Here's why:
* Engine Bay Size and Mounting Points: The 1985 Cutlass Supreme's engine bay is designed for specific engine sizes and configurations. Larger engines might simply not physically fit without significant bodywork modifications. Even engines of similar displacement could have different mounting points, requiring custom fabrication.
* Transmission Compatibility: The engine must be compatible with the existing transmission (or a replacement transmission you plan to install). Not all Oldsmobile transmissions will work with all Oldsmobile engines. You might need a new transmission, driveshaft, and potentially other drivetrain components.
* Wiring Harness and Computer Control: Older carbureted engines will require significant modifications to the wiring harness and fuel system. Even later, fuel-injected engines may require considerable work to integrate with the existing computer system or require a complete engine control system swap.
* Exhaust System: The exhaust system will likely need to be custom-fabricated or significantly modified to accommodate a different engine.
In short, while some Oldsmobile engines from similar eras might be relatively straightforward swaps, others would be extremely challenging and expensive projects requiring substantial fabrication and potentially significant modifications to the car's structure. You'd need to research the specific engine you're considering very carefully, consulting resources specific to Oldsmobile engine swaps and the 1985 Cutlass Supreme.
