hot air rises, so I would have thought topside sealing does matter...accepting that high pressure air will find the line of least resistance and that could presumably mean evacuation out the bottom of the engine bay.

maybe I'll just remove the rubber strips that go down each side of the front end if their only purpose is to seal the engine bay to reduce noise. The alternative is to just fit some small low profile vents into the bonnet (rather than a full-on bonnet scoop). I already have a digital temp probe under the bonnet, so it would be interesting to see what difference this makes to air temp although obviously the real issue is what difference it makes to engine operating temp which would require an EGT or coolant probe. My Td5 runs fine and doesn't have an overheating issues, but anything simple I can do to improve cooling is obviously desirable - starting with the removal of the plastic acoustic engine cover...