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?

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.

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.

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.

Pete.

Here's an example:

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

Same car, better view of the idea:

https://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.

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!

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.

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)
https://www.fuelly.com/attachments/fo...767f78dbd7.jpg
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)
https://www.fuelly.com/attachments/fo...e29662a0cc.jpg
however a special 4x4 ralley version had the sawtooth vents again along with side extractors
https://www.fuelly.com/attachments/fo...003cb9b600.jpg

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)
https://www.fuelly.com/attachments/fo...51de2d73bf.jpg

Just an idea, but what about doing something like this on your belly-pan:
https://www.hotrodlouvers.com/
On page 2, they even have a pan.

that's what I was thinking, samandw. louvers in the belly pan. louvers on the hood would require repainting :(

For now I'm revising my bellypans. Gonna go back to just a front and rear pan.

Mine are made fom ABS plastic and follow the suspension and undercariage grimaces to a gnat's butt. Needless to say it was a lot of work, but who knew?

Something to consider for anyone engineering a pan. Where's all that air gonna go?

I keep a thermocouple attached under my hood (on the firewall, just behind the motor). The other end is always under the dash, so I can hook it up to my meter any time I make a modification.

Last summer the underhood temps on my stock Geo never exceded 150 degrees F. on a 100+ degree day. I blocked the grill in stages, and found that blocking either section of the grill sent temps to over 220 degrees in short order.

After installing the full pan temps went to near 200F at times, and it's winter here!

From a mechanical standpoint, I have concluded that grill blocks are a shortcut. The better way may indeed be to allow all that air to continue to come in, then give it somewhere to go (i.e. vents in the hood or fenders, or at least allowing it to go under the car).

I know some of you may disagree. There seems to be a false notion that heat=MPG.

To some extent this is true. Thinner lubricants offer less friction, hotter air molecules occupy more space, etc. But if that heat has nowhere to go, your dead car will get 0 MPG. To me heat ultimately means shortened life expectency of major components.

Just food for thought. I think we armchair engineers should think ahead, towards building a better overall package.

For example if you want to run a hot air intake. We already established that the principle to "works", to varying degrees. The "right" way to accomplish this may be to isolate your intake air (in something resembling a heater box with an air filter, fed by radiator or exhaust heat), rather than by heating up your entire engine compartment unneccessarily as some are now doing.

Just rambling, mostly cuz I'm avoiding going out there under the car.

My belly pan is vented out the the front suspension/axle area. I also installed 5 4"x10" vents running along under the exhaust system. I tested my pan in Kansas city traffic all last year with good results. I was able to keep water temp under 200f when it counted the most. A manual fan switch would help though.

My secret to keeping under hood temps to near normal is an HAI that picks up air at the exhaust manifold. You might consider taping your exhaust manifold. But the saturn likes the HAI idk about the geo.

Nice work.

If you seal up the front undercarriage on a Metro the only openings left for cooling are the two slots that the tie rods run through. Not much area there.

I just gotta re-engineer the whole thing. The Metro's sills are flush with the floorpan, I need to extend them toward the ground and do something like you've done with your Saturn.

Exhaust manifold wrap ???
 

Another way to decrease underhood temperatures, is to wrap exhaust manifold with hi-quality wrap like racing cars have. Second way is to make coolant more efficient by changing ratio to 70% water/30% antifreeze + water wetter.

Mustanger, have you seen the mercedes e class underbody windtunnel pics. I think it was autospeed that posted them. Anyway Merc uses to portals about 8" in diameter just below the firewall area. You might consider looking into that style of underhood exhaust.

I was thinking about this a while back... it occurred to me that my mirror housings have about a 1/4 inch gap all round the glass (allowing clearance for adjustment I guess) and thus a 5x3 mirror has about 4 square inches around it. This is a low pressure area also. So began wondering if I could pipe engine bay heat into the mirror housings. This would provide some airflow bleed... and would defrost the mirrors in winter! (and maybe keep some rain off them too) There is a potential that with being a low pressure area, the 8 square inches for both sides, would vent much more air than 8 square inches of hood vent, and may negate some proportion of the base drag of the mirror housings....

If installing a belly pan, maybe making provision for air passage under the pan, all the way to the rear of the car would be a good idea, maybe plumbing it to come out of holes high in the bumper molding where pressure will be lowest.

Lol :D

:p

Well I always wondered if there was something a little extra in Timmies when Roll up the Rim was on, but I swear I haven't had one today.