Diesels are getting much lighter.
Volve makes an aluminum block straight 5, yes 5. Honda is promising an aluminum block v-6 that gets 60 MPG. Mercedes is building one too IIRC. I wonder about the reliability of an all aluminum turbo diesel though.
There are several diesels made of Compacted Graphite Iron or CGI. I've just started looking into CGI apparently it's stiffer than iron so they can use thinner castings, the blocks weigh about 25% less than iron blocks. The Ford/Jaguar 2.7 and 3.0 TDI CGI block V-6 is reported to weigh around 450 lb complete.
http://www.tupy.com.br/downloads/guesser/compacted_graphite_iron_for_diesel.pdf
