what are ported heads and why do they give a big power gain ?

 

Thats a pretty wide open question...

Essentially your engine is nothing but an air pump. To make power, you need to make tq/cylinder pressure from combustion. The more combustable mixture you burn with each power stroke, the more cylinder pressure/tq/power you make. So altering the dimensions, shape, and/or trajectory of the cylinder head/intake ports can increase both the efficiency, compacity, and quality of the air/fuel charge they're delivering to the cylinders. Then there's valve job prep, valvetrain components, valve timing events etc...

Thats beyond brief & dumbed down... hope it's halfway sufficient .