All the charts & photos on this page pertain to leaded fuel. Most of the data I have supplied is pre-unleaded. I know, probably 30% of the info here may be useless.

Reading a spark plug is a science all on its own and there is no way that I can pass on what I have learnt from 30 years of reading them. However I can give you some info I have gathered from all over the internet to help you. It won't make you an expert, but it may help you understand a little more about the humble old spark plug. Engine tuners do not use plug charts to diagnose problems, if they are good at their trade. They are better used as an aide to liase with customers.

Customers can see their spark plug condition on the chart as the Tuner explains the possible causes. Customers like to see in charts and pictures what is being explained to them, it gives them better understanding and assurance that their not being ripped off, after all this is a specialised field and there are a few Tuners out there that DO NOT explain the problems to the customer at a level they can understand.

 A spark plug has one of the hardest jobs to do in an internal combustion engine. If my maths is correct , in a 6 cylinder engine, a spark plugs fires 7 times per second, idling at 840 rpm and 42 times a second at 5040 rpm, under the most extreme and varied conditions. The spark plug the car manufacturers fit and recommend is an all round general use plug. This means you can cruise around, give it a hiding for short bursts and do a little bit of light towing and you should be able to get at least 10 to 20,000 kms of life from the standard plug and some precision metal plugs are good up to 100,000 Kms. The standard spark plug are a good plug for general use and will be the plug that is used for most of the engines life. However if you plan on towing for long periods under heavy loads, give the engine a constant hiding or install high performance cams, then the standard plug will overheat before its serviceable life has been reached and eventually will start to misfire. Under these conditions the plug heat range will need to be colder. So it is a fine line that the spark plug treads and it is the Tuners job to understand the owners driving habits so he can choose the correct heat range of the plug to try and get the best mileage from them. Unfortunately you can't get around this problem as there is no such thing as an adjustable heat range plug. If someone invents one, it will be the invention of the century.