[scottj]

Since aluminum grows more than iron, to have the same hot idle pressure, an aluminum motor will need to have tighter main bearing clearance when cold. The tighter main clearance can reduce oiling to the rods until proper temperature is reached. It's not uncommon to build aluminum blocks to have a little more clearance on the main bearings. This prevents then end user from kicking the rods out when he stands on a cold motor. As long as pressure comes up on throttle, all is well. Every aluminum race engines I've owned was built this way.