Ram clock speed locked

MSI MEDION – MS-7785 Z77
BIOS Version/Date American Megatrends Inc. E7785MLN.105, 09.05.2012

i recently put in corsair vengeance 1600's x 4gb
I check my clock speeds in the bios and they remain greyed out at 1333...
How do I change the clock speed to 1600?

Cheers
Dave