Spark Plug Heat Range

[Dave]

I hear people going on about using a "Hotter" range spark plug or using a "colder" one but what does it actually mean and does it increase performance? As well as having Hot, Standard and Cold spark plug ranges, what do people mean when they say they use a "harder" plug.
Any replies appreciated.

[Keith]

Hi
It is confusing. Basically a colder spark plug will bring down (slightly) the combustion chamber temperature of a hot running engine. A hotter spark plug will raise the temperature slightly. Most people seem to refer to a colder plug as a harder plug.
For NGK plugs the number in the type is the heat range. A B9ES is a colder plug than a B8ES.
Spark plugs work over a certain temperature range. Go outside that range and performance will suffer (and possibly engien damage will result). Too cold and the plug will oil up. Too hot and it will start to blister and burn the spark plug. Different heat ranges of spark plugs work by varying the amount of the inside of the plug that is protected by the ceramics. More ceramic protection means that there is less surface area that can transfer heat from the combustion chamber to the water jacket, and so the combustion chamber temperature will increase.
Will changing the plug increase performance? No, just like most other bolt on changes such as jetting, getting it right is important. Get it wrong and performance will suffer, and getting it too far wrong and you will wreck the engine.
Basically if you are doing alot of high speed riding, then you may be safer going up one heat range on the spark plug (indeed, Honda recomend doing this for sustained high speed use). If you are mainly going round town then stick with the stock plug.
All the best
Keith

[milo]

I've been thinking about getting a hotter one for the winter. in theory the bike would warm up quicker and as i do a fair bit on the motorway it could be better for the engine. on the other hand i really don't wanna melt the cylinder heads. been looking at the more modern split spark type plugs. menna be more efficient.
anyone else know more?

[Keith]

Hi
If you were doing lots of short journeys in winter, then maybe go for a hotter plug, but certainly not if you are going down the motorway. It is combustion temperature it affects, which is not the same as engine temperature.
Multi electrode plugs work OK, but you will only get a single spark from them, however many electrodes they have. The spark will just jump the one with the best gap / cleanest electrode. The advantage is that as the electrode that is sparking gets worn, it will just start to use another electrode. Again, you will get no power gain from chamging to a multi electrode plug (and in the USA Splitfire have been had up for there advertising claiming performance gains)
All the best
Keith