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Why was there a cowl hood without, and cowl hood with flapper offered?

20K views 58 replies 29 participants last post by  Stokerboats 
#1 ·
Just never thought too much about it, but what's the story with having two styles available from GM? Cost? Cheaper option to upgrade a Malibu? Why would GM offer an SS with two available hoods? What years did they offer the non functional domed hood?
I have the GM stock domed/cowl hood no flapper, and choose to run a 4" cowl Fiberglas. I like the look of the 4" myself. I keep the GM steel one safe and dry. I don't know the history of which cars got the dome and which got a dome and flapper. Why would there be two styles?:confused:
 
#5 ·
The domed hoods were ONLY on 70-72 Chevelles.
Just the PLAIN, DOMED hood was part of the STANDARD SS package, PLUS it was also on the Heavy Chevy option. It was a NON-functional hood, just to add to the SS appearance.
The Cowl Induction hood was an option, as mentioned, RPO ZL2. And was ONLY available on a big block car.
In 1970, ALLLLLLLLLLLLLLLLLLLL SS cars came with a big block only (396 or 454), PERIOD.
In 71-2, an SS was available with a small block (350)------------------------COWL INDUCTION HOODS WERE NOT AVAILABLE ON A SB CAR!
ALLLLLLLLLLLLLLLLLLL Cowl Induction hoods also came with hood pins. The CI hood is MUCH MORE than just a domed hood with a flapper valve. The complete pkg requires many additional parts such as a special air cleaner with a rubber seal to mate to the hood duct, a wiring harness, relay, switch on the gas pedal, etc.

The Cowl Induction hood was more or less an additional performance feature and SUPPOSEDLY, under wide open, full throttle conditons, it MAY have added 10hp (there is no factual documentation to support this).
Sooooooooooo, bottom line, a Cowl Induction hood was basically eye candy. For me, personally, I LOVE the CI hood and have added the complete system to my 70 Malibu conv.

This is our 70 Malibu conv with the CI hood added, and it is 100% fully functional.



https://www.youtube.com/watch?v=Xra1mG1yE8Y
 
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#8 ·
Im thinking its because they could turn one hood design into two options. The standard style hood and the optional hood with a functional cowl, and to allow those that didnt want to pay extra for the cowl induction on thier SS car but still wanted the SS and thus got the standard domed hood.
 
#10 ·
Here’s my understanding of it.
When other manufactures were adding hood scoops and or some type of ram air systems a Chevrolet engineer, I forgot his name, developed the concept of cowl induction. His theory was to use the high pressure air that builds up in front of the windshield or cowl area and allow it to route cooler air into the engine compartment. The cowl induction on the 70-72 Chevelle’s with the flapper door, and that door really is cool, took things a step further and routed the cooler air directly into the air cleaner.
The idea and use dates back to some early Chevelle and Impala designs where the air cleaner was ducted directly to the cowl area.
 
#12 ·
Here’s my understanding of it.
When other manufactures were adding hood scoops and or some type of ram air systems a Chevrolet engineer, I forgot his name, developed the concept of cowl induction. His theory was to use the high pressure air that builds up in front of the windshield or cowl area and allow it to route cooler air into the engine compartment. The cowl induction on the 70-72 Chevelle’s with the flapper door, and that door really is cool, took things a step further and routed the cooler air directly into the air cleaner.
The idea and use dates back to some early Chevelle and Impala designs where the air cleaner was ducted directly to the cowl area.
Oh, it goes WAAAAAAAAAAY further back than that. The big ole Pratt & Whitney engines on our A26 have almost exactly the same air intake system (functionally at least) as the Cowl Induction hood for 70-72 Chevelles-------------------INCLUDING a flapper valve!!!!!!

all the other GM lines offered a fresh air induction system...this was Chevys offering.
And that is exactly correct. Buick had a hood with a dual intake that fed directly into a dual snorkle air cleaner, Pont and Olds had FUNCTIONAL Ram Air hoods. Personally, I love the dual scoops on the Olds W30 hoods. Anyone remember the dual air intakes mounted UNDER the bumper of the mid-60s 442s?








 
#14 ·
The idea (at Chevrolet ) of the carb getting cooler air from the low pressure area at the base of the windshield goes back to the road race Corvettes of the late 50's.I think they were called the air box cars.
Also the 67-69 Z/28 had a Cowl Induction hood option.Same principle different method of delivery.
I can't remember the name of it but there's also a dealer installed air cleaner assembly that gets sealed to the cowl under the hood that was basically a road race piece....
http://www.bing.com/images/search?q...E4DB86B994E57493F0B2C767F2C5&selectedIndex=33
IIRC there was some kind of controversy if this was ever offered on 66 or 67 Chevelles.
So the CI hood of the 70-72 Super Sports had more to it than just a money making idea but if Chevy (back then) could provide extra performance and make money doing it,then more power to them. JMO
 
#15 ·
Interesting thread. If I remember correctly, I read a while ago that chevelles cowl induction was the best hood design (offered by GM) in introduce cooler air into the air cleaner. The base of the windshield was found to be the best place for it. The ram air on the 442's was also a good option but lost volume as speed increased due to the front of the car deflecting air upwards. The Buick GS and the GTO was least likely to "grab" cooler air at speed due to air deflecting off the front of the car and not skimming along the top of the hood. They actually mentioned, that at high speed, there was diminished air at the center of the hood (GS/GTO) with most air hitting the windshield.

I personally always like the the chevelles CI hood. And my second choice would be the 442 hood.
 
#17 ·
I heard the only "fresh air" setups that worked...were the under the bumper Olds set ups and the '70-'75 Firebird Formula hood set ups(when functional). I like to watch the flapper on my T/A open and close.... I really don't think ti does anything though...
 
#19 ·
The only thing cooler than a Chevelle with a functional Cowl Induction hood---------------------is two of them! Even if it does cost more.
By the time I ordered the new CI hood and ALLLLLLLLLLLL the related parts and had it painted to match, I had about $1400 invested in it---------------and that was back in 98. I have ZERO regrets today!

 
#24 ·
I have a functional CI hood with pins that was added by the previous owner, and I love it. Even when the flapper is closed when the manifold vacuum is holding down the flapper, is it still getting fresh air from the domed opening at the windshield? The domed hood without a flapper should be able to get fresh air to the underhood also ... right? If I shine a flash light from my drivers seat into the cowl area (flapper closed), I can see the top of my air cleaner and open element filter ... so I should be getting fresh air to the carb even when the flapper is closed?
 
#25 ·
I always thought the Air Grabber on the 1970 Roadrunner was super cool...
 
#26 ·
I also liked the way ford removed the inner headlights on the T-bolts ducting it into the carb on their drag cars. I've thought of doing this on my Camino but haven't done it yet, but I think of it from time to time. Like most have said the car would really have to be moving to get any effects from it.
 
#41 ·
Me too





 
#27 ·
Look at the scoop on the underside of a P51D Mustang and it looks an awful lot like that of a pro stock race car. They obviously understood all about the boundary layer effect and aerodynamics back in the 1940s. A lot of what we think of as new, really isn't at all. Nitrous oxide was used in fighter plane engines too for a burst of power when needed.
 
#30 ·
The openings on the 4-4-2 hoods are too far back from the front edge to be effective. They look cool, but the opening is in a low-pressure area and may actually reduce HP with higher speeds.

The Chevelle cowl-induction opening is in a high-pressure area, and helps push more air into the carburetor as higher speeds create higher pressure.
 
#32 ·
Because the air pressure is higher closer to the windshield.

Air pressure is more important than the direction of air flow. Air velocity is lower in a high-pressure area, and higher velocity in low pressure area. The air at the base of the windshield is fairly stagnant compared to air at the front of the hood, or over the roof.
 
#33 ·
trying to understand the pressure...

Wouldn't that flapper opening create a vacuum over the area behind it?
With the carb also sucking... makes me wonder.

Doesn't the flapper change the direction of the air? It seems like it would make the air flow over the windshield easier rather than flow into the hood.
 
#34 ·
Tom, I think you've got one classy looking Malibu and I like the CI, no stripe look, great color too. I understand all of the working components on a CI system, I have 3 cars with CI now. So I'm not sure you understood my question, so I'll try again:

If the 69 Camoro with CI has the flapper under the edge of the hood per your description, wouldn't the SS hood on a Chevelle be providing fresh air continually ( just without the flap of the 69 Camaro)?

I'm sure your familiar with the shape of a wing, here a clip for others to check out:
http://www.grc.nasa.gov/WWW/k-12/UEET/StudentSite/dynamicsofflight.html

Now if a Chevelle SS with CI is moving forward (imagine a side view of the hood of the car) with air flowing over it, as it hits the dome it creates turbulence at the cowl base as does the leading edge of a wing. This would produce a low pressure area. And to Shane's question the flapper opened would create even more of a low pressure area. I see his point.
If what you are describing is accurate, the Cowl Induction is reverse Ram Air. As the higher pressure is forced in thru the flapper into the induction area of intake.

Does anyone have an engineering drawing of the principal of CI?
 
#35 ·
#36 ·
From and engineering perspective, the non-functional hood was a cost reduced appearance option by defeaturing the CI system. This was a great fit for the Heavy Chevy (poor man's SS?). Sales folks like lots of options to sell the customers, with the nicer/cooler ones adding to the price.
 
#38 ·
Ron O,
The back side of the CI hood is open and there is an electric solenoid operated valve that opens when the gas pedal is floored (there is a switch on the gas pedal). Thus, the high pressure air at the base of the windshield is instantly drawn into the hood ducting. The OUTSIDE flapper door is more or less just eye candy-------------------the true functional door is the hidden valve UNDER the rear of the CI hood.

The back side of the NON-CI SS hood is blocked off. Thus, not only does it prevent air from being taken in from the rear of the hood----------------BUT THE NON-CI AIR CLEANERS ARE THE FULL ENCLOSURE STYLE. THEREFORE, EVEN IF AIR COULD COME IN FROM THE REAR OF A NON-CI SS HOOD, IT CANNOT BE DIRECTED INTO THE AIR CLEANER! The full enclosure style of air cleaners can ONLY take air in through a snorkle which faces FORWARD.

It's quite difficult to explain if you're not totally familiar with the two hoods. You need to observe a CORRECT non-CI SS hood/air cleaner and a CORRECT CI hood/air cleaner so that you can compare the differences, which will make it completely clear to you.
 
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