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Hood vents over hot sections

Hood vents over hot sections

Hood vents over hot sections

With today's trend for low sweeping hood contours and bumper level radiator cooling grilles, I wonder whether we should have hood vents, left and right, to allow convective cooling on shut down. This has to be particularly important for supercharged engines.

The functions I visualize are:
- closed during engine operation;
- open upon shut down;
- thermostatically closed after sufficient cool down;
- rain tolerant.

Any comments?
Replies continue below

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RE: Hood vents over hot sections

To start,in order to get the OEM's to do ANYTHING extra is a tough row.  A demonstrated value must be   present and as of now, even in turbo applications, it would be of dubious value overall. Obviously some applications would benifit but those applications at present appear in the minority.
Bottom line is now, as always, the overall cost/benifit ratio.  Some less costly alternatives would include an underhood electric fan that continues to operate after engine shutdown. This is a fairly common mod for the performance crowd and in my opinion also of dubious value except in the extreme cases. For turbos the aftermarket electric oil  pump is set up to circulate oil through the turbos bearings for a short cooldown period after shutdown. This item SHOULD be included in most turbocharged passenger cars but, again the OEM are reluctant due to the added cost(?).  At least I believe it should be installed by the enthusiast driver.  Beyond that, scoops, louvers, thermostaticlly controlled flaps and such just add yet another cost/failure item to an already overly complicated engine system.

Or we could all move to New England.  


RE: Hood vents over hot sections

A simple gap between the bonnet and the top rear edge of the inner guard would allow hot air to escape. Unfortunatly it also allows sound waves to escape and impact on the windscreen.

Surely there is no added cost in leaving a bit of a gap, if heat soak is more of an issue than noise, as it can be when vapour lock becomes a problem


RE: Hood vents over hot sections

Finally I get to disagree with you Pat. Lifting the back of the bonnet at the base of the windscreen causes one of two things that I have observed on several cars that I tried this on.  Using 'tufts' of yarn taped along the cowl edge as a means of 'seeing' airflow---MOST cars actually flowed into the engine compartment and several just 'fluttered' about and apparantly did not indicate ANY airflow pattern.
Basically I attributed this to the "high" pressure area at the base of the windscreen.  I did this as a result of reading in a Hot Rod magazine about "letting the hot air out" way back in the late 1950's, I think. In later years I used a manometer to check the pressure areas in my Lotus Cortina and found two areas suitable for carb inlet air ducting---base of windscreen and grill/headlight area.
Now, I know that the aero of cars in general has come a ways since 1961 (my new,then, Corvette was my first 'test') and I am open to any new test data that you would like to contribute.  I am not going to try it on the Money Pit Mini because , as you have seen from the photos, the front is a tilt setup but---I doubt that the fairly virticle windscreen would yield data substantially different from that of 1961.  I could be wrong but, I doubt it. Aerodynamics is definately not an intuitive science.  


RE: Hood vents over hot sections

I have installed such a 'vent' in the hood of my car and when tesed with wool tufts I found that air was entering the vent at 25mph or more. It appears that other changes I have made have induced a lower pressure under the car compared to over the hood. At rest a noticable heat haze appears to emerge throught the 'vent'. As far as actuation, why not bimetalic?

RE: Hood vents over hot sections

After reading evelrods post again I thought that I had better add the information that the rear of the vent is approx 45cm from the base of the screen and thus probably not in the stagnation zone/high pressure zone at the base of the screen (plus the screen has a very sizable curvature with the vent placed 20cm from the centre line of the car so perhaps making even less likly to be in such a zone?).
The top of the vent is flush with the panel and has 'vanes' of .5mm 5005 ali at an approximate 30 deg angle to the airstream. The original intention was to 'suck' air out of the engine bay...it does precisely the reverse!...little did I know that the nose treatment was so effective! Still a very worthwhile effect in traffic (I have jumped out of the car in standstill traffic to check!) as the heat coming out of the vent is impressive. The ali is too hot to touch after 5 mins in such traffic. Next is an NACA duct to get more air in there!

RE: Hood vents over hot sections

I think you missunderstood my post. Maybe I was not all that clear.

I meant the gap to be along the side edge of the bonet, away from the stagnation zone.

Also, if the problem is only after shutdown, there will be no pressure buildup at the base of the screen.

I raised the rear edge of the bonet on my sons Holden Statseman, with a 350 SBC fitted, by adjusting the hinges. It made the car noisy, but cured a vapour lock problem, that only really occured at very low speed and idle.


RE: Hood vents over hot sections

I'm puzzled-
The pic labelled "fuel rail" is the A.I.R. manifold.

What is this "vapor lock fan" connected to? The manifold in the other photo?


Jay Maechtlen

RE: Hood vents over hot sections

The Mitsubishi Lancer Evolution has a hood vent on the front of the hood.  It is louvered, but is always open.  (Rain will fall on the exhaust manifold and turbo when the car is parked...)

Here's a picture: http://store1.yimg.com/I/machevo_1747_74019

Mach V Motorsports

RE: Hood vents over hot sections

Is there an efficient "fender vent" design that allows heat to escape the engine compartment comprable to the direct overhead vent? I would like to avoid rain running directly over my turbo.

RE: Hood vents over hot sections

Given the ever decreasing size of engine compartments this is certainly an idea warranting some attention.

Ferrari used a set of vents on the (front engined  / rear drive 365GTC and GTB "Daytona" model) V12's in the 1970's to achieve exactly that result you mentioned.

At speed the air was extracted due to the relatiively low pressure zone just aft of the bonnet (hood) leading edge while at rest the natural convection effect of the still heated radiator matrix allowed air to circulate up through it and out into with no mechanical assistance.
There were sizeable fans fitted to help airflow at low speeds, stop start traffic and the like.

The bonnet (hood) was so shaped that any rain was directed into the space between the engine and the radiator and the electrics were located well away from any errant drops.

Cheers, Pete.

RE: Hood vents over hot sections

The vent plasgears is picturing is already manufactured, they used them in 1998 ZJ's or Jeep grand cherokees equipped with the 5.9L /360CuIn V8. They were thermostacically controlled and operated very well, all you have to do is look at how tightly that 360 is crammed in there and watch the heat come out while off road or after shutdown, you could almost make popcorn over it. Unfortunately cost is around $450 per vent. I am looking for something similar only manually controlled for my off roading when in 4wd, 4Lo and 1st gear crawling over obstacles in my 98XJ 4.0L 5sp Cherokee so that I can keep it closed on road and in inclement weather, so far I have not been able to find a company that makes such a vent, don't want something that looks like it belongs on a house foundation either .

RE: Hood vents over hot sections

The hood louvers on the 98 Jeep Grand Cherokee 5.9L are not thermostacically controlled. They consist of two parts a louver and water shield tray. Here is an article describing them.

January 19, 1998 Design News

Patented part adds more zip to Jeep engine

Detroit--At a quick glance, buyers probably won't spot the difference, but the 1998 Jeep Grand Cherokee's new 5.9l V8 features a plastic component that helps make the vehicle's performance even more impressive. The part: a molded functional hood louver assembly.

The assembly resulted from a coordinated effort between Jeep-Truck Engineering and the Interiors & Molded Products Div. of Cambridge Industries, Inc. (Madison Heights, OH). Consisting of two molded parts--louver and water distribution tray--the assembly keeps the engine cool while allowing zero water intrusion. Chrysler considered the combination unique enough to be patented.

The louver, which requires extremely high heat performance, is injection molded from AmodelĀ®, a 33% glass-filled polyphalamide from Amoco Polymers, (Alpharetta, GA); the water distribution tray is produced from MinlonĀ®, a polyamide/mineral resin from DuPont Engineering Polymers (Wilmington, DE). The louver nestles into laser-cut openings in the metal hood. The tray design and material cost-effectively disperse water to the outboard side of the hood.

The two-component assembly combines design engineering skill with production expertise for optimum performance. The louver material, processed at 625F, offers high heat resistance without distortion or sag.

Cambridge Industries served as the Tier I supplier for the project primarily because of its performance on the louver for the Dodge Viper. While the Viper component provided only air distribution, the engineering assistance and production quality insured the timely, appropriate response required for the Cherokee project, Pat Muldoon, Project engineer, reports. "Most important," Muldoon continues, "the performance requirements-- including zero water intrusion--were met, with considerable part cost savings."

RE: Hood vents over hot sections

so can someone advise where the best place would be to place hood vents on a vehicle like the Jeep cherokee.? (to get rid of heat?)  Thanks  Ben

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