head temperature increase with hotter spark plug

[jaguar]

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.

NGK___max temp
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 http://www.clubplug.net/champ_ngk.html
Champion L82C = NGK BP7HS
Champion L87YC = NGK BR4HS

Thermocouple at spark plug base:
Champ__max temp
L87YC___448F
L82C____418F

Thermocouple at front head nut/stud:
Champ__max temp
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)

 

[dirt bike dave]

Interesting data!

I am also quite surprised at the difference.

Is it normal for small motors to run such hot plugs?

Many bikes I've owned run an NGK '8' or even a '9' series, and the hottest I've used was a '7' in my old KDX250.

A 6, 5 or a 4(!) just seems crazy hot.

 

[jaguar]

well, with this new data people can make a more informed decision on what plug to run. When I ran my KDX200 in hot weather I had to run half and half antifreeze or else it would boil over. If I had known this I would of run a cold plug and just clean it every other time out.
The heat range I chose to test was rather arbitrary, dependent on what I had in my toolbox and what was available locally. The goal was just to find out what typical heat change each heat rating had.

 

[jaguar]

I'm not sure but it may be that most air cooled engines run hotter plugs than water cooled because water cooled heads shouldn't get any much hotter than 260 degrees (with a 15 lb radiator cap). An air cooled is OK with no more than 450 degrees, above which the aluminum is weakened.
I think I read somewhere that the spark more readily jumps from a hot tip than from a cold one. Also, hotter is more self-cleaning.