Bonnet Vents?

[Brookers]

I'm looking at adding some more ventalation to the bonnet of my RR to draw out some of the heat, I'm running tubular manifolds and the under bonnet temperature is getting a little high so I want to add some means of escape for the hot air!

First thought was if I place them on top of the bonnet it will let hot air rise but if I go in deep water I dont want them letting water straight on to the engine, Would I be better letting vents in to the side of the bonnet?

Thoughts, sugestions and pics please.

Brookers

[The Hatt]

if the water is that deep surely the water would have reached the engine from underneath or from the bonnet shut line?

Do you compete in trials events? Some regulatory bodies don't allow vents on the bonnet for fire risk reasons.

Apart from that a forward facing scoop will push air in and rear facing scoop should let air out. Not really sure how affective they would be though. Might just be worth adding some big electric fans to increase airflow thru the engine bay.

[DEANO3528]

Twin electric Pug 405 fans up front with these in the back of the bonnet, certainly helps keep the temp down

[HoggyN]

Apart from that a forward facing scoop will push air in and rear facing scoop should let air out. Not really sure how affective they would be though

Possibly not very. You have to be careful with bonnet scoops as they don't always do what you expect. The reason being that, with most cars, there is high air pressure at the back of the bonnet and low pressure at the front (which is why heater air intakes are very effective when positioned at the base of the windscreen). A low, forward facing scoop generally gets little, if any, ram effect - and the faster you go the worse it gets.

[The Hatt]

Possibly not very. You have to be careful with bonnet scoops as they don't always do what you expect. The reason being that, with most cars, there is high air pressure at the back of the bonnet and low pressure at the front (which is why heater air intakes are very effective when positioned at the base of the windscreen). A low, forward facing scoop generally gets little, if any, ram effect - and the faster you go the worse it gets.

I agree, I was meaning more like a Scooby style scoop. It needs to be above the boundary layer to be effective unless you plan on using a naca duct.

As for high pressure area, its more at the bottom of the wind screen than the top of the bonnet. The Mustang Cobra uses rear facing heat extracting bonnet vents:

But to use a rear facing vent as an intake source you'd need it much closer the windscreen like the cowl hoods:

[Hybrid_From_Hell]

I'm looking at adding some more ventalation to the bonnet of my RR to draw out some of the heat, I'm running tubular manifolds and the under bonnet temperature is getting a little high so I want to add some means of escape for the hot air!

First thought was if I place them on top of the bonnet it will let hot air rise but if I go in deep water I dont want them letting water straight on to the engine, Would I be better letting vents in to the side of the bonnet?

Thoughts, sugestions and pics please.

Brookers

OK

Not as simple as you might think

Are you talking about when driving (prob not), or (more likley) when slow off roading ?

If the 2nd then some things to consider.............

• You have no "Air flow" other than the fan(s)
  
• If off road the main exit (under the engine area), may well be blocked by earth
  
• The engine will be working hard, and no flow
  
• You have to get the maximum then from the air that is going in, and then get it out
  
• Or Get more air in
  
• And also get more of that out
  

Cutting holes in the bonnet is not simple,

for the application above the holes in the wrong place will actually make things worse - in the wrong place the air will simply come in,

bypass the engine and go straight out. The air going in has to :

• Cool the Radiator / Oil Cooler
  
• Cool the engine as well
  
• Then exit the RR
  

You must only cut holes where the air will have alreadt been over and around the engine, on the 90 I have air vents (see thread) at the backs of the fronts wings each side about 1/2way up, to get here the air has to come arpund the top and sides of the engine before it finds the exit

Other things that will help cooling :

• Fit a lower thermostat I had 88 and 84 D in the V8 a 72 dropped the cooling down hugely :0
  
• Block off any areas around the sides top or bottom of the rad that allows air to miss the rad and enter the engone bay, the more you can block off the better
  
• Ensure you have the engine viscous fan and cowling both make a huge difference
  
• Fit additional fans, the more the merrier, the air will find a way out even with the extra vents
  
• You will see a lot of comp 90 stylee racers with big holes down the back edsge of the sides of the bonnet - NOT good for slow sped work OK for comps
  
• RRs having similar holes in the back edge of the bonnet are very different - its the design / flow / way the air comes through the RR vs 90
  
• 2 similar 4x4s but differing body shapes = similar ........but different results !
  
• Ensure you have the optimum mix of antifreeze
  
• You can get a engine additive coolant dropper for radiators expensive but does help
  

HTH

Nige

[The Hatt]

http://www.demon-tweeks.co.uk/products/Pro...;pcode=RED80204

[Bowie69]

Erm.... that wouldn't make any difference... you still have a finite amount of heat to get out of the engine bay, how fast you get it out the engine makes no difference what so ever.

You still need air going through your radiator and out somewhere else.

[Brookers]

Thanks for the replies, the problem is not the engine temp its just the ambient temp under the bonnet when slow speed off roading. Since I fitted the tubular exhaust the temp under the bonnet has increased a lot (or so it seams to me) and I was wondering if vents in the sides would help some of the heat escape?

Brookers

[Hybrid_From_Hell]

Thanks for the replies, the problem is not the engine temp its just the ambient temp under the bonnet when slow speed off roading. Since I fitted the tubular exhaust the temp under the bonnet has increased a lot (or so it seams to me) and I was wondering if vents in the sides would help some of the heat escape?

Brookers

Forgot to mention you can "Wrap" the headers, just make sure you run the engine so its put away dry as they can rot headers if always left wet

Ran 'wrapped headers' on the hybrid, made a huge difference / also the 4x fans shroud and twin radiator helped

Nige

[HoggyN]

Source: Road Vehicle Aerodynamics, A J Scibor-Rylski

The diagram above gives some idea of where to cut cooling holes (OK it's not a Range Rover but it gives the gist). If you want air to go out make sure the holes are in the area with the arrows pointing away from the car. If you want air to go in... On a Range Rover the boundary layer is of the (very) turbulent variety so it won't make much difference whether the vents point forwards, backwards, sideways or normal to the surface. Vents on the sides of front wings are also likely to be in a low pressure area so will pull hot air out (like on a Mini or a Maxi - through the front wheelarch).

[Roverdrive]

HfH beat me to it.

There is a material available that you can wrap the headers with to insulate them, and help prevent the under bonnet temperature rise. Demon Tweeks do it.

http://www.demon-tweeks.co.uk/products/Pro...;pcode=NIM11151

Good thought about the rusting. Hadn't considered that at all.