Why does a boosted car need a retune immediately?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• P01, P59: EFILive's Custom OS and 2/3-Bar tutorial: after fitting a 2- or 3-bar MAP sensor you must rescale it — {C6301} MAP Sensor Scaler to 190 for a 2-bar sensor or 288 for a 3-bar sensor, then trim until the sensor reads the same key-on/engine-off barometric pressure as the stock 1-bar sensor; log boost with the SAE.MAP PID and set {C5501} MAP Rationality Test RPM High to 2000 RPM. (EFILive)

Confirm against the instructions for your exact parts before wiring. More: Computer and tune in our tuning guide.