Ignition leads – facts and myths

Silicone insulation is not automatically the best: it is hard to recycle and mechanically weaker than modern thermoplastics. Classes A to F under ISO 3808 describe temperature resistance, not quality, and there is no class G. For LPG/CNG engines, a low voltage drop matters more than temperature resistance, which is why many carmakers advise against carbon-core leads above 7 kΩ/m.

Are silicone ignition leads the best?

No – that is a myth.

Compared with modern thermoplastics developed in recent years, silicone insulation has many drawbacks. Silicone is one of the hardest materials to recycle, and it is mechanically weaker.

That is exactly why silicone ignition leads are reinforced with glass-fibre or nylon braid and are usually routed in special holders to protect them from damage. These holders also do another job: they shield the lead set from rodents, as for some reason at least some animals find silicone a delicacy.

Are lead classes A, B, C, D, E and F quality classes?

No – that is a myth.

• Under ISO 3808, these are temperature resistance classes.
• Class A corresponds to the lowest temperature resistance,
• class F to the highest. Quality and temperature resistance are two different things.
• Ignition leads can be of very high quality and still meet class C or D rather than E or F.

Are class E and F silicone leads the highest-quality leads?

No – that is a myth.

These are leads whose insulation withstands high temperatures (−40 °C to +250 °C). The temperature resistance class must not be confused with the quality of the lead – it is one of the less important factors affecting lead quality. Some companies label their leads as class G. This is purely a marketing trick, as no such class exists in the standards.

Does the outer diameter of a lead have no effect on insulation or conductivity?

It does – the claim that it has no effect is a myth.

Despite all technological progress, the laws of physics remain the same, and a thicker layer of the same material is always more resistant to electrical breakdown than a thinner one. However, the insulating properties of different materials vary, so smaller-diameter leads should not be rejected out of hand.

Properly made, they can also meet vehicle manufacturers' requirements. Otherwise everyone would still be using 7 mm leads and nobody would use 5 mm ones. Besides, thinner leads are usually routed in additional plastic holders that protect them from external factors and from contact with metal engine parts.

Do LPG engines need ignition leads with higher temperature resistance?

No – that is a myth.

It certainly will not do any harm, but it will not help either. Burning gas does produce a higher temperature, but the effects are barely noticeable outside the cylinder. The cylinder head near the spark plugs, and the plugs themselves, run only about 5 °C hotter than on petrol, which makes little difference to good-quality leads.

What really matters is that the ignition leads do not cause too great a voltage drop, because on gas the voltage at the spark plug electrodes must be noticeably higher than on petrol for the spark to jump. This is why many vehicle manufacturers advise against resistive (carbon-core) leads with a resistance above 7 kΩ/m in cars with LPG or CNG systems. See also: ignition leads for LPG/CNG engines.