Stage 1
Software only. A calibrated ECU map on stock hardware, usually worth 15–25 % more power with zero drivability compromise.
Stage 2
Software plus supporting hardware: intake, downpipes, intercooler. Expect 30–45 % gains and a car that still passes inspection in most regions.
Stage 3
New turbochargers or superchargers, fuel system upgrades and often internal engine work. This is where builds become projects.