Lifting Your Vechicle Bonnet To Keep The Engine Cooler

[ThaiLife]

Ive been seeing more and more vehicles just lately with their bonnets slightly lifted up at the rear ( windscreen end ) from what I can make out its some thing to do with keeping the engine running cooler and improving the air conditioner temperatures .

My theory is this , the air flows into the front of the vehicle at its air intake opening , the moving air then passes through the engine cooling radiator and into the engine bay compartment ( now hotter air ) , now with a small opening where the bonnets been lifted the trapped rising hot air can now escape to the out side , thus causing the engine bay temperature to be reduced , normally if theres no bonnet air vent , the hot air flow coming out of the engine radiator would just stay in the vehicles engine bay area ( hot air rises ) making the engine bay area temperature rise . also if there's a quicker path for the air flow to pass through the air conditioner unit cooler and out of the engine bay area through the lifted bonnet air gap , this helps the air conditioner cooling process .

I'm not an engineer and I may be missing a few technical things here , but it does seem to make sense to some how release the hot air temperatures that are in the engine bay area , especially when stuck in traffic .

just a very simple drawing of my theory

So What do you Think ?

:jap:

[hansnl]

If the bonnet is closed the ventilator will push the air out from under the bonnet to the underside of the car.

Driving around with an open bonnet might work, however, there are some issues of safety.

Standing still with your bonnett open and leaving the car alone is quite handy for the thieving guild.

But indeed, the temperature might go down because of the hot air able to escape.

The best way to minimise the temperature under the bonnet is to install a ventilator in the bonnet.

I have seen those ventilators installed on bonnets and on the roof of cars.

[MoonRiverOasis]

Having the rear of the hood cracked open like that will make the A/C worse, not better.

The plenum chamber for the A/C system is always located at the junction of the windscreen and the hood, as that's the place of highest positive pressure on the car. Venting your engine heat here will mean that the air coming into the car is hotter, and more than likely a lower pressure than normal too, due to both physical and thermal aerodynamics.

Also note that your diagram doesn't appear to allow for the fact that the bottom of the engine bay is open. There's more than enough positive pressure in the engine bay (from either forward movement or thermo fan) to force the hot air out through the bottom and/or wheel arches.

Edited July 12, 2011 by MoonRiverOasis

[katabeachbum]

I think it will provide some hot and dirty air to the passenger compartment vent air intake, especially on the pictured Golf MK1, which has a sccop for vent air, but as hood doesnt rest on gasket, vent sucks air from enginecompartment. smelly, warm, dirty

due to the GTI spoiler, i initalilly thought the Golf was a GTi, a classic, but its just a GTi frontspolier

BTW, aircon, intercooler, enginecooler and oilcoolers all become more efficient if sprayed by water. changed airflow is not needed for engine bay, larger liquid coolers may be needed

[kartman]

Doesn't matter if it works or not its for "The Look" 99.9% would not have had a cooling problem origionally , same as older cars no reg plates.

[soundman]

In addition, see plenty of buses motoring around with the rear cover open, and plenty of pickups sitting parked with bonnet open, engine going.

Just a pride/testosterone thing IMO.

[Semper]

It could help an air-cooled car engine in slow city traffic. But they are scarce nowadays.

[transam]

On my Pontiac l had a big problem with carb over heating and fuel purculation. The hood had a rubber seal at the screen end of the hood. Removed it and half solved the prob as the hot air rising was now forced out the gap. But had no A/C.

PS. A hood open will look a bit naff unless you can see a blown V8 shoe horned in .

Edited July 12, 2011 by transam

[thunder30101]

Automotive manufacturers spend millions engineering and testing their designs before production, I dont think you are going to reinvent the wheel anytime soon by leaving your bonne\hood open for more airflow.

[katabeachbum]

It could help an air-cooled car engine in slow city traffic. But they are scarce nowadays.

more efficient with a fan for oil cooler

used to have a 1973 beetle 1303S with a tuned 1900cc, all the others relocated top of engine cover, I installed a fan for oilcooler, so cops could not identify illegal engine so easy

[Semper]

"Ive been seeing more and more vehicles just lately with their bonnets slightly lifted up at the rear"

ThaiLife. Are these vehicles been driven by local people by any chance?

BTW. Nice drawings.

[juehoe]

IMHO I also believe that this is more fashion than a real technical issue.

This way was common for the Abarth-tuned sport cars back some decades ago. It therefore might work for older cars. Todays cars are carefully designed for each climate. I also assume that this fashion will disturb the designed air flow in the engine compartment.

Another of these silly Thai fashions is to "cool" down the engine bey running it several minutes after the stop. This is not necessary if there is no warning sign or the engine temperature is in the red zone.

[MoonRiverOasis]

Another of these silly Thai fashions is to "cool" down the engine bey running it several minutes after the stop. This is not necessary if there is no warning sign or the engine temperature is in the red zone.

Not only not necessary, but potentially damaging.. When stopping you want you engine to retain as mcuh heat as possible, so the upon next start the oil is a lower viscosity and starts lubricating sooner..

The only exeption is heavily modified turbo engines, where it's important to keep pumping oil through the bearings until temps are lowered below levels that would cook stationary oil, but that's a very rare car..

Edited July 12, 2011 by MoonRiverOasis

[monty1412]

Unfortunately it doesn't work that way. Engine layouts are designed such that there is a hoovering affect at any speed such that the heat is dissipated down wards through the engine bay. Changing that dynamic will affect most AC setups. Its like in PI were you see heaps pf new cars driving around at night with lights turned off as the old theory that if you turn your lights off it will help the bulb life.. only 30 year out of date.

Leave the bonnet where it should be its safer and will not disturb the intend airflow and cooling requirements.

[Semper]

IMHO I also believe that this is more fashion than a real technical issue.

This way was common for the Abarth-tuned sport cars back some decades ago. It therefore might work for older cars. Todays cars are carefully designed for each climate. I also assume that this fashion will disturb the designed air flow in the engine compartment.

Another of these silly Thai fashions is to "cool" down the engine bey running it several minutes after the stop. This is not necessary if there is no warning sign or the engine temperature is in the red zone.

This will be ok for the turbo engines. If driven very hard and then just kill the engine, what could happen is that the bearings in the turbo could get overheated. This is the reason to let the engine idle for a while and let the oil flow through the turbo.

Edit. But I guess that modern turbo engines have a system (electrical) that flows oil through the turbo to cool the bearings even if you shut the engine off.

Edited July 12, 2011 by Semper

[Mrjlh]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

[Semper]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

[BuckarooBanzai]

There is something called the venturi effect which amounts to a reduction in pressure where air is moving the fastest. As air enters a moving, and I stress moving, car from below and also through the radiator it has to speed up along the bottom of the vehicle creating a suction effect. If you open the hood you defeat this effect and create turbulence which is very counterproductive. Turbulence slows the movement of air. Write in to the myth-busters if you want proof. Cheers!

[wandrinstar]

A sign of a Nackered Car.. Toyota Hi Luxes with the undersizead rads started it here, the Heads Distorted on early ones.In 7/11 it was a regular sight back then. So i asked them why, thats how i know.

[MoonRiverOasis]

A sign of a Nackered Car.. Toyota Hi Luxes with the undersizead rads started it here, the Heads Distorted on early ones.In 7/11 it was a regular sight back then. So i asked them why, thats how i know.

First image is a current gen Ford Ranger or Mazda BT-50, so should have an overheating problem, thus probably just a deckhead.

Second image is clearly a try-hard wannabe. Plate inidcates it's European so let's go with Dummkopf.

Third image is a current gen Mitsubishi Triton, so no way it has an overheating problem, However that color has never been sold in TH so I'm not sure what language I should use to denegrate them

Edited July 12, 2011 by MoonRiverOasis

[Mrjlh]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

The vents in front of the windshield allow fresh air into the passenger compartment. Opening the hood (you Brits call a bonnet) allows engine fumes to escape upwards and could be sucked into the passenger compartment. On a long drive over time there exists the possibility of carbon monoxide entering the passenger compartment.:jap:

[transam]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

The vents in front of the windshield allow fresh air into the passenger compartment. Opening the hood (you Brits call a bonnet) allows engine fumes to escape upwards and could be sucked into the passenger compartment. On a long drive over time there exists the possibility of carbon monoxide entering the passenger compartment.:jap:

Yep, loved it in the Pontiac with the 110 octane. Better than air fresheners.

[Spoonman]

The reason why the bonnet (hood) is sealed along the plenum line (back edge) is that if you shit an engine coolant and possibly oil is not spewed onto the windscreen, it's all about safety.

[Mrjlh]

The reason why the bonnet (hood) is sealed along the plenum line (back edge) is that if you shit an engine coolant and possibly oil is not spewed onto the windscreen, it's all about safety.

That too!

[transam]

The reason why the bonnet (hood) is sealed along the plenum line (back edge) is that if you shit an engine coolant and possibly oil is not spewed onto the windscreen, it's all about safety.

That too!

Do Hotrodders care about that, eeeeeeeer, NO.

Is the hot dog stand still open is more important.

Edited July 12, 2011 by transam

[Semper]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

The vents in front of the windshield allow fresh air into the passenger compartment. Opening the hood (you Brits call a bonnet) allows engine fumes to escape upwards and could be sucked into the passenger compartment. On a long drive over time there exists the possibility of carbon monoxide entering the passenger compartment.:jap:

There is also the option (in most cars) to alter the incoming air to recirculating.

[WarpSpeed]

Having the rear of the hood cracked open like that will make the A/C worse, not better.

The plenum chamber for the A/C system is always located at the junction of the windscreen and the hood, as that's the place of highest positive pressure on the car. Venting your engine heat here will mean that the air coming into the car is hotter, and more than likely a lower pressure than normal too, due to both physical and thermal aerodynamics.

Also note that your diagram doesn't appear to allow for the fact that the bottom of the engine bay is open. There's more than enough positive pressure in the engine bay (from either forward movement or thermo fan) to force the hot air out through the bottom and/or wheel arches.

Agreed (though I think you mean negative pressure = vacuum to draw the hot air out in regards to the engine bay) and many manufacturers are now installing their ECU's in this area, known as the "cowling", under the overhang of the windshield, out of the weather, but where it can get clean, cool air and be more easily accessible for service. As previously mentioned, the engine compartment is designed with the air flowing underneath the car intended to create a vacuum effect between the traveling car and the pavement as the air flows over the bottom of the car and creates a suction effect that removes the hot air underneath as well.

As mentioned briefly, safety is another factor as hoods these days (for more then 30 years thanks to Ralph Nader) now have a safety catch at the rear mounting points to prevent the hood from being pushed back through the windshield and decapitating the passengers in a collision. This most certainly is bypassed when doing this so the heat removed wouldn't outweigh the loss of that safety feature for me especially since I have no issues at all with overheating or under cooling from a properly operating vehicle cooling system and A/C unit.

I'd also not want to have any extreme amount of rain water entering in this area as this is where most of the critical electronics and fuel system intakes etc. are located in the engine compartment to sheild them from extreme water saturation and could get seriously flooded out in heavy rains..

Edited July 12, 2011 by WarpSpeed

[WarpSpeed]

I think it will provide some hot and dirty air to the passenger compartment vent air intake, especially on the pictured Golf MK1, which has a sccop for vent air, but as hood doesnt rest on gasket, vent sucks air from enginecompartment. smelly, warm, dirty

And potentially life threatening with CO (Carbon Monoxide) entering the passengers compartment.. Which also applies in all cases as that is the primary reason this air space is segregated from the engine bay..

Edited July 12, 2011 by WarpSpeed

[WarpSpeed]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

Correct, on more reflection though now, the safety catches were more necessary back in the day when the windshields were not made of the high impact glass they are currently made of and that makes me wonder if this still applies as critically as it used to.. Not up to crash testing to see how that plays out anytime soon though ..

[WarpSpeed]

What if you were to run into something? Instant guillotine. You might end up head less.

Plus like someone said now you get all the engine fumes being sucked into the engine compartment.

It was designed to be closed for a reason, leave it alone.

The vents in front of the windshield allow fresh air into the passenger compartment. Opening the hood (you Brits call a bonnet) allows engine fumes to escape upwards and could be sucked into the passenger compartment. On a long drive over time there exists the possibility of carbon monoxide entering the passenger compartment.:jap:

Yep, loved it in the Pontiac with the 110 octane. Better than air fresheners.

Accounts for a lot, I'm gaining more and more understanding ..