The same two letters, in the same order, can be added to the words below to form new words - but the letters have to be re-arranged. What are the two letters, and what are the new words?

GRAIN
TIMES
DREAD
ROWED
RATIO
PRATE
MARINA
REFUEL

HINTS:
#1: The added letters will be the first two letters of the new words.
#2: One of the added letters is H.

Answers:

The added letters are CH:

CHAGRIN
CHEMIST
CHEDDAR
CHOWDER
CHARIOT
CHAPTER
CHAIRMAN
CHEERFUL 

Credit: Will Shortz
NPR Weekend Edition Sunday Puzzle
July 13, 2014 - CH-CH-CHANGES