Ignition Leads

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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Julian
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Ignition Leads

Post by Julian »

A question was posed to me earlier today about what igntion leads to use.

The question was simply should I be using leads like these (name hidden) or should I be using something proven. The leads in question are rather "fancy" looking items that feature very low resistance and four cores that have been spirally bound inside the insulation.

Personally I've always used a particular (proven) brand that typically have a resistance of roughly 6k ohms over the length of a lead. The values typically found on OEM items is around 3k ohms while the leads in question are typically 500 ohms significantly lower than either prior example.

As I understand leads the primary factor is making sure that there is sufficent voltage at the plug to initiate a spark which for the average car seems to be around the 20kV mark. The voltage at the coil is typically 30kV so the leads can afford to be relatively high resistance - with the use of silicone core leads this is just as well. If the lead can carry the necessary voltage without breaking down then it should be more than adequate for the job. The problem is that I have no idea how critical the resistance is for a lead or what the real benefits of a lower resistance will be.

My first reaction was to sit down and do some sums (scratchpad style - apolgies for all the working out):

v = i * (rc + rp + rn)
where rc is the resitance of the air gap in the cylinder (this differs between initiation of spark and discharge of spark), rp is the resistance of the plug and rn is the resistance of the different ignition leads.

v est at 30000volts

rn:
r1 = 6000ohm (preferred brand)
r2 = 3000ohm (oem lead)
r3 = 500ohm (proposed brand)

rci est at 22000ohm to initiate spark
rcd est at 0ohm to discharge spark

rp est at 8000ohm

i1 = 30000/(r1 + 30000)
i2 = 30000/(r2 + 30000)
i3 = 30000/(r3 + 30000)

i1 = 30000/36000
i2 = 30000/33000
i3 = 30000/30500

i1 = 0.83A
i2 = 0.91A
i3 = 0.98A

i1d = 30000/(r1 + 8000)
i2d = 30000/(r2 + 8000)
i3d = 30000/(r3 + 8000)

i1d = 30000/14000
i2d = 30000/11000
i3d = 30000/8500

i1d = 2.14A
i2d = 2.73A
i3d = 3.53A


The deduction from this is that the current drawn is obviously higher for the low resistance leads, significantly so during the discharge period. Given that the total discharge (power) from the coil is going to be a constant for all three examples it seems to me that the high resistance lead would provide a slightly weaker but longer duration spark while the low resistance lead would give a stronger but briefer spark.

From this it would seem that a higher resistance is advantageous but only to a limit since the coil must be capable of providing the necessary voltage - if the resistance is too high the coil will fail or the insulation will break down (assuming neither is modified to compensate).

Are these assumptions valid? Am I barking up the wrong tree?

I'm gathering there is more to this subject than I have skimmed over but I am sadly ignorant of what the finer points might be.
SteveNZ

Post by SteveNZ »

The resistance of a HT lead is relatively unimportant, assuming they are in good working condition. Current does not flow through an HT lead like it does in a copper wire. HT lead resistance does not directly apply to Ohms law.

Try this link. It is marketing, but id say the science is true.

http://www.nightrider.com/biketech/truth.htm
Guy Croft
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Post by Guy Croft »

Current does not flow through an HT lead like it does in a copper wire. HT lead resistance does not directly apply to Ohms law

I'd like to know more about this, can you elaborate Steve?

GC
Julian
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Post by Julian »

So basically what you are saying is that the HT circuit is more of an AC power circuit than the DC circuit it first appears as?

Looking at the figures quoted in that article it seems my maths wasn't too far from the mark though so I don't feel too much of a dunce.

The article itself is very good although it does turn into an advert for the last few paragraphs - I'm guessing someone has just copied some promotional material.
NickRP
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Post by NickRP »

Unfortunatelly, electric arc cannot be described using DC (i.e. stationary state) equations. There are numereous IEEE papers on automotive ignition system, where the maths has been explained to detail, and/or relevant equations specified. To mention one: GJ Rohwein, SR Babcock, MT Buttram: Advanced automotive ignition systems. Quite old though, but relatively easy to understand.

So, considering the example Julian has given, there are two factors not taken into account. First one being resistance of the HT wires. DC values (i.e. ones obtained by measuring resistance using simple Ohm meter), which are by the way specified by certain manufacturers are not to be used at all. They tell nothing. And why? Because in ignition event, so called "skin" effect applies. Current tends to flow only using thin surface layer of the conductor, and in that way resistance get increased say an order of magnitude or more (really rough assumption, depends heavily on conductor construction). And AC characteristics of the HT leads are not published anywhere. Second thing is the internal resistance of the secondary winding of the ignition coil. Having said that, in reality we are talking of currents of 100-500mA on secondary side of ignition system.

When we consider energy available at spark plug, every resistance found in path from coil to the electrodes is a bad thing. However, the resistance must exist, purely due to the fact that the energy needs to be transformed and transported from place A (energy stored in core of the ignition coil) to place B (energy that will create and keep the arc).

Second problem is EMI and RFI. Since HT leads are acting like antennas, they are making a "shower" of electromagnetic radiation to the environment. I'll give an example: with specially designed low AC resistance ignition lead and 50mJ spark energy, an average home PC with casing open (in near proximity of the arc, say within 1m) would instantly crash, due to interference!!! Now this fact has to be biased against the energy available at the spark plug. This is why resistance is a good thing. Manufacturers today have advanced technology to get the best of both worlds, and keep the manufacturing costs low (not to be underestimated!). However, it is often the case that most advertized brands don't have cutting edge technology, but they rather choose to go on market stating not so scientifically correct results of their "latest design" HT leads. I don't want to mention brands here, but go to websites of the companies that are involved in producing items for high-budget racing (not to be exchanged with stickers found on racing vehicles), and look for facts they claim about their products. I am sure nothing so spectacular will be found there, in comparison to 100x smaller manufacturers (having small R&D teams, suspicious quality control standards etc), selling their "super effective product"!

So, Julian, instead at looking at useless DC resistance figures, rather choose HT leads from reputable manufacturer, that proved well in racing.

Regards,
Nikola Radenkovic
Testament
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Post by Testament »

I don't know much about the electrical side of things, but it would make sense to me that the resistance of the lead would be at least an order of magnitude smaller than the resistance of the spark plug gap, and therefore not being a key factor on it's own.
SteveNZ

Post by SteveNZ »

Spark plug gap resistance changes as the spark occurs. A larger voltage is needed to start the spark but once it has started the voltage can be a lot lower.

I don't know how current travels through a lead. I tried to find an answer to HT lead resistance a while back, but I could not find any simple specific explanation. The main point I discovered was, resistance is unimportant as the current does not even flow through that resistance.

As nick pointed out, current flows in the "skin" of the conductor. As I understand it, this is more like an electric field flowing around the core rather than electrons flowing like water through a pipe.
NickRP
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Post by NickRP »

Testament wrote:... it would make sense to me that the resistance of the lead would be at least an order of magnitude smaller than the resistance of the spark plug gap...
Actually, when an arc is present (i.e. current flows through the system), arc resistance is a few orders of magnitude SMALLER than resistance of the leads!

There are a few formulas present to calculate arc resistance - however too many factors are playing role there (many of them being time variant).

Regards,
Nikola Radenkovic
Guy Croft
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Post by Guy Croft »

Nikola,

can you tell me the reality about 'crossfire' and crossover' from HT lead to lead please?

Thank you, and thank you for sharing your very expert knowledge here,

GC
NickRP
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Post by NickRP »

Guy,

the reality is that such thing can exist, but very seldom happens on a well mantained engine.

When one cylinder is approaching TDC in compression stroke, due to high pressure the voltage needed to make an arc across the spark plug gap is much higher than in other cylinders that have pressures far closer to atmospheric. Once the coil generates high voltage neccessary for spark, electricity will try to find shortest path to close the circuit. If the insulation of the leads is inappropriate or/and if the conductor (HT lead) of the spark plug that is supposed to 'fire' is bad, it could happen that an arc between that HT lead and one of the adjacent HT leads (or ground for that matter) is initiated. This will inevitably prevent correct plug to fire, and might also cause incorrect plug to fire. This tendency increases with increased pressure in cylinder and with increased fueling (i.e. bigger air contamination), and with reduced distance between HT leads (lead to lead, and lead to ground).

This "crosfire" effect can be also present in worn distributor cap.

Regards,
Nikola
Guy Croft
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Post by Guy Croft »

Thanks Nick

I seen 'prevents crossfire' cited in plug lead adverts for years and wondered what they're referring to, because, as you say quite rightly, it never/rarely happens on a well maintained engine even with quite cheap HT leads. Personally I have never seen it happen, thought i might be missing some other phenomenon.

GC
johnfound
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Post by johnfound »

Recently I made giant SPICE modeling work on the ignition systems.
As a result there is one conclusion: Using resistive leads and sparkplugs have the only purpose to reduce the radio interferences. As a result, resistance in the circuit reduces both the spark energy and spark duration. The simple explanation is that some part of the energy accumulated in the coil turns to heat in the cables/sparkplug resistance instead of spark ionization in the spark gap.
Removing resistance from the ignition cables and sparkplugs definitely leads to the biger spark energy (at least twice) and longer spark duration. But, also to great radio interference, that may disturb any electronics inside/around the car.
Greater sparkplug wear is expected as well.

P.S. In some (near/far) future I will write an article about this work - unfortunately not in english - but with full SPICE models included.
Guy Croft
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Post by Guy Croft »

Well done John, I figured that was the case.

I've done a lot of work with 2 stroke outboard powerboats and certainly those motors just won't run well with resistive leads or plugs, just fouled up all the time.

GC
SteveNZ

Post by SteveNZ »

What was the difference in spark duration for low vs high resistance leads?
johnfound
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Post by johnfound »

SteveNZ wrote:What was the difference in spark duration for low vs high resistance leads?
Actually it highly depends from sparkplug air gap. For 0.9mm air gap, the difference between case with regular resistance (approx. 10kohm in leads and 8kohm in the sparkplug) and without resistance at all (only the resistance of the secondary coil winding - 3kohm) is approximately twice - 500us vs 1ms.
The main problem in this kind of simulations is that there is no models of real sparkplug, so I used some "mean" sparkplug in the SPICE models. But the difference is valuable and I don't think several percents more or less will change the picture.
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