Strong portfolio returns helped large U.S. defined benefit pension plans overcome a drop in corporate bond yields in September.

The “funded ratio” at a typical plan rose to 67.3% at the end of the month, up from 66.9% at the end of August, according to consultants in the Stamford, Conn., office of Towers Perrin Forster & Crosby Inc.

The index stood at 85.8% stood at Sept. 30, 2008.

Towers Perrin bases pension status figures partly on the performance of a hypothetical benchmark pension plan. The firm also tracks the actual pension results reported by 300 large U.S. companies.