This article from ‘Journal of Academic Librarianship’ looks at the Engineering literature:

John J. Meiera and Thomas W. Conkling, Google Scholar’s Coverage of the Engineering Literature: An Empirical Study, The Journal of Academic Librarianship, May 2008.

Google Scholar’s coverage of the engineering literature is analyzed by comparing its contents with those of Compendex, the premier engineering database. Records retrieved from Compendex were searched in Google Scholar, and a decade by decade comparison was done from the 1950s through 2007. The results show that the percentage of records appearing in Google Scholar increased over time, approaching a 90 percent matching rate for materials published after 1990.