Try this calculator: http://www.rpmoutlet.com/dyno.htm

For a Datsun 1200 with A14/A15 engine, say 1660 pounds. 150 pound driver (total: 1810 pounds). 120 gross HP which is maybe 90 rwhp, the calculator says estimated ET of 15.8 seconds. For 120 rwhp, it's 14.4 seconds.