Bonnet - is it a low or high pressure zone?

[JimAttrill]

We have a GM V8 6.2 diesel Defender which is running too 'hot' (95c), what with the rather small radiator possible, two oil coolers and the air con condenser all in the rather small space. There is a big mechanical shrouded fan.

The question is, if we make holes in the bonnet, will air come out or go in?

[silvio]

I think air should go out, Jim.

Give it a try: remove spare from bonnet, if present; open bonnet and just lay it down, without having the lock catch; speed up to 120km/h....

Bet you get the bonnet flat open on the windscreen!

[Quagmire]

Think of a plane wing, the air travels further and therefore faster over the top of the wing which causes lift. The air in you engine bay would be going slower than that over the bonnet i would imagine, and so would go out through the bonnet holes...

Could be wrong tho- flame suit on

[paul stage1v8]

hot air rises, also under the bonnet has a lot of air pressure from the air coming thru the radiator.

[GBMUD]

Think of a plane wing, the air travels further and therefore faster over the top of the wing which causes lift.

An aeroplane wing is a aerodynamic shape though, a Landrover is not!

Chris

[tacr2man]

It depends what part of the bonnet you are talking about, and it will vary with speed . Vents cut on top of bonnet will likely let hot air out at low speed but as speed increases this may well stall out . The best way to see pressure?air flow over bonnet is to drive in falling snow (if we get any this year). I know that spare wheel on bonnet causes air flow reversal at bottom edge of windscreen at high speed (washer water blows forward) .

I think a more reliable method would be to get extra cool air in from wing area ( eg vents ala late defender TD5) in front by headlight, wing top scoop etc. Also try and move cooling radiators so they are not in each others air flow as the air will heat up progressively as it passes thru one to the other, thereby decreasing the temperature differntial between what you are trying to cool and what you are cooling it with. a lot of 4x4 in OZ have to turn aircon off when in sand dunes as the condensor is in front of engine rad and causes overheating . HTSH

[JimAttrill]

I think we'll do the old aircraft air flow trick - cut a hole in the bonnet and glue little bits of wool around the edge. Then look to see if the wool goes in or out. I do know some bad mistakes have been made in car design by just assuming that a certain area was either low or high pressure. A good example is the Defender heater intake which should be at a high pressure area but quite obviously is not

[ThreeSheds]

I think that there is often a low pressure area near the front of the bonnet and a high pressure area at the rear centre, just in front of the windscreen - which is why manufacturers put 'hidden' heater intakes there (a la Disco) - i.e. they don't have to be scoops to get air forced in.

my 2p's

Rog

[pist0nbr0ke]

The air will be drawn out by the air travelling over the bonnet (when your driving along).

When you are stationary the heat will rise

cheers

Neil

[T1G UP]

I would go for an aditional elecrtric cooling fan in front of the rad set on a stat and jack the rear edge of the bonnet up on the hinges to allow the air to escape...see if that works before cutting any vents in there.

I run a vw golf track car and the bonnet trick works well to eliviate excess heat .

[Steve King]

You could try one of THESE

[Dave W]

On one of the 6.2 supercharged Ibex we cut a series of holes across the back of the bonnet and a triangular section out of the sides. Seemed to make a huge difference in the sand with the engine running a lot cooler.

Are you running a full aluminium radiator ? If not, you should get one made, much more efficient than traditional or standard LR radiators. Don't suppose you've considered ditching the air con ? It won't be helping the airflow...

[JimAttrill]

Yes, full aluminium radiator. Yes, the air con is really the culprit although there are two other coolers in the front. We are thinking of moving the bull bar and winch about 2" forward which will allow us to put a pusher electric fan between the aircon and the rad. We aren't really interested in how well the aircon works.

It also seems that the engine only holds about 6.5l of oil, which is not really enough for such a large engine. Also the cooling system holds about 15l which is a lot less than the 26l of a Hummer.

I rather like the idea of lifting the bonnet at the rear as it is less destructive than sawing holes in the bonnet.

[Hybrid_From_Hell]

This is a catch 22 question ! - Having had V8s in various ststes of tune and uses I have done much around this sort of prob, hope the stuff below is of help

re Overheating / getting temps down :

There is a difference 'on' and 'off' road

and there is a difference at 'staionary' / working hard and 'cruising' at speed difference

On and off road.

Prob here is when off road the exit for heat under the LR is cut off, thus the heat cannot escape downwards and the heat engine issues rises

When on road or at least not deep in goo off road, the heat can get out from underneath

Stationary vs Speed

When stationary holes cut in say the rear curve of a 90s bonnet does allow air out, but, and its a big one it can be air that is coming into the bay area then going up and out without hitting the engine and cooling it much, at speed this is exactly what happens via the pressure and flow of the outside air passing over the bonnet pulling the air up and out rather than over and across the engine and out

So, whats the answer(s) ?

About as complex as it gets ....but there are some key things you can do

Ensure the maximum amount of air coming in comes in

and comes in and then goes where you watn it to

Ie unblock cut holes etc in the front panel, ensure that the surface area to the front of the LR has as much exposure to air and is as open as it can be. Oil filters / oil coolers in front of the rad do not help move it somewhere else so the rad is open to the cool incoming air

BLOCK OFF all area to the top bottom and sides of the rad, thus the air as it hits the front of the LR HAS to go through the rad, not taking a easier route (which it will if it can) in byopassing the rad and going through a hole / gap

Use a Engine / Radiator cowling, and the viscous fan or engine fan if you can, the cowling again directs air OVER and INTO the engine and makes a vast difference

Holes as I found using smoke canisters means the best place is the backs of the 90s wing about half way up, this allows heat OUT but it does go on and through the engine 1st

Use a lower or the lowest themostat in the engine, this makes the engine cool better, and if you fit additional fans place on the outside of the rad so as to keep the cowling. Also do what you can to reduce underbonnet heat / ie try if you can to wrap the headers in heat tape. Some people also play with the waterpump pulleys so as to up the cooling folow speed and fan speeds, just don't go too fast or you get cavitation

Bigger raditors also make a huge difference, not just size in terms of width vs lebgth etc but the number of cores (depth) and the openness of the cores, I have a TD rad on mine as big WXH as I could get in but it a 5 row and bigger tubed type (increases water capacity too), this is good but the above can reduce the temperatures hugely, so don't just assume Bigger Rad will solve it all. Make sure the antifreeze mix is as high as you can go for max cooling effect, you can also get additives which reduce temp when added to the rads, not snake oil - they do work but are expensive

Ali rads are again another step forward, but as with this post should really be at the end of all of the above, and they are expensive and a bit more delicate than a qulaity well built non ali one

Nige