[CigaR007]

On some cars, the upper grille is purely cosmetic. Blocking the lower grille will require that you monitor the coolant temperature via a scangauge, ultragauge, etc.

Ideally, you want to optimize the opening to get adequate cooling. The opening will vary from one car to another. An accepted rule of thumb is to leave the middle of the lower grille open and block the sides.

You can get a good cooling airflow with an even smaller lower grille opening, but it will require ducting to the radiator.

To give you an idea of optimization, my current lower grille block was not efficient at speeds higher than 30 mph. I added a small coroplast deflector which redirects the incoming air towards the radiator. It has since then been working as it should, keeping the coolant temp stable at 188F, making it efficient at any speed. Trial and error experiments are usually the way to go when installing a grille block.