* Torque Capacity: The 3.8L engine produces significantly more torque than the 3.4L. A transmission designed for the lower torque of the 3.4L would likely be damaged or fail prematurely under the higher stress of the 3.8L's output. The internal components, like the clutches and shafts, aren't built to handle the increased load.
* Gear Ratios: Even if the torque capacity were similar (which it isn't), the gear ratios in the transmissions are likely optimized for each engine's power band. Using the wrong transmission would result in poor performance, potentially leading to overheating and damage. The 3.8L might need lower gears for better acceleration, while the 3.4L's transmission might be geared for better fuel economy at the expense of acceleration. The mismatch would negatively impact drivability and efficiency.
* Input Shaft Diameter/Spline Count: The input shaft that connects the engine to the transmission may have a different diameter or number of splines between the two engine types, physically preventing them from mating correctly.
In short, while the Impala may look similar externally regardless of engine, the internal drivetrain components are engineered specifically for the engine they were paired with during manufacturing. Swapping them leads to serious mechanical problems and is strongly discouraged.
