[Barry_R]

Aluminum blocks work just fine for street use. Nothing wrong with them. But all things being equal an iron block will make more power - if maximum power is a factor for the application. The weight advantage can be significant in a Cobra.

All NASCAR blocks are iron, and most if not all Pro Stock blocks are iron. They've actually gone to higher density, heavier grades of cast iron (compacted graphite or CGI) to add strength and for more power yet. Somebody mentioned the architecture and they were correct, along with thermal stability.

I'll give you a single simple example - there are several others - but you can actually measure and visualize this one. If you set up your motor for zero deck clearance and say 10.5:1 compression. As the all aluminum engine warms up your deck clearance will increase by about .010 and your compression ratio will drop by a corresponding amount. You can see this in the valve lash settings on an all aluminum combination - my aluminum head stuff will grow by about .009 cold to hot - the all alloy stuff needs to be near zero lash cold.