HT leads with resistors in them

[thexav]

I saw a set of HT leads today that have a set of resistors in them and they cause the spark to be bigger by delaying the spark until the current builds right up.

Are these worth the money? They were about £50 for a set.

What are inductive and resistive HT leads?

 

[lutonmatt]

i have never heard of these before and any thing that stops it firing when it needs to cant be to good

normaly if you want better you go bigger

 

[obi_waynne]

It sounds a bit pointless to me buddy! A spark is a spark, a resistor or capacitor (which is more likely) will shorten the duration of the spark and not really give a bigger spark.

Stick with good quality HT leads but a lead is a lead and there is little to be gained by paying over the odds.

 

[HDi fun]

Resistive plugs and leads have been used for years to reduce radio interference. That was never their primary purpose.

In the days when we had mechanically controlled ignition the resistance of the leads was designed to prevent early sparking. Essentially the resistance of the leads if far far lower than the reistance of the spark plug gap. This means that a greater magnetic flux (and therefore voltage potential) builds up in the coil(s) secondary (HT) winding before finally being able to overcome the reistance of the plug gap. Thus a better spark is produced. This not really a problem today as many cars use coil per plug systems.

 

[obi_waynne]

So in a way, cheap leads with high resistance might be better then?

 

[HDi fun]

So in a way, cheap leads with high resistance might be better then?

Probably not. HT leads designed to work with the rest of the engines HT ignition system will be the best.

Single coil and rotary distributor systems are rare now - using the distributor to control ign timing is ancient. Even my old 1986 Montego had a crankshaft position sensor and ign ECU (analogue in design) to control ignition timing.

 

[thexav]

I would like to know the comparison of the performance between copper core and high performance silicon HT leads - are there benefits?

How are leads shielded to stop interference.

 

[HDi fun]

I would like to know the comparison of the performance between copper core and high performance silicon HT leads - are there benefits?

How are leads shielded to stop interference.

You can't really compare the two types as different ignition systems are designed to work with different leads anyway: Using the wrong ones will lead to a degradation of performance. Bear in mind that at 30kV 10 megaohms resistance in the lead is nothing compared to the resistance of the spark gap. Without going too far here, resistance is not really the issue. It's dynamic impedance that matters.

as for shielding - no HT leads are shielded. They are single conductor insulated wires.

Many cars now use a coil per plug setup so the delivery to the coil top is only around 40-50 volts anyway. Some use 'long spark' systems where the spark is actually a series of several dozen sparks delivered over a few milliseconds.

FOr 'conventional' ignition systems the shileding is done by the bonnet lid. All that happens is that makers now fit good earth bonding wires so that the bonnet works as an effective Faraday shield (as does the rest of the bodywork) against spurious EMFs.

THe biggest source of interference now is likely to be from the ECU itself with RF emissions being the biggest factor. Good electrical design with decoupling capacitors elimiates most of this. Again, it's all dependent upon earth impedance.

Impedance is a reactive phenomenon and cannot be considered to be the same as simple DC resistance.