so TVs weren't bothered. I suppose that the industry then adopted that as a design-to criteria, and now parts off-the-shelf require this.

fantastic discussion! while i have you on the phone...

here is a cloth covered plug wire and a brass nut 'cap', both available from lowbrowcustoms.com.

http://www.lowbrowcustoms.com/p344/...sets-vintage-style-assorted-colors-available/

http://www.lowbrowcustoms.com/p338/...ted-ring-terminals-for-your-spark-plug-wires/

these wires claim not to be suppressor core wires and could cause interference with electric ignitions. does that mean they are low or no resistance? people seem to be buying these things so how much of a problem could potentially happen?

the brass nut replaces the cap so i would imagine there would be no resistance there either. could that be compensated by using a resistor type plug?

theoretically if you use both items combined, is there an even higher resistance plug that should be used to compensate?

thanks! i hope this info is valuable to someone else out there too!

TwoManyXS1Bs said:

Ooooh, a brain that wants more input!
Pamcopete and I are saying the same thing, just sort of a theoretical/applied physics viewpoint.
Typical plug wires nowadays are carbon-core, 5k per foot. You're running about a foot, so 5k.
Hot spark is good, two plugs better. Why doncha try those iridium plugs?

Click to expand...

TwoMany,

That is not true for XS650 motorcycle spark plug wires. Most are copper core with effectively zero Ohms per foot. Any resistance in the circuit is in the caps or the plugs.

The problem with Iridium plugs is that with our dual output coil, one of the plug wires is positive and the other is negative. The operating principle of the Iridiums relies on a negative voltage, so effectively, if you use Iridiums, you only benefit from one of the plugs, and they are expensive.

Ford Motor Co. recognized this dilemma and their engines are equipped with a different plug for the positive and negative plug wires.

deadchef,

If you are going to use those wires, then use a resistor plug, like NGK BPR7ES.

Dunno about the wires, cut one open I guess, see if it's stranded wire. Looks like they're trying to recreate the yesteryear nostalgia, with cloth-covered wiring and such.

I remember folks complaining about tv interference. That was then, AM equipment, vacuum tubes, doghouse front-ends that needed twice-yearly cleanings, 300-ohm twinlead threaded all throughout the house, going out some window, up to creaky/crusty roof antennaes. My dad could turn on his electric razor and piss-off the whole neighborhood.

Now everything is FM, VHF, UHF, UHFI (ultra hi frequency indeed!), Fibre-optics, Vertical and Circular polarization. This stuff is pretty immune to the old spark emissions, except maybe the stuff in your dashboard (proximity). But rules are rules, so mfg must comply. Just for fun, run some old points ign non-suppressed bike around your neighborhood and see if anybody complains (except for the geezer in his ham shack keying out morse to Hawaii).

BUT, if your new whiz/bang electronic ignition system calls for specific suppression, heed it!

Pamcopete, thanx for the info on the xs650 ign lines, thought maybe they went resistor core like cars after I left. AND, the info on the Iridium plugs is priceless, and eye-opening. Methinks will look into that...