[MetroMPG]

Driving efficiently when nobody's looking? C'mon! :P

Cold air is good for power. The idea behind a warm air intake is to intentionally DEpower the engine, which requires a wider throttle opening to do the same amount of work. A wider throttle opening means reduced pumping losses. One of several reasons diesel engines are more efficient is because they have no throttle plate.

A grille block is just that: closing up some of the front of the vehicle ahead of the radiator. A significant amount of the total aero drag of a vehicle is airflow through the radiator. If you don't NEED the airflow, closing off that opening will help. Don't melt your engine though.