debate starter-bonnet vents
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  1. #1
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    Default debate starter-bonnet vents

    Cant get a defined answer on this one...half the world says that to get hot air out from under a bonnet the vents should face rearwards [common sense ]
    and should be situated at the REAR of the bonnet.
    The other half say they should be nearer to the FRONT of the bonnet to rid hot air [ evo are a good example they say ]
    Who is right?...anyone want to thro their 2c worth in.

    cheers jr

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    going back to summernats days in canberra just about every hot V8 there has spacers under the rear of the bonnet lifting if up a bit to rid hot air from the engine bay

    this would mean that air entered via the grille and pushed the air out through the raised bonnet

    whether it worked or not i have no idea but everyone seemd to do it
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    ...apparently, one of the esiest ways to went hot air (is to stir the cyclists on AF...kidding ok)..is to remove the weather strip at windscreen end of bonnet....
    But really,...it shouldn t matter where the went is if you r using a negative pressure style scoop (ie facing back)....but common sense would dictate that the went/scoop should be towards the back...ie air flows through grill/hot air rises therefore backward facing scoop at back of bonnet would make sense??? I like the street race look of spacers at back of bonnet...although it needs to be done with finnese so the gaps arent stupid and out of alignement...in most cases all that is required is a few mm....multiply by width of bonnet an you will usually end up same breathing area as a scoop...


    cheers


    dino
    Last edited by dino; 4th August 2004 at 02:09 AM.

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    There are two main types of vents. There are the ones you're talking about (have a look at an E-Type Jag for the most classic version). These are meant to relieve the pressure that'd normally build up at the bulkhead and then would have to pass out the bottom of the engine bay...plus the air flowing over the vents on the outside creates a low pressure area for a sucking effect. The other type that face forward usually let air in for a reason, such as to go through a radiator, intercooler, or feed the induction system. Not always, but usually. And there are others that do nothing...

    Stuey

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    .....
    Last edited by winnie; 4th August 2004 at 06:40 PM.

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    I need to get some heat out from under my bonnet too. I plan to raise the rear a few mm, with the combination of a NACA duct on top of the headlight. It's not a Froggy, but see the pic below.

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    Remove the bonnet ?

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    Default Vents explained

    Have a look at www.autospeed.com
    Thet have a series of articles in thier technical section about Undertrays, Spoiler & Bonnet Vents. Interesting read.
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    so according to the three articles on autospeed (as posted above) raising the rear of the bonnet is actually detrimental to under bonnet airflow due to the high pressure build up at the windscreen.

    When they mention the addition of the undertray provided an 85% increase in flow through the front radiator ducts, where does this air then exit? i assumed modern cars were designed to expell the hot air underneath the car (which is bad) - then where else can it go?

    -Q.

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    The Autospeed article was the thing that got me going on this and hasnt really helped. It has muddied the waters. The 206 has the vent at the rear for a purpose...Hi-performance cars normally have them in the first 30% ??
    This vent question is more intricate than one thinks especially taking into account hi/low pressures etc.
    Any good reading about it somewhere ? cheers jr
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    When building the Dauphine I moved the radiator up front and needed to vent the hot air out the bonnet.
    So I applied a bit of common sense. Obviously you need to vent at a low pressure area, a lot of cars have their interior air intakes at the base of the windscreen so I figured this must be a high pressure area (generally). So I placed my vents about half way.
    I tested my theory by tieing lengths of wool to the vents and going for a drive. Sure enough there was a good stream of air coming up out of the vents at about a 45 degree angle for about 40-50 cm then backwards.

    Ross

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    Quote Originally Posted by jr20516v
    The Autospeed article was the thing that got me going on this and hasnt really helped. It has muddied the waters. The 206 has the vent at the rear for a purpose...Hi-performance cars normally have them in the first 30% ??
    This vent question is more intricate than one thinks especially taking into account hi/low pressures etc.
    Any good reading about it somewhere ? cheers jr

    There is plenty of good technical reading on this topic, but you won’t find a definitive answer in the ‘comics’. The optimum position for a vent will very from vehicle to vehicle (their shape plays a big part), and the position to fully utilise the pressure distribution may be in conflict with what is going on under the bonnet. ie the vent may want to be in one position and the greatest source of under bonnet heat could be some distance from there.
    If under bonnet heat is a problem I would also consider a small electric fan with a thremo switch to assist which ever vent you decide on.
    Vents near the windscreen are in a high pressure vone and used by manufactures as inlets. Renault Clio Super 1600 has it’s cold air intake ducted to this location.

    Which car in particular are you talking about??
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    does annone know where i can get an air scoop from an r12G or an r5gt turbo, i think they are similar, or one of similar size and shape off another make/model?

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    ..... The 206 has the vent at the rear for a purpose

    Isnt that vent used to duct fresh air in to the air con system??


    i like the removal of bonnet idea.. problem solved..

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    Quote Originally Posted by bowie
    Isnt that vent used to duct fresh air in to the air con system??


    i like the removal of bonnet idea.. problem solved..
    Yes, the 206 is there for venting fresh air into the cabin. Although, I looked and looked at that duct and nder the bonnet the other day and could not see that vent leading anywhere but a dead end.... I must be blind.

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    If you look closely down that little area where the bonnet scoop vents to you'll see there is a pollen filter that sits parallel to the ground. But strangely there is a big area below it where it looks like the air would get ducted to. Maybe they put this in to create turbulence?
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    either way the only performance it will increase is the efficency of the Air Conditoner

    and fair enough..

    Bowie... who still realy loves the no bonnet idea..

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    I think the vent in the centre of an E type jag bonnet would actually push air into the engine bay, possible to the carbys. the hot air exit vents are mounted about 2/3 back from the front on either side of the power bulge and facing backwards. They are fine louvres.

    If you look at old photos of racing cars from the 50s and 60s and a lot of british sports cars they often had louvres about halfway along the bonnet and facing backwards.

    The base of the windscreen is definately a high pressure area and if you look ad cars like the A9X Torana and the VK group A Commodore and other cars like them, you will often find the rearward facing bonnet scoop directing high pressure air to the air filter ( unless it is a dummy scoop put there my some w*nker who thinks it looks cool )

    The rear facing scoop seems counter-intuitive but it does work at pushing high pressure air forwards into the airbox.
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    Quote Originally Posted by Pugnut403
    I think the vent in the centre of an E type jag bonnet would actually push air into the engine bay, possible to the carbys. the hot air exit vents are mounted about 2/3 back from the front on either side of the power bulge and facing backwards. They are fine louvres.
    I've recently finished helping a relative restore a '68 E-type. There isn't a vent in the centre, unless you mean the one at the very back of the bulge, which you can only see from the back of the bonnet. The louvres are at the back of the bonnet, about 10-15 cm from the back edge (and consequently, just above the bulkhead). They certainly let heat out - I think they were added during development to aid cooling.

    The air filter is actually low on the RHS near the bulkhead and is fed air from the grille.

    Stuey

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    Winnie, with respect m8...it doesnt really matter which car i am asking about as surely the science/basics will be the same.
    So in short again i say...can someone point me to info which explains where the best place on a bonnet is to VENT out hot air?

    cheers jr
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    I still say it's at the rear (say the last 1/4) of the bonnet. I agree that cars have their cabin vents at the base of the windscreen because of a slight high pressure, but this usually only works properly (without a fan) where it's close enough to the windscreen to benefit from the air banking up on itself slightly, as it changes direction, and causing a slight high pressure region. 30-40cm further forward from this (depending on where the rear of the bonnet and the bulkhead are) would be the prime spot.

    Stuey

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    Quote Originally Posted by jr20516v
    Winnie, with respect m8...it doesnt really matter which car i am asking about as surely the science/basics will be the same.
    So in short again i say...can someone point me to info which explains where the best place on a bonnet is to VENT out hot air?

    cheers jr
    With reducing respect, behind the main radius of the bonnet (where it stats to become flat), re RSC super 1600, Xsara rally car in last months Racecar Engineering and a truck load of others. The main cause of under bonnet heat would be the radiator (it's job after all is to reject waste heat) having the vents not too far from the radiator, possible with some simple ducting to encourage the air flow to the vents, or even small fans re the Xsara mentioned earlier.
    Last edited by winnie; 12th August 2004 at 09:31 AM.
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    Yep negative pressure vents are the way to go for relieving heat from an engine bay. I have a lot of turbo mates who do a fair bit of track work and drift and that sort of thing and constant high revving builds heaps of heat underneath the bonnet as you could imagine. Another trick they use is the crank the heater, but when racing who wants to open a window for air..Drag uleh drag.

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    The vents on the bonnet of the Xsara WRC are actually there for a reason other than aiding the cooling through the radiator. Behind the main grille is a kevlar curved vane the leads straight out of the bonnet and the only thing in there is the intercooler raked at a 45degree angle.

    BUT Citroen would still be placed them in a logical / technical specific place. So if only i could find some replica's......

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    First of all, that gaping hole under the bonnet is a low pressure area and most hot air gets sucked out under the car when on the move.

    When stationary (or at low speed) I imagine a vent anywhere on the bonnet is going to let hot air out, and by the time the car is travelling fast enough to produce a significant high pressure area at the base of the windscreen (not really a big factor on alot of todays cars) , most hot air would be getting sucked out under the car.

    If you really want your vents in the right spot,(as I think Stuey said) I would tape down about 50 tufts of string (of contrasting colour to your car) evenly across the bonnet (1 tuft per 15 cm sqaure), and get a mate drive infront of you while his passenger videos the front of your car as you cruise down the highway at 100 kph.

    In low pressure areas the string will look quite turbulent and curly, and in areas with good laminar air flow, the string will be dead straight.

    I would put a reverse facing vent in an area of the bonnet that has good laminar air flow so that the it produces a vacuum and sucks air out from the underbonnet low pressure area.

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