To those reading that don't know of Amdahl's Law, it basically says there are limits to the efficiency of multi-core systems, and past a certain point you can damage the efficiency of your system by adding more cores.
Like the other gentleman stated concerning the pieces of code that cannot be made parallel, there is a simple equation that allows you to determine the speedup:
Tp = 1/s * Ts + (1 - 1/s) * Ts/p
