890GX Extreme 4 - Can't set BIOS for Memory mHz

Hi all, I have a Asrock 890GXExtreme 4 R2.0 motheboard and 16GB of Corsair Vengeance 1600Mhz RAM. I have worked out the the speed the Ram should be running at per channel is 800Mhz but can't seem to set the BIOS to correspond. I could be doing something wrong. In CPUz the memory is shown as 500Mhz per channel not 800Mhz. I changed the Bus speed in the BIOS to 400 (400x2 = 800Mhz) but can't start the computer at that. I lowered the HT Link to 1860mhz and reduced the CPU multiplier to x15 so i get just over 4Ghz for the CPU. It seems the PC won't start if the Bus is faster than 270. I have also tried increasing the voltages but that doesn't seem to help. Any ideas?