Mapping optimises the power output of the engine by adjusting the fueling and ignition timing at various "load points" and RPMs.
Basically it means you don't use too much (rich) or too little (lean) fuel to achieve maximum power and/or torque. If you run lean you will get "pinking" under load which can destroy your piston lands or melt holes in your pistons at high revs. If you are running VERY rich you can get bore wash where all the lubrication gets removed from the rings, also you emissions will be very poor. Both of these can reduce the power output of your car. Turbo cars are often set up to be rich at high rpm because the excess fuel is used to "cool" the cylinders keeping the exhaust gases lower and thus helping to produce more power without damaging the engine.
If you change something major on your engine like new throttle bodies, new design exhaust primaries, fuel pressure regulator, injectors or high power fuel pump you can get you ECU re-mapped to take advantage of the new performance parts.
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