Spark plug heat range for bolt-on mods?

[Piloter]

I've been going back and forth on this one. Logically, increased airflow would lead to increased fuel flow, and therefore you'd want hotter combustion to extract more power, thus spark plugs one step hotter...

But then again, all the sites out there suggest one range colder for extended driving at speed or heavy loads. Increased air and fuel may not be increased to match, so leaner, so burns hotter, so you'd want a range colder to keep combustion temps under control.

I can see both sides of the argument, and various ASE techs at various parts stores have been about evenly split. Help! *L*

(BTW, mileage record is up to 39.8 for a tank of regular and a 250-lb passenger and mountains. I'm getting the hang of this.)

[Flashmn]

and therefore you'd want hotter combustion to extract more power, thus spark plugs one step hotter...

A sparkplug wont add or subtract power if its working, heat ranges dont work that way. You cant make a sparkplug "Add" a hotter combustion by using a hotter plug, the heat range of plugs isnt about the temperature of the combustion happening, its how hot the plug will get under operation.

A plug is measured to have a certain heat range so that under normal operation, the plug will get hot enough to burn carbon deposits off the insulator and tip. The range is determined by the head design.
Now a too cold plug will get sooted up and it wont clean itself out, because the plug is too cold. Thus this plug will quickly clog up due to soot and misfire.
A too hot plug will quickly overheat, causing pre-ignition or knock and this will kill the engine permanenty.

The reason why for example turbocharged cars have colder plugs than N/A cars, is because the pressure inside the cylinder during ignition alot higher, thus causing more heat. I believe that 1bar boost is double the combustion temp. Now your hot NA plug will quickly get so hot that it will ignite the mixture due to the temperature of the plug tip, not the actual spark. PING PING BOOM. Two plug ranges colder plug at the same theoretical situation, might be ok, it will still be so hot that it will burn off the soot, but it wont start to act like a glow-plug under boost.

Hope this helps a bit.

[Piloter]

By hotter combustion, it was meant that the hotter-range plug removes less heat from the combustion chamber via the longer insulator.

[Flashmn]

Plug temp doesnt affect combustion temperature, its a whole different ball game. The insulator length only determines the time it takes for the plug to cool off. Read my previous post about it.