good question. Dont think they have changed the 4.6 that much. Good reliable motor. I was talking to my buddy who was telling me that they went plastic because of heat build up and that it is lighter than straight aluminum. Dont think the weight would be that much of an issue though. Seems to me that aluminum would be the better alternative... but then again, what do i know...... oh thats right, that plastic p.o.s cracked.