Hood vents

MOTORHEAD · #1 (**permalink**) 08-11-2013, 07:33 AM

I have seen louvers/vents on some cobras, both replicas and original, in the aft 1/3 of the hood on either side of the hood scoop.
Thinking of adding them to my car to get rid of some of the underhood hot air.
Wondering if anyone here has these on their car, and if so, what is your assesment of their functionality ?

CHANMADD · #2 (**permalink**) 08-11-2013, 08:02 AM

Aesthetic.......

bobcowan · #3 (**permalink**) 08-11-2013, 10:55 AM

If you have a windshield, the area in front of the windshield is a high pressure zone. Any vents or holes are purely asthetic.

I have driven my car at triple digit speeds with the hood unlatched. The hood did not lift. That tells me there is no pressure under there. No point in putting any functional vents there.

OTOH, since that is a high pressure area, it would be a good spot for an air intake. Like a '65 Mustang.

rodneym · #4 (**permalink**) 08-11-2013, 11:34 AM

Motorhead,

I assume you mean sitting heat. You shouldn't have any issue on the hwy.
I too want to get rid of some heat, too. Sitting in traffic is a killer.

Morris put vents on his KMP racer. Maybe we should ask him.

MOTORHEAD · #5 (**permalink**) 08-11-2013, 12:07 PM

Saw in a recent thread someone had posted a pic of his car with vents in hood. Car was dark blue. Can't remember name of thread, and couldn't find it in search. But "Dallas" has them, and I forgot about Morris. I read that he and Tom Kirkham had done some airflow testing with yarn tufts. They would be the "authority" on the subject. That said, just getting rid of some heat while sitting at stops would be a good thing. I know that the escaping heat comes in over the w/s, and drops in on you ! Florida in the summer is a be-ich !!!!

ps: I always pop the hood after a run to help cool it down, + put a box fan in front of the radiator if at my garage, and leave the cars fan running till the temp stabilizes .

427 sam · #6 (**permalink**) 08-11-2013, 01:13 PM

hi i have vents on the bonnet and find effective in removing hot air.also i have nose vents to force more air through the engine bay. regards Sam

fordracing65 · #7 (**permalink**) 08-11-2013, 01:14 PM

Quote:

Originally Posted by MOTORHEAD

Saw in a recent thread someone had posted a pic of his car with vents in hood. Car was dark blue. Can't remember name of thread, and couldn't find it in search. But "Dallas" has them, and I forgot about Morris. I read that he and Tom Kirkham had done some airflow testing with yarn tufts. They would be the "authority" on the subject. That said, just getting rid of some heat while sitting at stops would be a good thing. I know that the escaping heat comes in over the w/s, and drops in on you ! Florida in the summer is a be-ich !!!!

ps: I always pop the hood after a run to help cool it down, + put a box fan in front of the radiator if at my garage, and leave the cars fan running till the temp stabilizes .

You think Florida is hot, come to Arizona,

I'll say one thing, I think they look cool on the car, very racey looking...ps I saw a car on here with them, blue car no windshield, looked cool...

LMH · #8 (**permalink**) 08-11-2013, 06:18 PM

CSX3243, one of the two original cars used in the movie "The Gumball Rally" has hood vents. Might give you some ideas.
Larry

Focus-SVT · #9 (**permalink**) 08-11-2013, 08:42 PM

Hey 427 Sam, I really like your chin spoiler! Can you share any info on it?

427 sam · #10 (**permalink**) 08-11-2013, 11:11 PM

Quote:

Originally Posted by Focus-SVT

Hey 427 Sam, I really like your chin spoiler! Can you share any info on it?

Hi, aftermarket product supplied by DB replicas DB Replicas Cobra Build Agent £ 60,00 uk, the picture taken just before i glassed into place. not sure what shipping to the USA would cost, regards Sam

fordracing65 · #11 (**permalink**) 08-11-2013, 11:41 PM

Quote:

Originally Posted by LMH

CSX3243, one of the two original cars used in the movie "The Gumball Rally" has hood vents. Might give you some ideas.
Larry

I want hood vents now, sooooo coooool...

Ron61 · #12 (**permalink**) 08-12-2013, 03:04 AM

Here is a link to Morris and his Cobra build. It is several pages long but he explains everything with pictures included and a page or so past this he has the pictures and explanation of the testing with the tufts of yarn. Morris is great at answering any questions so don't be afraid to ask him something if you want to know.

KMP 259 Phase 2

Ron

strictlypersonl · #13 (**permalink**) 08-12-2013, 04:49 AM

From the pressure diagrams that I've seen in various automotive aerodynamic books, the lowest pressure on the hood is only a few feet back from the front edge of the intake. From there back to the base of the windshield, the pressure increases. You can see the application of that phenomenon on the GT40 and the Cobra Coupe, where the vents are put only a short distance behind the radiator. I mention this because, on the CSX shown above, the vents are likely placed in a non-optimum location. If you want to maximize the pressure effects, put the vents about mid-way (or forward) on the hood.

MOTORHEAD · #14 (**permalink**) 08-12-2013, 06:07 AM

Thanks to all for your input and pics !
Ron: thanks for the link to Morris' thread on the Cobra Registry. Strangely enough, Morris keeps refering to the area at the base of the windshield as "low pressure". That can't be right !?
Fordracing65: "It's not the heat, it's the humidity !"

dallas_ · #15 (**permalink**) 08-12-2013, 06:35 AM

Motorhead, I added them to our hood to try to help minimize the heat sink on the Webers. I have no before and after temperature readings so I can't say for certain that they help reduce under hood temperatures, but I have never had the heat sink issues on our carbs that others have reported.

strictlypersonl · #16 (**permalink**) 08-12-2013, 08:16 AM

Quote:

Originally Posted by MOTORHEAD

Thanks to all for your input and pics !
Ron: thanks for the link to Morris' thread on the Cobra Registry. Strangely enough, Morris keeps refering to the area at the base of the windshield as "low pressure". That can't be right !?

You are correct. He isn't.

rodneym · #17 (**permalink**) 08-12-2013, 08:43 AM

Motorhead,

Do you want cooler temps while cruising or sitting?
The vents that Dallas has will definately let some heat out in traffic.

I'm no aerodynamacist, but I play one on the net

We know that with speed, the car wants to lift due to low pressure over the car. When air is in motion (us, really), the air pressure is less, with a couple of hot spots of pressure, like the front and windshield. Add to that the big gaping mouth and it would seem to me that pressure would build in the engine compartment (maybe not enough to lift a heavy glass hood), hence Morris' hood vents and the idea of the turkey pan (separating engine compartment pressure with the outside, trying to 'force' air in with the scoop at speeds). Don't we know that the hood scoop with no turkey pan isn't really helping?

We have Magnahelic gauges at work. I'll try and find one with +/- and have my wife hold it, with the tube sitting in the engine room.

Morris · #18 (**permalink**) 08-12-2013, 01:35 PM

Quote:

Originally Posted by strictlypersonl

You are correct. He isn't.

I get a kick out of folks trying to figure out what we have done...... and all my National Championship cars have been fun as folks are always trying to figure out what is going on and why the cars were so fast. A few comments I have on this subject since I'm trying to pass on the knowledge is as follows....

To compare any Cobra to KMP259 is like comparing Apples and oranges .... they are not even close ....

Your drawing is of a GT car which we built many and a Cobra is not even close on that drawing ....

6" windshield ..... 4" tall reversed hood scoop.... air vents in the hood .... complete flat bottom.... splitter ..... rear wing.....suspension mods.

My point is each car is on it's own merit and needs..... depending on how you anticipate to use the car is how you prepare it.... KMP259 was prepared to run on a race track and is very difficult to drive on the street as the motor is unruly and Sunoco is quite expensive at $10.00/gal.

Some of the things you are saying are right ...but it depends on what speed you are running.... some of the thing are just not right and should not be judged unless you know what you are talking about....

Many folks don't take the time to think out what is going on when one change is made and the effect is has on the other parts.

All of my references are to KMP259... and without a big windshield all of your theory's go out the window.....

Infact the above drawing really doesn't apply to a cobra except for the hood and on a Cobra you have a lot more lift under the car as that is where the air goes and lifts the car as it has no other way to escape the engine compartment unless you have the wheel well open for it to get out.... this creates a higher pressure under the hood and doesn't allow air to flow thru the radiators like it should.

As soon as you pack the front opening with air and it fill the engine compartment and the only way for it to get out is the wheel wells ....as you go faster you are putting more air in under the hood then can escape...... and so you start lifting the car as the only other place for the air to get out is the bottom of the car..... that's why many Cobras get lite at 150mph.....
There are ways to create a lower pressure under the engine area so you get air flow thru the radiator but there is only so much air you can get out of the car as the speed goes up.

When you get up in the 160mph plus range you have to control the air just like a suspension and get it to balance front to rear so the car tracks and brakes down a straight away under control and with a solid stance.

Those tricks are proprietary to each racer and only a few share their knowledge with others.

I guess to say I'm wrong is fine with me ..... but until someone can beat us.... then I'm not changing anything and know right now it works at very high speeds ..... and would be happy to compare lap time anytime.

strictlypersonl · #19 (**permalink**) 08-12-2013, 02:23 PM

Morris,
You are right that a lack of a windshield changes things radically, but I don't think that that situation was mentioned by the O.P. I suspect that my comments are still quite applicable.

Speaking of hoods rising, when we raced a Healy (without a full windshield) back in the 70s, we replaced the original rear hood latch with some loose belts at each rear corner. At speed, the hood came up about an inch because of the inside/outside pressure differential.

RET_COP · #20 (**permalink**) 08-12-2013, 04:28 PM

Ok, so I just ran to the garage and looked under my hood. From this info I would benefit the most by removing most of my fiberglass inner fender. I can see where this would allow the escape of air pressure under the hood.
The pressure starts with the air riding up over the radiator and into the engine compartment. Since there is high pressure above the hood, vents at this location would not allow the escape of air...but the side of the car does not have the buildup of pressure so the escape of air out the wheel well would work.
Am I right?
Now this does not matter too much below 130MPH. am I right again?
So I can keep my inner fenders for now, but it is nice to know a fix for a fast car float.

My radiator is laid back several degrees which promotes the air flow over and not through.

For those of you that have had the car at high speeds, at what speed does things get dicey.