Bonnet Vent Basics

[Simsy85]

Carves had gone off his pills again...

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

[carvesdodo]

Originally Posted by bruggz351

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 ... ... 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 ...

Originally Posted by bruggz351

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.

Originally Posted by bruggz351

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 ...

Originally Posted by Simsy85

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.

Originally Posted by Mur

listen to carvesdodo, he knows what he is talking about

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

[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]

Originally Posted by carvesdodo

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 ....

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

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

[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]

Originally Posted by 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.

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.