Spark plugs: heat range, iridium and platinum, and what actually matters

A spark plug has one job and it is harder than it looks: put a spark across a gap, in a cylinder full of compressed mixture, tens of times a second, for years, without wearing out or getting hot enough to light the mixture on its own.

Most of what gets argued about — iridium against platinum, brand against brand — matters far less than two things almost nobody checks: the heat range and whether the plug is actually the one the engine was designed around.

Why the electrode got thinner

The original article makes an argument that is worth keeping, because it explains the whole modern plug.

Modern engines run leaner than older ones. A lean mixture is harder to light — there is less fuel near the electrode to catch — so the risk of a misfire rises exactly as manufacturers chase economy. And a misfire is not just lost power: unburnt fuel goes down the exhaust and into the catalyst, where it burns and destroys it (see the catalytic converter article).

The fix is geometric. A thinner electrode ignites a lean mixture more reliably, because the spark is concentrated and because a fine electrode absorbs less heat from the young flame kernel — a thick one acts as a heat sink and can quench the flame before it establishes.

The problem is that a thin electrode in ordinary nickel alloy erodes fast. So the fine wire has to be made of something that survives: platinum, or better, iridium, which has a very high melting point and resists spark erosion.

That is what you are buying. Not power — the ability to make the electrode thin and still have it last 100,000 km.

The power question, answered plainly

On a healthy standard engine, fitting iridium or platinum plugs in place of good conventional ones will not give you power you can feel or measure — one to two per cent at the very most, and generally nothing.

Where a plug change does produce a real improvement, it is because the old plugs were worn out. That is a restoration, not a gain, and a set of correct conventional plugs would have restored it just as well.

So the honest case for the expensive plug is service life and lean-mixture reliability. If a car specifies iridium, fit iridium — the interval assumes it. If it specifies conventional plugs, fitting iridium buys you a longer interval and very little else.

Be sceptical of anything sold as a performance plug on the strength of electrode shape — multiple grounds, split centres, V-grooves, surface-gap designs. If a geometry reliably produced power, engine manufacturers would specify it, and they specify what they specify.

Heat range — the property that actually matters

This is the one the original article does not cover and the one that causes real damage.

Heat range is not how hot the spark is. It describes how quickly the plug can shed heat from its tip into the cylinder head. It is set mostly by the length of the insulator nose: a long path holds heat in and makes a "hot" plug; a short path conducts it away quickly and makes a "cold" one.

The plug has to sit in a narrow window — hot enough to burn deposits off its own tip, cool enough not to become an ignition source itself.

• Too cold for the engine, and the tip never reaches self-cleaning temperature. Deposits accumulate, the plug fouls, and you get misfires — typically worst on short journeys and at idle.
• Too hot for the engine, and the tip glows. It lights the mixture before the spark does, which is pre-ignition, and pre-ignition destroys pistons. This is not a performance compromise; it is engine damage.

A hotter plug does not make more power. A wrong heat range reduces it, in one direction by fouling and in the other by forcing the knock control to retard timing — assuming it does not hole a piston first.

Use the heat range the manufacturer specifies. The exception is a genuinely modified engine — forced induction, raised compression — where more heat has to be removed and a colder plug is correct. That is a decision made with the rest of the build, not one made at the parts counter.

Gapping, and when not to

Conventional plugs are gapped to the figure in the service data, checked with a wire gauge rather than a flat feeler, and adjusted by moving the ground electrode only.

Fine-wire iridium and platinum plugs are different, and this is where people ruin new parts. They arrive pre-gapped, and the manufacturers explicitly say not to check or adjust them unless instructed. The centre electrode is a fine wire with a precious-metal tip welded to it, and a gapping tool — or the common habit of tapping the plug on a bench — chips or snaps it.

NGK's own guidance is that a fine-wire plug slightly out of gap will still outperform a correctly gapped conventional one, so there is nothing to gain by interfering.

Reading a used plug

A plug that has been in an engine is a record of what has been happening in that cylinder, and it is free diagnostic information every time you change them.

• Light tan or grey tip, dry — the engine is running correctly.
• Dry, black, sooty — running rich, or the mixture is not burning: over-fuelling, a lazy oxygen sensor, a dirty air filter, or too cold a heat range for how the car is driven.
• Wet, oily, shiny black — oil is getting in past rings or valve stem seals. This will poison the oxygen sensor and the catalyst too.
• White, blistered, or with erosion of the electrode — running too hot: too lean, wrong heat range, or timing too advanced.
• Melted or broken electrode — detonation or pre-ignition. Stop and find out why before fitting new plugs, because new plugs will go the same way.

Compare all of them. One plug that differs from the other three tells you which cylinder to look at, which is worth more than any of them individually.

Fitting

• Torque them. Over-tightening distorts the plug and its seal; under-tightening leaves it unable to conduct heat into the head, which converts a correct heat range into too hot a one.
• Anti-seize is usually wrong on modern plated plugs, and where it is used it changes the effective torque. Follow what the plug maker says rather than what the workshop habit is.
• Fit them cold. Removing a plug from a hot aluminium head is how threads get pulled.
• Replace HT leads or coil boots that are cracked while you are in there. A lead with a hairline crack in its insulation tracks to earth rather than firing the plug, and it will often do it only in damp weather — which reads as an intermittent fault with no pattern until somebody notices it rains on those days.

Related codes

Misfire codes are the usual reason to be looking at plugs: P0300 for a random or multiple misfire, and P0301 onwards for a specific cylinder. A single-cylinder code plus one plug that does not match the others is about as clear as diagnosis gets.

The coil codes belong here too, and this is the part worth carrying away from the whole article. P0351 through P0354 name a coil, and the corpus is explicit that a worn plug is a genuine cause of a failed coil rather than something merely found alongside one: the gap widens as the electrodes erode, the coil must produce a higher voltage every cycle to jump it, and coils fail from being asked to work beyond what they were designed for.

So a coil replaced on a car whose plugs are long overdue is a coil that will fail again, and a set of plugs costs a fraction of the second coil. If you are fitting a coil, fit the plug that killed it at the same time — and if a workshop quotes for a coil without asking when the plugs were last changed, that is a question worth asking them. Ignition leads covers the boot and lead side of the same circuit.