Some interesting reading:
Air/ fuel Ratio
In most spark ignition, internal combustion engines, the mixture is combustible within an AFR (air/fuel ratio) range of roughly 9 to 1 to about 17 to 1. 9 being very rich, 17 being very lean. 14.7 is the stoichiometric ratio (chemically correct) for lowest emissions. Best power is obtained at around 12.5 for most naturally aspirated engines.
Pistons
Most automotive engines produced today have cast aluminum pistons as opposed to forged aluminum pistons used in race applications where engines produce high continuous hp levels. Forged pistons transfer heat much faster than cast ones and retain their strength at higher temperatures however, the increased fitting clearances required makes them an unattractive choice for production car engines where noise, emissions and longevity are important and the average hp is relatively low in the automotive application. Aluminum loses approximately 50% of its strength at 400F and this is a pretty typical temperature for a piston crown operating at medium to high power settings. The strength rapidly diminishes above 500-550F so they cannot be allowed to get this hot or deformation and fatigue will set in due to the high pressure they are subjected to. In other words, they can become somewhat plastic and weak if they get too hot.
Pistons are usually the first parts to fail from excessive thermal loads on automotive engines. The solution to this for almost all auto OEMs is to run the AFR very rich at WOT (wide open throttle) and high rpms corresponding to high hp output. The excess fuel drops the combustion temperatures, EGTs and cools the piston crowns themselves, keeping them within their safe thermal limits. While best power is obtained with AFRs around 12.5, most OEMs run the AFRs at between 10.5 to 11.2 at WOT. This is where you need to tune your AFRs also. While there will be a slight hp loss and you'll use more fuel, this is the only way the pistons will survive for very long running at continuous high power settings. I suggest a nominal target of 10.5 to 11.0 for power settings above about 75% and no leaner than 11.5 from 50% to 75% power. I'd also mention that detonation is most likely to happen at AFRs of around 12.5 as well if there is excessive spark timing and/or low octane fuel.
Valves
Automotive engine valves and seats vary in material and quality between engine types and brands. Again, these parts have thermal limits and high EGTs can decrease the life of these components. Rich mixtures decrease EGTs and exhaust valve temperatures. While most valve failures are slow and gradual compared to piston failures, we don't want degradation here either.
We have the capability of checking Lamda against EGT, never thought of doing this might be interesting to see the relation between the two.