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A lean mixture means that the air-fuel ratio is too high, causing the mixture to burn like a cutting torch, melting the psitons, instead of cooler burn which causes expansion to move the pistons. If cylinder temps are too high, the mixture can detonate before it's had a chance to be ignited as a controlled burn, this causing pre-ignition. Egt's are widely discussed, some like to aim for 850 as max temps, but others push further to 900 or even 950. Ignition advance affects the point at which the spark ignites the fuel, get it wrong and you will either loose power and response (too much retard), or get det (too advanced).
You will aslo notice a difference in turbo response, as the hotter exhaust gasses move faster, thus far better transferring their kinetic energy onto the turbine wheel, so timing and fuel with have an affect on that also.
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