An ECU is short for 'Engine control unit'. It monitors various things like the air fuel ratio, it sends fuel according to the readings sensors provide it. The throttle position sensor tells the ecu how much the throttle plate is open and the MAP sensor tells the pressure of air thus calculating the density. The ECU also controls the idle speed and the ignition timing. Ignition timing is when the spark plug sparks. The ecu also controls the VTEC engagement. The D16W9 has a 3 stage VTEC system, meaning the first stage is at 2500 rpm and the second at 5500 rpm. The D17 has a single stage VTEC, engages at a different rpm, I don't know when. The ecu of the d17 can only engage it. And I heard that there's an EGR valve in the D17, not sure, but the D16 doesn't. So that won't work with the D16's ecu. The rest can be better explained by seniors who have knowledge about this and have experienced it aswell.