Fors and against Bonnet scoops

**rick130** · 20th May 2007, 07:36 AM

Originally Posted by incisor

positive engine bay pressure has some good features, but venting can make a big difference cooling wise. mate had a rangie that he vented and it made a significant difference temerature wise and he never looked back... a defender, i would have tought would leaked air all round the bonnet, why not just adjust your bonnet stops and latching pin up 5mm ? but there is the bling factor :P

Originally Posted by DirtyDawg

Good Idea Dave, No bling for this dawg...just another thing to break as I use my bonnet as an observation post when deciding directions

I got the idea from Sith who has put a nissan one on his RRC as for the facing it will be on back wards as not to push air/water in but let hot air out,,,just a thought at this stage hence the poll

First up, a general comment on letting air out of the underbonnet area.
1. Raising the trailing edge of any vehicles bonnet doesn't let any air out. The base of the windscreen is quite a high pressure region. If you raise the trailing edge air is actually forced back in under this edge and into the engine bay. I've proven this on a number of vehicles over the years with wool tuft tests.
Remember back in the day how the A9X Torana had a rear facing bonnet scoop for the carby ? It was for this reason. Even earlier, HDT used to pack the trailing edge up on the XU1's to feed colder air to the carbies.

2. Without a wind tunnel, the best place to let hot radiator air out is fairly well forward in a place of laminar flow. Look at where the bonnet cut out is on a Lancer Evo VII, VIII, or IX or a Panoz ALMS car. Also look where the small outlets on the outside edge of the bonnet are on the Suby WRX STi WRC cars.

I've actually done wool tuft testing of the airflow over and around the bonnet of the 'fender and it throws up some interesting anomalies.
Not a lot bleeds out from around the edges of the bonnet. It appears the air flow in these regions is too strong from the front, impeding air trying to exit.

I removed the small plastic cover on the drivers wing and the flow here is strange, depending on speed. There was quite a bit of flow reversal and turbulance. Not a good spot to extract air.

The centre of the bonnet, at a point just above the t/stat housing on a 300Tdi had excellent laminar flow at all speeds, and would be my pick to place a cutout or rearward facing scoop.

**George130** · 20th May 2007, 07:59 AM

I voted yes as long as it does help. Guess thats the problem knowing it helps.

**rick130** · 20th May 2007, 08:16 AM

anyway, I say do it, as I reckon the Defender has a problem with airflow through the radiator caused by high pressure air in the engine bay being unable to escape.

I'd use louvres or an Evo style cutout, though.

Also, remember that if you are pulling hot air out of the underbonnet area and you open your vent flaps under the windscreen, it could get a little warmer inside.......

**p38arover** · 20th May 2007, 08:25 AM

Originally Posted by rick130

The base of the windscreen is quite a high pressure region. If you raise the trailing edge air is actually forced back in under this edge and into the engine bay. .

One can see this is how it is done for the Discovery 1 cabin air intake.

Ron

**JDNSW** · 20th May 2007, 09:11 AM

Originally Posted by rick130

anyway, I say do it, as I reckon the Defender has a problem with airflow through the radiator caused by high pressure air in the engine bay being unable to escape.
............

I'd like to know what you base that on - there are wide gaps each side of the engine, even on a V8 leading into a large open area under the body/chassis/gearbox, which because of the shielding effect of the front axle, I would expect to have relatively lower pressure at speed, certainly lower than the pressure of air forced by the ram effect through the radiator. And this under body volume has a large exit area each side, even though the rear axle impedes the exit there.

Certainly the cross section area of the escape path down the sides of the vehicle and out underneath is vastly greater than any possible scoop on top. Of course you could always leave the bonnet off, but that has its drawbacks as well.

There may be a patch of stagnant air in the top rear of the engine compartment, but with the overall airflow through the engine compartment I rather doubt it - that air bleeds down over the bell housing and gearbox and out under the sides of the vehicle - if you don't believe me, feel the floor and transmission tunnel after a run, and no, that is not the gearbox getting hot - if its hot, it is probably from this hot air.

John

**101RRS** · 20th May 2007, 09:28 AM

Anything to improve the looks of a Defender has gotta be a plus

Gazzz

**dobbo** · 20th May 2007, 10:03 AM

In one of the pommie mags this month theres a company that makes scoops similar in look to the bonnet buldge on the new Defender

www.riversideallterrain.co.uk

Thing is I cannot find the scoop on there website.

**p38arover** · 20th May 2007, 10:08 AM

Originally Posted by garrycol

Anything to improve the looks of a Defender has gotta be a plus

And this from a "hippo" driver!

Can you imagine any other manufacturer taking the P out of themselves like this?

Ron

**dickyjoe** · 20th May 2007, 10:20 AM

I guess it depends if it would really help. I would imagine that a bonnet scoop would not help airflow through the radiator. I have seen a lot of cars with scoops in the bonnet to aid cooling but it can actually make it worse creating vortexs in the engine bay and stopping efficient airflow through the radiator. I would be very wary of it. Does your fender actually overheat?

**D110V8D** · 20th May 2007, 10:38 AM

I had a forward facing WRX scoop on the V8 Deisel Defender I used to own.

It didn't do much to help the cooling issues. I ended up cutting holes in the inside wheel arches, 9 each side, with a 52mm hole saw.

This improved it a little. I think what was occuring was a pressure lock in the engine bay, caused by the scoop (facing forward) The holes in the wheel arches helped to relive the pressure I guess.

I would definetly have the scoop facing backwards if I was going to do it again.

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