When you overclock a laptop with Intel smart boost, from 2.4ghz to 3.4 witch it is advertised will it do it only to certain cores or to all of them.Will it shut down other cores?
The boost scales with the load. It won't go all the way up to the max with all four cores, in other words. To use hat-picked and not-at-all accurate numbers, if the load is only using one core, that one core will boost to 3.4 GHz. If the load goes to two cores, then those two cores will boost to, say, 3.1 GHz. If it's three cores, the turbo clock will go down to 2.8 GHz. Again, don't quote me on the numbers; I'm describing the principle.
The idea is that the CPU doesn't need as much cooling if all cores aren't utilized, and so the CPU can afford to increase its partial-load temperature to give you a situational speed boost.