Ignition Leads - Mick's Tips of the Trade

By: Mick McCrudden

Presented by

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Dressing up your engine? Let's talk leads...

We thought we’d get on to ignition leads today – or at least the older style, which feeds the power from a coil to the cylinders.

Many modern vehicles run direct fire ignition, which is a coil per cylinder. It’s more efficient, but when they fail – and they do – the expense is horrendous compared to an older-style lead.
Once you understand the basic principles of an internal combustion engine, it doesn’t matter whether it was built in 1939 or 2017, the principles remain.

For example, when you’re out there driving and you accelerate hard and there’s a miss, it is 99 per cent of the time the lead breaking down – not a spark plug. A plug will give you a miss at idle and when you put it under pressure it may backfire.

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Though you shouldn’t do it, you can isolate which one is failing by earthing a plug against the block and testing each lead as the engine is turned over. (Make sure you have all the plugs out first, as you don’t want it starting!). A multimeter is a better way to go, as you can actually measure the resistance, which will tell you what’s going on. If one lead is breaking down, you should replace the set, as it’s likely the others won’t be far behind.

What hurts the performance of a lead is resistance, which is measured in Ohms. A race lead may read as low as 500 ohms per meter, while 6000 is regarded as good for street performance. A bog stock family car might read as high as 16,000.

When you install the new leads, make sure you get a nice positive click with all the connections as they go on. Something I’ve recently discovered at an NGK seminar – here’s proof you’re never too old to learn! – is a substance called dielectric grease. It’s very effective and CRC makes a version of it. What you do is apply just a little inside the plug cap, with a small paintbrush, and this allows it to slide over the top of the plug easily so you can feel that all-important click.

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What about appearance? Let’s say you go to a lot of trouble building a nice pretty engine, and get caught with leads scattered over the top of the engine. You can lay them down underneath, but check the clearances with your headers. You need to use lead spacers – lots of people make them – we put a heat shield over the lead. Use a little talc powder inside the shield and slide it over. It’s very time consuming to go to this sort of trouble but the visual results are great.

You don’t have to go to that extent, but it is well worth doing a couple of things. One, get good quality leads. If it’s a meat and three veg kind of car, go for original equipment or Bosch or NGK. People get obsessed about putting on fat leads when they may not be required. A lot of them have the same core but a thicker insulation layer, and offer no real benefit. Look for the resistance number.

Second, spend some time installing them. Yes, you can slap them in, but it’s worth taking time to ensure they’re laid out properly in the engine bay, using locator clips and the like, and each connection gets that crucial click when it goes on.

These things don’t have a set service life, much depends on the conditions under which they’re used. The biggest problem is human intervention (rough handling, poor installation) and heat. They should last nearly a lifetime.

Happy sparking!

Mick owns Glenlyon Motors, an expert workshop and car storage facility in Brunswick, Melbourne. Call him on (03) 9380 5082.


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