Don't get me wrong, I would pick Al for sure, but the HP loss is real. When I bought my Al block I used the same rotating assembly, same compression, heads, clearances, everything to do a back to back comparision to the iron half filled block. The weight differece was 90lbs the et loss was 2 tenths, I got to within about 1.5 tenths tightening the lash and leaving with some more heat. That being said, the advantages of durability, repairability, and lightening the front end in my opinion outweigh the HP loss.