Reading sparkplugs ???

[eggman918]

How does spark plug heat range affect reading plug regarding mixture

[knightfire83]

http://en.wikipedia.org/wiki/Spark_plug#Heat_range

[fastEdsel]

A rule of thumb we use in our aircraft engines is "hot plug for low compression, cool plug for high compression." That also was the part of the lecture given to us by a representative of Champion Spark Plug Company at an FAA sponsored seminar last spring.

[eggman918]

A rule of thumb we use in our aircraft engines is "hot plug for low compression, cool plug for high compression." That also was the part of the lecture given to us by a representative of Champion Spark Plug Company at an FAA sponsored seminar last spring.

Good to know
So does that mean that running hotter plugs in a motor that was difficult to warm up was a bad idea

[Dragon]

The hot plugs did not affect cold starts. In a new race motor I would put what we called warm up plugs in the engine normal for that engine is way too hot for racing. But for initial break in and tuning they are perfect. The race plugs and Ford followed this rule too If a 9.5 to 1 engine uses BF42 (old autolite designation) then 10.25 uses BF32 1 step down at 11.25 to one my FEs got BF22 and the 13 to 1 427 I built got BF12. Following that rule a rarely cooked a plug or had evidence of detonation unless the engine suddenly went lean in a race.

The hot plug retains more heat then a cold one. So in a high compression motor, that actually builds more compression with cylinder stuffing, would compress the fuel and find the hotter part of the plug and between compression and that hot part fire at the wrong time. A colder plug draws heat away from the tip and shell faster so the plug has less chance of creating heat ignition. As you compress any substance heat rises. Compressing air 13 to 1 raises its temperature about 5 kw or a small electric space heater.