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Old 02-12-2008, 03:32 PM   #1
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Venting underhood pressure

I want to experiment with venting the engine compartment into the airstream over the top or sides of the car. Reason being that my belly pan is generating alarming underhood temps, but I don't wanna give it up.

Since the Geo metro's design doesn't lend itself well to fender vents (strut towers and firewall ahead of upper-rear portion of fender) it looks like vents just aft of the leading edge of the hood are the alternative.

The cons of the hood vents as opposed to fender vents are realized in bad weather, road grime, heat and rainwater directed toward the windshield from the vents, or rain water pouring on the motor when parked. So some sort of baffling is needed beneath them, which will efectively limit their airflow.

Most internet info is related to race cars where they simply cut openings in the hood and cover them with mesh, or with open louvers

Anyone have any ideas on how to get a baffled, reasonably high flow vent or two within the confines of a Geo hood? Ever seen anything in a boneyard or on a production car that you thought would work?
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Old 02-12-2008, 04:06 PM   #2
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engine panel vents

I'd have made small vents to let the air in from underneath. My father's stock mazda 5 has this solution in engine panel.
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Old 02-12-2008, 04:30 PM   #3
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How does the hood mount? You could space out the hinge at the hood and effectively lift the windshield side of the hood. In some cases, this could be done with just some washers and some longer bolts.

Sometimes there's a piece of weather stripping there too on the hood or mounted to the body that you could take off. I don't know if it would help the aerodynamics by introducing that stream through there, but it might allow an escape for the hot air.
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Old 02-12-2008, 06:43 PM   #4
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Quote:
Originally Posted by SD26 View Post
How does the hood mount? You could space out the hinge at the hood and effectively lift the windshield side of the hood. In some cases, this could be done with just some washers and some longer bolts.

Sometimes there's a piece of weather stripping there too on the hood or mounted to the body that you could take off. I don't know if it would help the aerodynamics by introducing that stream through there, but it might allow an escape for the hot air.
Not a good plan since the windscreen end of the hood is at high pressure, that would just let more air into the engine compartment. See this autospeed article for the placement: Undertrays, Spoiler & Bonnet Vents, Part 3

The best place on the hood is to the front where there is a low pressure zone that can suck the air out. You can do your own testing fairly simply to work out the highest pressure differential across the hood in order to place the vent fairly easily as described in the article.
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Old 02-12-2008, 06:47 PM   #5
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Ferrari used the type of vents you are talking about in some models from the 1970's but they were built in from the factory and had a radiator tilted about 30 degrees or so to aid cooling when the vehicle was not moving.

Perhaps a few slots in the underbelly pan may help.
Make them across the car and with a small lip located ahead to provide some negative pressure region.

Another alternative is to cut a small vent into the hood just ahead of the windscreen but as close as you can get to the pillar holding the screen.

The airflow around the pillar normally creates a low pressure region so venting into this area should be a help.

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Old 02-12-2008, 06:55 PM   #6
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Here's an example:

http://www.caraudiomag.com/v_feature.../photo_09.html

Same car, better view of the idea:

http://www.caraudiomag.com/v_feature.../photo_10.html



What it does is to use the relatively low pressure air flowing over the mid-front hood surface to extract the higher pressure air accumulated under the hood, allowing it to continue in the airstream over the car. Fender vents accomplish the same thing in the same manner, minus most of the hood vent issues, but aren't practical on my particular car.

Getting the air IN isn't the problem. I'm not familiar with the Mazda setup but I'm guessing it doesn't have a full undertray, and the air exits behind the drivetrain and under the car. If not, enlighten me. I'm open to anything.

Lifting the rear of the hood is a good suggestion experimentally. On some cars this results in high pressure air from the hood/windshield junction being rammed back INTO the engine compartment. At any rate I'm looking for something cleaner and more permanent. I already tried the weatherstripping trick and it made no difference. I may try your suggestion to get an idea of the vent area required.

Aero is beginning to make me less crazy.
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Old 02-12-2008, 07:25 PM   #7
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Jaguar used a vent system on the V12 E type (called the XKE in the US I think) where they fitted rain shields underneath the vents to deflect any water coming while the vehicle was parked.
There was a gap of about three inches or so at allow air out between the shield and the vent in the hood.
The shield was stainless steel and had a shallow V shape pressed in it to form a gutter to direct water away from any thing vital.

A good move considering the quality of standard Lucas electrics of the time!
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Old 02-13-2008, 01:00 PM   #8
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It may help and I suggest trying it just don't lift the hood *too* far as there's usually a hook on the hinge mount to catch it if you get in an accident. otherwise the bolts shear off or tear out and you get a mouthfull of hood.
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Old 02-13-2008, 02:32 PM   #9
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here's some examples i know of on opels

the first are on a ascone 400 with a manta in the back (late 70's to mid 80's)

both had a nice sawtooth pattern with a mesh in it just behind the radiator. these where limitid run 4x4 ralley versions, the standard cars didn't have this

second is the GSI version of my kadett wich had two resesed vents at the side of the hood... looks pretty attractive and actually pretty hard to spot (mid 80's to mid 90's)

however a special 4x4 ralley version had the sawtooth vents again along with side extractors


the last ones are on the astra gsi (britich vauxhall badged version here), the successor of the kadett. this one featured a more bulged design with openings to the side... my least favorite of the 3 actually (90's)
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Old 02-15-2008, 08:36 AM   #10
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Just an idea, but what about doing something like this on your belly-pan:
http://www.hotrodlouvers.com/
On page 2, they even have a pan.
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