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Message: [QUOTE="jaguar, post: 1432909, member: 19340"] Nowhere have I ever read (from a spark plug manufacturer) that hotter spark plugs increase cylinder head temp. I thought that they couldn't because ceramic (the insulator) is a very poor thermal conductor. If I was right about the ceramic then the small area of the center electrode wouldn't be enough to absorb a lot of the heat. But I was wrong. The ceramic is not a typical kind of ceramic and does conduct a lot of the heat to the body of the plug and to the head. Here are some tests I just did: 55cc ported for 8300 rpm. 18mm Mikuni with reed valve. non-squish-band slant plug head with cranking pressure of 140psi and spark plug screwing in at the rearward intake side of the combustion dome. Jaguar torque pipe. Fiber head gasket insulating the head from the cylinder. Fiber washer insulating the thermocouple from the cooler cylinder studs/nut. Thermocouple connected under a forward head stud/nut leading to digital readout at handlebars. [U]NGK[/U]___[U]max temp[/U] B5HS__299F B6HS__268F --------------- Second test: Same engine but with squish band head with 165psi. Metal head gasket. Champion plugs tested which spanned 3 NGK heat ranges. The #87 had an extended tip (protruding more into the combustion area). from [url]http://www.clubplug.net/champ_ngk.html[/url] Champion L82C = NGK BP7HS Champion L87YC = NGK BR4HS Thermocouple at spark plug base: [U]Champ[/U]__[U]max temp[/U] L87YC___448F L82C____418F Thermocouple at front head nut/stud: [U]Champ[/U]__[U]max temp[/U] L87YC___399F L82C____333F So basically there was a 10 degree change per heat rating at the spark plug, and a 20 degree change per heat range at a forward stud. (there were 4 studs) [/QUOTE]

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