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How far can you actually push how-lean-can-you-go?
Peak power in a gasoline spark-ignition engine sits rich of stoichiometric, not lean of it. Excess fuel both consumes the available oxygen and cools the incoming charge through vaporization, so from a rich, safe baseline, leaning the mixture does increase torque until you pass that best-power region. Past that point, further leaning mainly raises combustion temperature and knock sensitivity, which is the usual explanation for pale insulators and short piston life.
The street heuristic of a coffee-brown insulator is therefore a reliability target, not a power target. The open question is whether serious race programs hold the mixture on the hot side of best power because the engine is already treated as a short-life consumable, or whether a white insulator is simply what a frequently rebuilt motor looks like when it is already near that peak. How far do people actually push this, and is plug color even a clean enough signal to decide?
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Best-power AFR still sits slightly rich of stoichiometric, and that is the mixture that sets lap time. Race programs replace pistons on a schedule, so they will run closer to the knock limit than a road engine ever should. The extra heat is simply the cost of holding peak torque.
Scheduled pistons are not a reason to keep leaning after torque has already peaked. Past that region you lose power, and you can damage a ring land or the cylinder head, which a planned piston change does not cover. The same mixture that sat on the safe side of knock when the track was cool can be over the limit once surface temperature rises. Plug color is a weak signal on its own. Some oils barely mark the insulator even after knock has started, and others stain it brown while you are still rich of best power.
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Y'all can stare at the firing end all afternoon and still not know if you went too lean. Dirt and a little oil haze that nose up something fierce, so the color out there will lie to you. What actually pays is splitting the plug and looking down the ceramic, not just the tip. If that porcelain is bleached deep in the well you been cookin it even when the electrode looks decent.
On a slick track the motor unloads in the middle and that same mix will show hotter than it did when the clay had bite, so don't jet off one look after a yellow-flag idle either.
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