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Hot/cool spark plugs: what does it all mean?

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Joined: 13 Mar 2003
Odometer: 22856
Location: State of Confusion

PostPosted: Mon Sep 15, 2014 11:23 am    Post subject: Hot/cool spark plugs: what does it all mean? Reply with quote

I've been struggling with this one since I discovered that modern spark plugs are manufactured without glaze on the inner ceramic part so will foul up very quickly if the engine floods or is running rich and often cannot be cleaned up. This is, apparently, exacerbated by the additives in modern fuels including ethanol.

One problem can be that if you use a plug with the wrong temperature range it will not burn off residue properly. In theory you can use a plug which is either hotter or cooler to compensate but the description is counter intuitive, varies from make to make and you can easily make the wrong assumption.

I was going to try to explain it myself but this link is so good replicating it is pointless

Heat range is the measure of how fast the spark plug tip dissipates combustion heat. It must do this in a precise and controlled manner so the spark plug will:

* Stay cool enough to avoid pre-ignition and/or electrode destruction due to detonation.
* Run hot enough to burn off combustion deposits that would otherwise collect on the insulator tip and cause fouling that results in misfire.
* Adapt to specific engine characteristics and widely varying driving/load conditions.

In my case study, you can see from this photo the difference between the 2 plugs.

On the left is the D21 and on the right is the D16 (actually the equivalent NGK AB6)

You can see that the D21 is finer and described as a 'hotter' plug which will retain more heat and therefore burn off deposits more easily. This will suit engines that are run for shorter periods and under less load. The terms hotter and colder do not refer to the engine temperature!

Poking the Grim Reaper with a stick then running away. The devil made me do it but God said it was okay with him.
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