Nissan Patrol 4x4 (Africa)

Looking at the post on the Patrol Gazelle (How I wish...) I noticed the bonnet mounted air intake or front hood scoop.

I see its facing forward, Ive also seen them face backwards and flat panel mounted ones..

Is the idea to force air into the intake - if facing front? If so at what speed are they most efficient?

Is the idea to allow hot air under the bonnet out at the highest point? Facing rearwards.. and is there a "drag" effect created sucking hot air out by the wind at speed?

With the flat ones, i presume they only let hot air out and water in?

What's your thoughts on these?

Are they worth the cost and effort of fitting and do they actually work?

Have you had a overheating problem ?

Basically, yes. Front-facing scoops are usually there to collect air for a specific purpose, such as an inter-cooler or brake cooling ducts. Rear-facing ducts (whether raised or louvered) are typically there to extract hot air from under the bonnet.

For either to work, the pressure on the inlet side of the duct needs to be higher than the pressure on the other side.

Therefore I think these things need to be carefully considered and ideally tested in a wind tunnel.

Of course, few of us have easy access to wind tunnels. There are however old-time tricks which can help.
For example, short pieces of wool taped to the bonnet will give you at least an idea of which way the wind is blowing...

I would say that any car designed within the last 20 years or so would have been fairly carefully tested in a wind tunnel, and any modifications to the air flow under and over the bonnet probably requires some careful thought. Of course, this includes things like bull bars and bash plates, so once again, I am not entirely practicing what I preach...

I have recently learnt that covering my bash plate with an impermeable plate can reduce engine temperatures but conversely (possibly) increase engine oil temperature...

I also suspect that relieving air out the top of the bonnet might well increase temperatures in the foot-well by reducing airflow around the gearbox?

No not at all Tinus, I haven't even got back to SA to collect my Troll yet. I did have air vents on a Jeep I had and always wondered how effective it was.

My thinking was if the vent was facing backwards the airflow through the front grill would work its towards the rear of the engine compartment and exit at the top of the bonnet thus expelling some of the hot air out of the top of the engine bay along with maybe a suction effect from the wind flowing over the outside of the vent.

I can understand if a forward air vent was piped into a air cleaner etc so kinda force feeding the engine but don't see how it can help otherwise while driving, evidently it will allow hot air out once the vehicle is stopped.

A top mount cooler makes less air go via radiator ...when rubbers are missing on side of wheel arches it makes them run hotter