The V8 Owners Forum -

, hi guys, Can anyone tell me when fitting a bonnet vent, to cool down the underbonnet heat, is it best to fit it facing the windscreen to take the hot air out !!. or facing the front to feed the cold outside air in to push the hot air out??. if it will do that. help please paul

The vents are normally fitted near the rear of the bonnet (closest the windscreen)

The vents are then open to the rear - when seated you should be able to see through the hole into the engine bay
This way will also add a small low pressure area immediately behind the upstandanding part when the car is moving

Air will come in via the radiator and exit out these over the screen.

Someone will say "but the air pressure at the bottom of the screen is a high pressure area" well I believe you will also find the pressure under the bonnet is also high and infact higher than that above the bonnet (Easy enough to check with a manometer (U shaped tube with fluid in and see which way the fluid moves when driving)

Ian

(Describing this is not too easy)

hi ian, thanks for the info, i too think that its best to fit it that way round, paul

Ian Anderson wrote:The vents are normally fitted near the rear of the bonnet (closest the windscreen)

The vents are then open to the rear - when seated you should be able to see through the hole into the engine bay
This way will also add a small low pressure area immediately behind the upstandanding part when the car is moving

Air will come in via the radiator and exit out these over the screen.

Someone will say "but the air pressure at the bottom of the screen is a high pressure area" well I believe you will also find the pressure under the bonnet is also high and infact higher than that above the bonnet (Easy enough to check with a manometer (U shaped tube with fluid in and see which way the fluid moves when driving)

Ian

(Describing this is not too easy)

Most bonnet vents tend to be near teh middle of tbe bonnet. Not the rear.

At the rear you generally have the air intake for the heater etc.

rear middle???

The middle of the bonnet is recognised as as being the low pressure area but there are disadvantages to doing this as the hot air and fumes will then go straight into the heater inlet.

An alternative and highly efective solution is to cut some vents into the inner wings to allow air to escape into the front wheel arches which are a much greater low pressure area.

In my MGBV8 even when stationary you would not believe tjhe volume of hot air that comes out if the wheel arches when the fans cut in.

Additional bonus with the B anyway, is that with the air escaping from the wheel arches is that it is not being forced under the car and causing lift so improves stability and helps with the aerodynamics.

Kevin.