[cameron02]

I made the vents on my DRB functional. I cut out the accessible 1/3 and made fibreglass tunnels to the engine bay. Simply cutting out the 1/3 will not have any affect as the engine bay is still blocked off, it will simply pull air from the wheel arch area. I would post pics but my car is in for painting at the moment.

The vents offer virtually nil cooling when the car is stationary. As far as I know they work like this.

As you are driving the air passes over the vents. Because the vents are rear facing the air passing over them has a pulling effect (suction). This results in air being pulled out through the vents.

Ceramic coating you headers will also help reduce under bonnet temps but if your engine is overheating then the cuuses could be multiple. Air in the system, unsuitable radiator etc. You could try fitting a thermo fan to the front or rear of the radiator for extra cooling.