From the member 'therieldeal' on another mazda forum.
ENGINE ELECTRONICS:
The engine wiring harness and ECU from a 91-94 EGT will interchange with the 90-94 protege. all the major sensors, airflow meter, injectors, etc are all the same as well. one exception is the coolant temp sensor for the dash gauge, as it has a different resistance range. if putting a mazda engine in an escort, simply re-use your stock sensor, otherwise your temp gauge will not read properly. this is the very small 1-wire sensor that screws into the drivers side front corner of the block. in 95 the escort switched over to a 4 wire oxygen sensor, all other years used a 1-wire o2 sensor. you just have to use the proper o2 sensor to match your wiring harness/ECU. In 96 ford switched over to OBD2, so the ecu, wiring, and more of the sensors are going to be different. this OBD2 system is not interchangable with the protege.
ENGINE INTERNALS:
The engine block itself, the head, manifolds, cams, pistons, rods, etc are all identical. the one difference worth noting is that escort engines do not have a MBSP (main bearing support plate). this is a stamped steel plate which is sandwiched between the oil pan and block, and bolts to 4 of the 5 main bearing caps. it helps to prevent crank flex at high RPMs and high loads. if you can get one with the mounting bolts from a protege or miata BP it will bolt right up to your motor. its a nice thing to add if you have to remove the oil pan for some other reason.
TRANSMISSION / DRIVETRAIN:
For the most part everything is the same. transmission, mounts, clutch, axles, clutch master/slave cylinders, shifter linkage, etc. the only difference is that the speedometer sensor and cable are different. you can, however, put an escort speed sensor in a mazda transmission or vise-versa, but your speedometer may be off by a few MPH.
STEERING:
The escort steering system is also different from that of the protege. the steering rack and its mounting bushings are different, as are the tie rods. all 91-96 escorts use the same lower ball joints, which interchange with 90-92.5 proteges. 92.5-94 proteges use a larger diameter ball joint. the only time this would matter is if you are replacing your spindle with one from a protege, you would need to make sure you use the correct ball joint to match that spindle.
