Is it necessary to keep the blanking plates fitted to the two bonnet vents? Surely you will get better cooling in the summer by removing them, or is there some sort of venturi effect type thing going on?

 

Have them off and feel how much heat comes out after a spirited drive.
If removed it can be an issue with water if car is fitted with exposed air filter especially the non oil type.

Rgrds

 

Use some washers to create a space between the existing blanking panel and the vent.

I used plastic washers about 5-6mm thick to allow improved air flow on the passenger side (as I had a scoop on the drivers side!) but still prevent water from ending up on the battery!!

 

If your engine bay is standard then removing the vent blank(s) will result in an increase in under bonnet temps. Totally against what you might expect but after careful experiments using lots of temperature logging that is the results.
The air comes in from the front, over the engine and with forced air from the scoop it all exits from the transmission tunnel.
Remove a blanking plate, or both plates and air is sucked out through the bonnet louvres by the low pressure area that exists on top of the bonnet when moving and this disrupts the intended air flow under the bonnet.
What is beneficial is the removal of the rubber strip on the bonnet, on the drivers side, above the headlight.

If you have a non standard set up then there MAY be an advantage in removing the blanking plate.
At present I have relocated my air filter to sit between the engine and off-side wing and then built an air box round the filter which seals quite well to the bonnet, bukhead and chasis. Air is forced in from the inner wing area and sucked out from the open louvre without disturbing the rest of under bonnet air flow.