Reply to "Header temps"

Idle.

A hotter plug is one that permits a higher combustion temperature. A colder plug colder combustion temps.

Adding more fuel to the idle mixture would have a similar effect of cooling the exhaust.

Looks like it will be a combination of both in this case.

It could also be that I have some sort of an anomaly here. I'm using data (plug heat ranges) that was for leaded fuel and unleaded with ethanol has this "slightly" different result with higher exhaust temps. What's the difference in temperatures? 200 F degrees?

Makes sense in a way. Obviously the main reason for unleaded fuel was to remove the lead but raising the exhaust temps also reduces certain emissions so that's two fold effect.

Shelby stopped putting headers on the cars after 66 because Ford had added air injection to the exhaust to comply with California clean air numbers and they thought steel tube headers were "incompatible" with that.

"Incompatible" might just have meant the "steel tube headers will melt" from the heat? Cast iron would last longer. That was still leaded gas and they hadn't yet seen the effect I've got here now.

When I was racing it was still with leaded fuel. The AF22's were fine on the track but would foul at idle on the street. Come to think of it, the plug was never tried with unleaded to my memory?