Theres's some discussion in earlier pages regarding which portions of the HCHII grill to block first. We're in the temperate west coast region, and have had a very mild winter to date. The lawn is getting gangly, flowers and trees are sprouting, etcetera. No sign of snow, and not much on the local mountains, where something's been going on for the last few days... Anyway, with the moderate temperatures I've only been grill blocking the lowest row on our bumper. Our block heater plug has just developed a break, so I rewired it with a new plug. In the process I pulled out the grill block. Since the block heater cord is routing out through the lowest grill row, I decided to try blocking the upper grill row for a change, the openings just under the hood front edge. My first impression is that it's at least equally effective during driving, but better at retaining heat if you're parked for a while, since it more effectively traps warm air in the engine bay.