Do I have to rescale the tune when changing injector size?

Yes. The computer calculates fuel from the injector data you give it, so new injectors need new data.

• Flow rate is the foundation of the fuel calculation. If it's wrong, the engine never runs right, or the error hides a mechanical problem (LSX Magazine). HP Academy puts injector setup among the first steps of any tune.
• Enter the full characterization, not just flow. In LSX Magazine's 1,000cc injector swap, the tuner entered the supplied flow rate, offset (dead time) and short-pulse adder data.
• Match the fuel pressure. Injectors are rated at a set pressure, commonly 43.5 psi (3 bar) or 58 psi (4 bar), and flow scales with the square root of the pressure difference. Set the fuel-pressure reference correctly (atmospheric vs manifold-referenced), because flow and dead time both change with differential pressure.
• Use characterized injectors. Summit's LS guides recommend fully characterized injectors so the tuner can get idle and drivability right.

Want to do it yourself? See who tunes it, and how to learn.