I’m looking to change my 2.0L mazda3 engine to a newer 2.5L turbo Mazda engine and I was wondering if I would need to completely swap the ecu or if my stock ecu could run this engine.
Just guessing with a conservative opinion as ECUs aren't universal when switching engine displacement and adding turbocharging. A stock 2.0L without a turbo is naturally aspirated/vacuum mode; intake air is sucked in when the engine's running. An engine using a turbocharger requires a different ecu with tuning (fuel mapping) because the engine is naturally aspirated before flooring the pedal to bring on turbo boost which creates pressurized air/boost mode. The positive pressure forces air into the intake manifold, changing a n/a engine into a boosted one. The boost in intake pressure air means more fuel is needed to maintain stoichiometric air/fuel ratios. In EFI systems, a 1-bar map sensor is used for n/a engines measuring vacuum pressure in a running engine. while a turbo engine uses a 2 or 3-bar map sensor to measure vacuum and positive pressure (when the turbo is creating boost pressure), hence the tuning part of an ecu to take into account engine running from n/a (suction) mode to forced air (positive air pressure/boosted) mode. Without the tuned ecu and correct map sensor, the extra air simply leans out the a/f mixtures resulting in an inadvertent case of a blown engine or melting pistons from the non tuned ecu not programmed to detect boost pressures and a 1-bar sensor unable to detect positive air pressures. The ecu and map sensor (there may be other sensors needed) from the larger engine must be used otherwise you're likely to blow up the engine using the original ecu not tuned for turbo use. The 2 or 3-bar map sensor measures vacuum and positive air pressures in the intake manifold as a load sensor, sending signals to the ecu to ensure the a/f mixtures are uniform between n/a (vacuum) and boost (positive) modes.