EIST / SpeedStep controls CPU speed. If it's disabled you'll always have your set clocks (3.4GHz stock + turbo, or whatever you OCed the multiplier to)C1E controls Vcore. If it's disabled, doesn't matter if Vcore is manual or offset, you'll allways have your Vcore constant, but if you use manual Vcore, C1E won't work and your Vcore will remain constant even if C1E is enabled

Anyway Dazz, As you originally posted, no difference in performance between the two modes just a cooler running CPU when not under full load! Thanks Andrew

Sorry to hijack the thread but I have similar questions. Right now I'm following this guide: http://www.clunk.org.uk/forums/overclocking/39184-p67-sandy-bridge-overclocking-guide-beginners.html which I've heard good things about. Right now I'm trying to get my i7 2600k (asus p8p67 deluxe, 8gb 2000mhz cl9, running at 1333mhz same cl while OC:ing) stable at 4ghz before I head upwards. The guide says to use about 1.120 vcore but im still getting bsod 101/124 at 1.145. When looking at the vcore in cpu-z when stresstesting in prime95 i sometimes goes down to 1.04v and doesnt want to stay stable at 1.145 all the time, is this normal? LLC high.Sorry for bad grammar etc, im writing this on my iphone :)