Mapping a motor is just fancy talk for setting timing and fuel requirements at different speeds and load factors. From the manufacturers the engine is mapped for a combination of economy and drive ability without over stressing throughout the normal rev range. An after market "chip" is basically a piggyback computer that interferes with the standard map from the engine control unit. In my experience there is no such thing as a free lunch and the same applies to engines, there is usually a trade off for the extra power gained in certain areas whether it be economy , longevity or just loss of power in a different rev range, the secret is to know the cost of the modification. In my plat karre I have programmable ecu's and I can alter the mapping as I choose and have 3 different maps that I have put on them and can change as the situation dictates, not something I would do on my pajero, here reliability remains top and I am not even thinking about altering the chip myself.
PS petrol engines can be set up for the same parameters as diesel you just change different settings
get your mixture too lean and you burn valves or run the timing wrong and you have detonation as bad if not worse than high egt.
In short this is an area that it would be wise to tread lightly
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