Engine blocks used to be made of cast iron, but they've pretty much all moved to aluminum because of 1) weight, and 2) that higher thermal conductivity. IANAME

Engine blocks used to be made of cast iron, but they've pretty much all moved to aluminum because of 1) weight, and 2) that higher thermal conductivity. IANAME, but IIRC the problem with iron's lower thermal conductivity is that it created hot spots which could cause pre-ignition, so you had to use lower compression ratios to avoid this. Aluminum has more uniform temperature so you can boost the compression ratio, which gets you more efficiency and power.