Valve timing diagram of naturally aspirated four stroke engine.

Valve timing diagram of turbocharged four stroke engine.

In both cases the inlet valve open before the exhaust valve. The period in which both valves are open is termed as overlap period.
In naturally aspirated engine the overlap period is 20° before TDC+ 20° after TDC that is 40°.
In pressure charged or turbo charged engine the overlap time is 80° before TDC and 60° after TDC that is 140°.
The large overlap period of pressure charged engine is to provide time for complete expulsion of exhaust gases from the cylinder.
This overlap period provide enough time for cooling by fresh air of inlet valve, exhaust valve, cylinder.
This help in reduction of thermal stresses of these parts as it help in reduction of value of surface temperature.
When exhaust valve closes there is greater mass of air present in the cylinder to start the compression.
The overlap period is also referred as scavenge period in turbo charged engine.

Importance of fuel valve timing.

Early injection of fuel result in back pressure as the fuel ignites early and restrict the motion of piston.
This in turn increase the vibration and reduce the exhaust temperature.
This could result in damage to cylinder, cylinder head, or even bend rod.

Effect of exhaust valve opening early and closing late.

Small roller clearance result in early opening of exhaust valve this result in escape of hot gases which will result in loss of power of the engine, damage to valve face and seat.
When exhaust valve closes late this will result in dilution of charged air and ultimately result in loss in combustion efficiency.

If air inlet valve open early this will result in high pressure of the exhaust and will also contaminate the charge air.
There are also chances of scavenge fire.
If inlet valve closes late this will result in escape of charge air when piston is about to initiate compression stroke.
In both these cases volumetric efficiency is low and performance is poor.