Bonnet Vent Basics

[carvesdodo]

Bonnet Vent: ..... Lets air out.

1/. So as to increase radiator air intake ...
a). Vent is best when in front area of bonnet ... removes air heated from radiator quickly ... with reduced airflow to the rest of the engine bay allowing a build up of engine created heat.
b). Vents located on the sides of raised bonnets or high on the guards or rear corners of the bonnet will allow heat to flow out ... and be drawn out at speed ... as will;
c). raising the rear of the bonnet ... with consideration to/steps against,induction air effects at higher speeds.

2/. To let heat rise out of the engine bay ... ( vent becomes scoop at speed if placed incorrectly )

3/.. For whatever, other perceived reason a person may think.

Bonnet Scoop ... To force air in. ( forward or rearward facing )

1/. For engine induction (airfilter / turbo intercoolers etc) ... not engine bay cooling in general.

2/. For whatever, other perceived reason a person may think.

[carvesdodo]

What we all think is happening with airflow.

Air in the front past the engine and under the car.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35535&stc=1&d=1253123632


What is actually happening

Air in the front and underneath ... and swirled around coming down into the plenum chamber intake for the interior vents.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35536&stc=1&d=1253123632


Forward mounted vent is generally the best way to increase radiator air intake

This can affect the air temp the fanclutch sensor "sees" and reduce airflow needed to remove engine generated heat.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35537&stc=1&d=1253124171


Rearward mounted vent at slow speeds

Air is drawn in by vehicle movement / fan ... Basic physics allows the hot air to escape up through the vent. Any air escaping through the vent from fan pressure is costing airflow/cooling to the engine bay/drivetrain.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35539&stc=1&d=1253124325


Rearward mounted vent at high speeds

Downwards air from the windscreen can be forced into the vent (or raised bonnet) giving lower engine bay temps at the cost of lesser air intake at the radiator ... and affecting the temp dependant fanclutch sensor.

High CFM electric fans may alleviate the loss of rad airflow ... compared to the variable operation of a fanclutch or lacklustre performance of smaller CFM fans.

The causes of the engine bay temps havent been resolved ... the cooler temps are only a result of cooler, introduced air .... mixing with the underbonnet air.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35540&stc=1&d=1253124325

[carvesdodo]

"Induction Bonnet" at slow speeds.............

http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35541&stc=1&d=1253124585


"Induction Bonnet" at high speeds.............

The causes of the engine bay temps havent been resolved ... the cooler temps are only a result of cooler, introduced air .... mixing with the underbonnet air.
http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=35542&stc=1&d=1253124585


Raised Bonnets ...............

Raised bonnets work the same as an induction bonnet ....

Hot rising air will escape at the rear ...
... and at speed ( 30kph in the case of the vehicle tested ) outside air begins to enter the engine bay.

Placing a barrier, centrally at the rear, under the raised bonnet to minimise/stop induction air at speed ... still allows engine bay air to escape from the rear corners at both low AND high speeds.

For an induction bonnet to be of any real benefit in providing useful airflow to the transmission tunnel ... it would require ducting or deflector plates to send the air in that direction rather than just wafting around the engine bay.



Pictures are drawn by myself based on past experiences and some research ... If anyone has some fact based data / research proving the drawings wrong ... let me know ...

Everybody can chop holes in their bonnets if they want ... I really dont care .... but at least be aware of what ALL the results can be ... ;)


Some of the available internet information:

http://office.autospeed.com/cms/A_2159/article.html
http://office.autospeed.com/cms/A_2160/article.html
http://office.autospeed.com/cms/A_2162/article.html
http://www.oman4x4.com/hoodvents.htm
.

[XJKEE]

Carves
You've really been putting in the hard yards on this one

KEEP UP THE OUTSTANDING WORK!

[Ben]

http://www.oman4x4.com/hoodvents.htm
.

I remember way back when the guy on Jeepsunlimited originally posted that up, he spent ages measuring the temp characteristics, etc. Good to see it's still around.

[Mur]

listen to carvesdodo, he knows what he is talking about:D :D :D

[bruggz351]

hey carvesdodo, not being a smrtar5e, what would you suggest? i didnt read the links but going by the pics, it looks like a rearward facing vent would be the shot?? if fitted, do you know, does a rearward facing bonnet vent rob airflow to the plenum intake for interior vents??

i havent had much thought on this subject as i seem to be on top of the overheating thing at the moment but, i havent done any slow or laboured driving as yet:) eg. beach or towing.

cheers

[Simsy85]

Carves had gone off his pills again...

But seriously good post.. what do you think about the AEV bonnets?

[carvesdodo]

hey carvesdodo, not being a smrtar5e, what would you suggest? i didnt read the links


Suggestions is all I can make ... as everybodies vehicles will be different ... as will their use of the vehicle ... Its why I labeled the thread "basics".

Vent placement / usefullness is totally dependant on the fitting location ... Its why I posted the links - so the variations could be seen ... Unfortunately its not as simple a matter as choosing new seat covers. :(

My thoughts on why there is a heat build up under the XJ bonnet is this:

1/. Clutch fan doesnt move enough air to push the heat pockets out.
2/. Electric fan doesnt come on until temps are too high.
3/. The bonnet is too close to the engine.
4/. The vehicle is designed to meet emission burn off temps ... not safe and practical running temps. (coolant or air)


To lower the under bonnet temps I have done this:

1/. Have the electric fan running almost constantly - creating more airflow.
(currently fitting a spare ambient temp sensor under the bonnet to provide some temp figures)
2/. Removed the bonnet padding (presuming that would create a little more space and allow better airflow) ... For the odd, frosty winter mornings ... I just chuck a blanket over the bonnet, the night before.(vehicle brand regardless)
3/. Dumped the black plastic lunchbox sitting over the throttle body.

To further lower the under bonnet temps I am doing this:

1/. Heavy Duty fan-clutch to increase airflow (now I know they are available)
2/. Keeping the electric fan over-ride.
3/. Constantly running, small Extractor Fan at rear of engine bay (temp testing needed to prove any value)
4/. If the cheap fan test doesnt work ... ducting and guard vents will be experimented with ... maybe connected to the extractor fan. edit:... It worked ... dropped temperatures somewhat .... untill it melted ... :lol::lol: ... so back to raising the bonnet as usual for summer.

5/. If the result still isnt good enough ... I will rob a bank and get a cowl bonnet sent over and block the holes for daily driving / highway use.

Item 1 should increase cooling capacity of radiator somewhat - which may affect my coolant temps ... meaning I will probably have to swap the 180' t'stat back to a 195' t'stat ... maybe ;)

Item 5 aint likely to happen ... ;)

i didnt read the links but going by the pics, it looks like a rearward facing vent would be the shot??

Only if it doesnt cause any effects at normal driving speeds. XJ vents mounted in the "popular" location can create airflow variations after certain speeds are attained ... If your only ever going to drive at slow speeds ... They shouldnt affect the vehicle airflow. Have a look at how many blue arrows are drawn in as examples ... on each picture.

How much airflow interference occurs at the higher speeds also depends on the fan setup used in an individual vehicle i.e. CFM moved and for how long ... and the vent opening size. e.g. fans and vehicle speed pulling enough air through radiator ... to count-act the downward air movement from the windscreen.

Same thing occurs with spacing the bonnet hinges ... too big a spacer and the radiator airflow is losing the battle against the windscreen airflow - at speed. Having tried spacers on toyotas and the XJ in the past ... I would consider them an equivalent ... if not better (and cheaper) than bonnet vents ... for slow speed driving.

Note ... Fitting the vent at the front of the bonnet should improve radiator cooling and blow out some hot air ... but because so much of the small amount of "normal" XJ airflow would escape at that point ... I suspect the rear of the engine bay would be worse off than what it is already ... requiring more mods and alterations.


if fitted, do you know, does a rearward facing bonnet vent rob airflow to the plenum intake for interior vents??

At a guess ... No .... the plenum chamber should be getting the central force of the windscreen downward air pressure. Remember too - the plenum chamber entry is over the wiper motor ... on the drivers side ... The passenger side is blocked.

And then of course ...
there is the effects of front bar changes ... and
changes in wind drag / airflow because of lift kits ... etc etc ...

All much fun ... **)


Carves had gone off his pills again...

But seriously good post.. what do you think about the AEV bonnets?

AEV should have some data / testing to show what the vent is designed to do - and whether it does ... Otherwise they are just guessing like the rest of us. ;)

listen to carvesdodo, he knows what he is talking about:D :D :D

As agreed ... $5000 ... AJOR Casino dollars is on its way .... :lol: :lol: :lol:

[itchy]

Very interesting read Carves, after i just went out and cut my bonnet up?

[mac1340]

G'Day some very interesting reading.But still no real answer to the problem.i own a 2006 Wrangler .That runs at 100deg no matter what.I would like to cool the under bonnet temp down more than the motor.mainly to make things under there last longer & not burn the paint off the bonnet.Obvieously the reason the motor runs so hot is to meet emission standards.

[bruggz351]

Suggestions is all I can make ... as everybodies vehicles will be different ... as will their use of the vehicle ... Its why I labeled the thread "basics".

Vent placement / usefullness is totally dependant on the fitting location ... Its why I posted the links - so the variations could be seen ... Unfortunately its not as simple a matter as choosing new seat covers. :(

My thoughts on why there is a heat build up under the XJ bonnet is this:

1/. Clutch fan doesnt move enough air to push the heat pockets out.
2/. Electric fan doesnt come on until temps are too high.
3/. The bonnet is too close to the engine.
4/. The vehicle is designed to meet emission burn off temps ... not safe and practical running temps. (coolant or air)


To lower the under bonnet temps I have done this:

1/. Have the electric fan running almost constantly - creating more airflow.
(currently fitting a spare ambient temp sensor under the bonnet to provide some temp figures)
2/. Removed the bonnet padding (presuming that would create a little more space and allow better airflow) ... For the odd, frosty winter mornings ... I just chuck a blanket over the bonnet, the night before.(vehicle brand regardless)
3/. Dumped the black plastic lunchbox sitting over the throttle body.

To further lower the under bonnet temps I am doing this:

1/. Heavy Duty fan-clutch to increase airflow (now I know they are available)
2/. Keeping the electric fan over-ride.
3/. Constantly running, small Extractor Fan at rear of engine bay (temp testing needed to prove any value)
4/. If the cheap fan test doesnt work ... ducting and guard vents will be experimented with ... maybe connected to the extractor fan.
5/. If the result still isnt good enough ... I will rob a bank and get a cowl bonnet sent over and block the holes for daily driving / highway use.

Item 1 should increase cooling capacity of radiator somewhat - which may affect my coolant temps ... meaning I will probably have to swap the 180' t'stat back to a 195' t'stat ... maybe ;)

Item 5 aint likely to happen ... ;)



Only if it doesnt cause any effects at normal driving speeds. XJ vents mounted in the "popular" location can create airflow variations after certain speeds are attained ... If your only ever going to drive at slow speeds ... They shouldnt affect the vehicle airflow. Have a look at how many blue arrows are drawn in as examples ... on each picture.

How much airflow interference occurs at the higher speeds also depends on the fan setup used in an individual vehicle i.e. CFM moved and for how long ... and the vent opening size. e.g. fans and vehicle speed pulling enough air through radiator ... to count-act the downward air movement from the windscreen.

Same thing occurs with spacing the bonnet hinges ... too big a spacer and the radiator airflow is losing the battle against the windscreen airflow - at speed. Having tried spacers on toyotas and the XJ in the past ... I would consider them an equivalent ... if not better (and cheaper) than bonnet vents ... for slow speed driving.

Note ... Fitting the vent at the front of the bonnet should improve radiator cooling and blow out some hot air ... but because so much of the small amount of "normal" XJ airflow would escape at that point ... I suspect the rear of the engine bay would be worse off than what it is already ... requiring more mods and alterations.




At a guess ... No .... the plenum chamber should be getting the central force of the windscreen downward air pressure. Remember too - the plenum chamber entry is over the wiper motor ... on the drivers side ... The passenger side is blocked.

And then of course ...
there is the effects of front bar changes ... and
changes in wind drag / airflow because of lift kits ... etc etc ...

All much fun ... **)




AEV should have some data / testing to show what the vent is designed to do - and whether it does ... Otherwise they are just guessing like the rest of us. ;)



As agreed ... $5000 ... AJOR Casino dollars is on its way .... :lol: :lol: :lol:


Thanks for the very informative answers Carvesdodo. lots of food for thought.

By the way, item 5....... need a driver????:rolleyes:

[turbomart]

I would think that the main problem would be working hard at low speed ,therefore the rear facing rear vents would be most effective.
Interesting to note ZJs have no such issues but run same engine & fans, so its probably not fans.

[carvesdodo]

I would think that the main problem would be working hard at low speed ,therefore the rear facing rear vents would be most effective.
Interesting to note ZJs have no such issues but run same engine & fans, so its probably not fans.

There is no doubt that the vents offer heat a point of escape at low speeds ... The point of the thread is to highlight the repercussions at other than low speeds or if they are incorrectly positioned.

ZJ has the same clutch fan as the TJ according to US forums.

Bigger blades, bigger radiator ... and bigger fan-clutch than an XJ. Didnt help with the TJ I had tho ... ;)

Still waiting on some local confirmation as to whether the TJ/ZJ fan-clutch does interchange with the XJ fan-clutch.

[Jeeps]

I really want to reduce the engine bay temp. When my TJ is cold and i start the engine, i can run my engine bay mounted ARB compressor for ages and do dozens of tyres. However after the TJ's engine been running for say 30 minutes i only get a max of 3-4 tyres before the ARB's thermal cutout switch kicks in because the ARB compressor was obviously stinking hot before it started up.

I did the washers in the bonnet hinges trick a while back but didn't notice much difference.

Maybe i'll get a panel shop to fit a vent similar to the AEV hood ones...

http://www.jeepkonection.com.au/prodimages/AEV_HEAT_REDUCTION_HOOD_TJ_2.jpg


cheers

[carvesdodo]

This post is a "work in progress" .... and will be added to / updated, etc. as time & interest allows.

Supply of others temps and the weather/veh details welcomed ... to include as comparisons ... especially stock XJs


OK ... been watching things for a while now .... instead of getting more important things done :roll:

Soooo ... Time for some comparisons. ... Its all a bit basic but only intended as a guide so should be ample.

Temps are averages written as:
a).. ambient daytime temp / coolant temp / underbonnet temp
b).. The underbonnet temp is NOT the inlet manifold temp provided by the computer ... that temp is way different in most all cases.

Use ya noggins - and pay particular attention to the ambient temps before getting excited one way or the other, about the coolant & underbonnet temps. ;)

Lots of fluctuations on the spreadsheet but you would have to have been in the passenger seat, looking at the driving terrain, to understand it all. Ask away tho if something seems confusing or you want more info .... ;)

Under bonnet sensor located here .................................................. . Opening created with 1/2" spacers
The terminal thingy on the end of the grey wire.

http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=39393&d=1270801570 ................................ http://www.ausjeepoffroad.com/forum/attachment.php?attachmentid=40262&stc=1&d=1274556005


The bit of foam padding ziptied to the bonnet release cable .... is because I dont want "imagined" induction bonnet "benefits".

With a good condition fanclutch ... air is pushed out the rear corners at "crawlin" speeds .... aux leccie fan on helps a bit more with dvrs side airflow/venting but didnt make a lot of difference to the actual temps ... and at higher speeds the vehicle "slipstream" effect assistes further by drawing underbonnet air out..

Bigger spacer may help with further lowering of temps at slow speeds ... but the small gap is my preference as it allows "excess" heat to be cleared from the corner "collection" areas ... whilst still maintaining a main airflow down the gearbox tunnel as required.


Average Temps - "Stock"

OK ... take one completely stock XJ - apart from the extractors ... Wasn't going through manifold changes again just for you lot ... The thermostat and bonnet padding were bad enough .... :p :D

12yr old vehicle and stock cooling system in what appeared to be - good nick ... towed a loaded car trailer 800klm through Shytney traffic & over the range with it on a previous trip ... and it got max 118'c temps on the hills ... so I guess it was as "normal" as any ;)

Averages:
0-25kph incl 4x4 ................ 25-75kph .................. 75-100kph
19'c / 98'c / 49'c ............... 19'c / 96'c / 25'c ........ 19'c / 95'c / 30.5'c

Test period min & max
Ambient ............... 15'c - 23'c
Coolant ................ 92'c - 106'c
Under Bonnet ........ 21'c - 75'c
High U/B temps occurred when stopped or at speed under load.

Above vehicle temps are unladen vehicle ... 2 people / esky / 1 wks worth of clothes ......... and the usual 2 x milk crates full of tools and jeep bits ... :D
-----------------------


Average Temps - With some changes

A/. New Adrad radiator / New "stock" Waterpump / New "stock" Fanclutch / Hoses / 195'f Tridon thermostat & new housing / Flushed, and Tectaloy 60 cordial premix.
B/. 1/2" bonnet spacers and induction barrier / Bypassed radiator, AW4 gearbox heat exchanger / 1994 TB airtube fitted.

Averages:
0-25kph incl 4x4 ................ 25-75kph .................. 75-100kph
39'c / 97'c / 70'c ............... 39'c / 95'c / 44'c ........ 39'c / 95'c / 42'c

Test period min & max
Ambient ............... 36'c - 43'c
Coolant ................ 94'c - 101'c
Under Bonnet ........ 38'c - 75'c
1/..High U/B temps only occurred when stopped.
2/..The 101'c coolant temp was reached after the vehicle was left to idle for 30min in after starting up on completion of cooling system reassembly ...
3/..Whilst driving, temps were about 20'c maximum over ambient at 0-25 and comparable with ambient at higher speeds.


Mudgee/Hargraves/Hill End

Averages:
0-25kph incl 4x4 ................ 25-75kph .................. 75-100kph
22'c / 00'c / 00'c ............... 22'c / 92'c / 27'c ........ 22'c / 91'c / 24'c ....

Test period min & max
Ambient ............... 21'c - 23'c
Coolant ................ 90'c - 95'c
Under Bonnet ........ 24'c - 30'c ....


Sydney

Averages:
0-25kph incl 4x4 ................ 25-75kph .................. 75-100kph
29'c / 00'c / 00'c ............... 29'c / 92'c / 33'c ........ 29'c / 00'c / 00'c ....
Underbonnet sensor relocated to behind radiator.

Above vehicle temps are unladen vehicle ... 2 people / esky / 1 wks worth of clothes ......... and the usual 2 x milk crates full of tools and jeep bits ... :D
-----------------------

And just remember when searching for comparison temps ....

Comments like .... "The temps have dropped heaps" or " Temps are 50' lower now" ... are useless without knowing what they were originally ... or what the ambient is ... or even how the vehicle is setup ;)

[carvesdodo]

Oooooooo but i am excited :p !!!. Now that i know what you have been filling your time with, lol, you truly are a cooling system slurry ;).

You're way too easily excited mashed ... :p


1/. So .... what would be your overall opinion of coolant temps from stock to "re-coed" ?.
2/. Whats your conclusion on the info at hand thus far OR dose the quest for knowledge continue ?.

hmmmm ... probably be a good reason for another thread ;)

1/. Short and sweet ... better

Stock - its a barely adequate cooling system, made to run at a high temp ... that cant be trusted to maintain temps without the cooling components being in peak operating condition... Typical of imports but acceptable if you acknowledged the design and drove it accordingly.

Wonder what the factory option copper/brass Saudi market XJ radiators were like ???

2/. hmmm ... Its amusing looking at the spreadsheet and then reading the temp drops, vents and cowl bonnets are supposed to provide.

... but I havent finished playing yet ;)

[bonza]

I would have thought that bonnet vents are only usefull when stationary or near still to let heat escape. once moving hot air should be forced out below the engine and along the transmission tunnel as passing air is also required to remove heat from the gearbox and transfer as well

I recall years ago when a certain vehicle had gearbox failures. the same box was used in another model vehile without any problems. the one with the failures had a very narrow tunnel and not enough air was passing over it to transfer heat away from the gearbox. the solution and fix was to use a heaveier grade oil in the transmission and it worked.

I once considered bonnet vents on my TJ, but dont like the look of the add on types. was considering haveing them pressed in so they look something like the old FJ40 Landcruisers along the bonnet flank curves but it came to no avail

[Gravel]

Great read Carves,

But I'm sure somewhere in the ADR's there is a clause that makes rear facing vents now illegal in Aus... Their reasoning was that if; the radiator or something else blew (engine) under the bonnet that all or any of the gunk that would follow would not be sprayed directly on to the windscreen causing as much of a visibility problem and the likely hood of heat causing the windscreen to crack or shatter...

But hey... I may be wrong.

This is probably the most likely reason that AEV hoods (the Hummer) had the vents facing forwards... The old FJ vents were most likely placed on the curved side of the bonnet which would force any hot fluids etc down the sides of the truck and minimise impact to the windscreen.

[carvesdodo]

Great read Carves,

But I'm sure somewhere in the ADR's there is a clause that makes rear facing vents now illegal in Aus... Their reasoning was that if; the radiator or something else blew (engine) under the bonnet that all or any of the gunk that would follow would not be sprayed directly on to the windscreen causing as much of a visibility problem and the likely hood of heat causing the windscreen to crack or shatter...

But hey... I may be wrong.

This is probably the most likely reason that AEV hoods (the Hummer) had the vents facing forwards... The old FJ vents were most likely placed on the curved side of the bonnet which would force any hot fluids etc down the sides of the truck and minimise impact to the windscreen.

Hey - we can all be "wrong" ;) ......... :D

ADRs .... pfffft .... bunch of end of the world / doomsday rules applicable to "what if" and nanny state scenarios ...... applied to everyday driving circumstances ... Where for the most part they are irrelevant.

But the rules have to be applied to the biggest loose nut behind the wheel ... and we all suffer for it :rolleyes:

ADRs and scoops/vents .... been rules around for a while ... never took a lot of notice of them.

Rearward facing ones throwing goop on the windscreen ??? yeah ... just goes to show that even the "experts" dont know much ... It will happen but is veh speed dependant & goop movement pressure dependant in regard to the actual result ... All to do with that downward moving air that fills your plenum chamber for you.

Proper induction bonnets will let virtually nothing escape the engine bay as the bonnets/vent bottoms are designed/shaped to fit around an air intake ... and goop should ... end up in your inlet manifold in that case.

Big, unmeshed, forward facing ones are dangerous because I think ... they may suck up & swallow little babies crossing the street in front of your vehicle ... :rolleyes: ... and some other little bits & pieces ;)

Had goop on my windscreen one night ... Old V8 1 tonner ... 12hrs into a 16hr drive and forgot to put the oil filler cap on properly ...... :oops:

The oil came up out of the tappet cover filler hole .... flowed forward along the bottom of the bonnet, exited out of nosecone/bonnet gap, blew back and smeared the windscreen ... but that was that vehicle .... and dont particularly want to try it in another .... wasnt liters of it but enough to cause a vision problem ... ;)

Your heat / cracked windscreen comment is a good one ... bit like the boiling kettle forcing steam on a kitchen window on a frosty morning ... a random occurence but entirely possible ... On a moving vehicle - that plenum chamber filling air becomes a factor again tho.

[Gravel]

Hey - we can all be "wrong" ;) ......... :D

ADRs .... pfffft .... bunch of end of the world / doomsday rules applicable to "what if" and nanny state scenarios ...... applied to everyday driving circumstances ... Where for the most part they are irrelevant.

But the rules have to be applied to the biggest loose nut behind the wheel ... and we all suffer for it :rolleyes:


No arguments from me... I agree with what you said but the powers that be; that come up with all this gobbily gook; unfortunately pay far too much attention to bean counters than real life engineering... :hammer:

Consequently if you go hunting in all the normal places for bonnet vents you will notice they all open forwards...

[carvesdodo]

Bit more on why its important to carefully consider changes to bonnet setups

Link--> http://www.ausjeepoffroad.com/forum/showpost.php?p=1128399&postcount=40
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