Ecu mapping is a very specialist area. Essentially and broadly speaking the ecu manages the air/fuel mixture, fuel injection and spark based on input parameters it receives from various sensors monitoring air tempt, air pressure, crankshaft position sensor, camshaft sensors, engine speed, knock sensor, lambda oxygen sensor measuring exhaust gas oxygen content.
To truly tune an engine a rolling road setup is needed and the ecu hooked up to a pc via which various parameters can be altered to improve power, mileage, etc. at various engine speeds and loads.
A Stock ecu can also be "reflashed" ie a new engine management software be downloaded to improve performance or also an additional hardware box be piggybacked onto the stock ecu to do the same job. These two are the easiest options compared to doing rolling road tuning with a programmable ecu. The rolling road tuning with programmable ecu will give far better results but not worth it for an ordinary road car. Also the programmable ecu is coupled with other upgrades such as throttle bodies, turbos, superchargers, reboring, hot camshafts, etc etc etc to name just a few.
I run a DTA fast programmable ecu on one of my cars and it is truly an amazing piece of kit pushing bhp up by almost 40 horses on a moderately modified engine.
An excellent read is Engine management by Dave Walker.
If you want to do re-mapping, then you have to find out the logic of the ECU. And find out the boost pressure of Turbo Charger. Increase this step by step and see how the engine is behaving. But be carefull
My understanding is remapping can be done. but however its best tuned on dyno isn't it. doing it as a DIY is pretty dangerous. please correct me if i'm wrong!!. would like to know more from people who have hands on exp with this. i know remapping chips that are there in the market for petrol as well as diesels but those are pre programmed.
It is sufficient to change the following parameters in EPROM
Part throttle injection
Fuel during acceleration
Have never tried it on own. My colleague is an automobile engineer. He has done it for many (Ok, only three since people are generally afraid to mess up ECU :-) ) friends who have turbo charge engines.
1. getting the car tuned through Dyno is better or doing it yourself is better which gives better results. as i had read somewhere that tuning is always suppposed to be done on dynos only, for accurate results.