I know the principle of OVERLAP- the time when both intake and exhaust valves are open. But, why is it good for NA applications but bad for FI. My thinking is because the intake charge is now pressurized ABOVE atmosphere, the need to "scavenge" the charge isn't necissary? Please excuse my ignorance
TIA

 

when n/a, you need it for scavenging, correct, but with boost, when both valves are open, the pressure just forces the mixture right out the exhaust.

 

it mainly has to do with that N/A cars are running with vaccum pressure and FI cars are running with positive manifold pressure, ie boost. so with the overlap of the valves the exhaust helps creates more vaccum pressure thus getting more air/fuel mix into the cylinder for more power.