Here's why:
* Engine Block: The 2.7-liter engine in the 1990 Legend is a completely different design from the 2.5-liter engine in the 1987 Legend. They have different bore and stroke dimensions, meaning the cylinders are different sizes and the pistons travel different distances.
* Mount Points: The engine mounts and their locations are different between the 2.5 and 2.7-liter engines.
* Accessories: Many engine accessories, such as the alternator, power steering pump, and air conditioning compressor, are mounted differently and likely won't be compatible.
* Electronics: The 1990 Legend used a more advanced engine management system than the 1987 model, meaning the wiring harness and sensors are incompatible.
You would need to do significant modifications to even attempt to install the 2.7-liter engine into the 1987 Legend, and it's highly likely that it wouldn't work properly.
It's always best to stick with the original engine for your model year. If you're looking for a performance upgrade, consider finding a used 2.7-liter engine from a 1990 Legend or a compatible aftermarket performance engine that's designed for your car.
Remember, before attempting any engine swap, consult with a qualified mechanic to determine the feasibility and potential complications.
