I mean on like old cars, a regular hood scoop let air flow into the carb. and even on cars that have fuel injection, it still helps keep the innards cool(not really, but yeah.) But cowl induction, the vent is blasting on the windshield, the only thing I can think of is if you have a big butt engine, and you need the room, but something about the word induction makes me think otherwise......
Surely you guys know! :-)
Thanks.

 

It alll comes down to aerodynamics, Tom.

Although the opening faces to the rear, this design actually forces air into the area surrounding the airfilter & carb as a result of the aerodynamic pressure created as the vehicle moves at speed. It is not a vent- it is forced air induction.

 

Ya see, Tom, if you go to college, you can learn these kinds of things, and the business and liberal arts guys, well, they can read it out of car magazines. :D :D

Like Macster said, cowl induction takes advantage of something that is otherwise all bad. The windshield deflects air flow coming over the hood up and over the top of the car. This causes aerodynamic drag, but fluid flow being what it is, it also creates a region of high static pressure at the base of the windshield, where the relative flow velocity is essentially reduced to zero. This converts all of the dynamic pressure or energy in the flowing air into increased static pressure. Since this increased static pressure is higher than the static pressure under the hood, the air from the base of the windshield can flow forcibly under the hood to the carburetor venturis if a properly designed channel is available.

Cowl induction is superior to hood scoops. Hood scoops are for "show", cowl induction is for "go". Even if you "ram" the air through the limited open area of the hood scoop, you still have to have some kind of efficient plenum device that will declerate the flow and convert the dynamic pressure into static pressure which is the only thing you can really use anyway. Cowl induction is a way of taking something negative (parasitic drag) and turning it into a positive (cool, high pressure air at the carburetor) with no additional expenditure of energy. :smash:


[Modified by Chuck Sangerhausen, 12:55 PM 8/7/2002]

 

Easy for you to say....... :p:

 

Yeah...what he said!

 

This is EXACTLY the same system used by the NASCAR cars. In 70-72, a Cowl Induction system (NOT just a hood) was an option on Chevelles with a BB option, as well as 69 Camaros with some of the optional engines. And, of course, it was standard on mid-70s Vettes.

But the trickest (is that a word?) of all the CI hoods was the 70-2 Chevelles because it had an outer flapper door which opened (when functioning properly) under heavy acceleration, plus they had a chrome COWL INDUCTION emblem on each side. I'm not aware of any other car which had a functional OUTER door. I have a 70 Chevelle with a CI hood and I love it.

 

----------> <-----------

 

Oh, ok, I gotcha, even without a windsheild there, it would still work pretty damn effective, my friends 87 Toyota Truck has part of the rear window gone and let me tell you, plenty of air flows in, going forwards, I guess in a way, that is the same principle. Cowl Induction owns.
So do you guys. :-)

 

Believe it or not, even an '81-82 Citation X-11 had a fibreglas cowl induction hood

 

The 73 Corvette had a subtler way of getting the same effect. The flapper door wasn't exposed, but was hidden under the cowl induction hood, so the majority of the time, when the loud pedal wasn't mashed to the floor, the cowl induction operated normally. When things got hot and heavy, the flapper door opened for the extra blast of cool air directly to the air filter. Here's a picture (photo from The 73 Spot) !!

 

Nope.

Tom, my boy, yer not payin' attention. I can see my long dissertation on how cowl induction works went right over your head like the air flow over a windshield. Cowl induction works BECAUSE the windshield is there...no windshield, no decleration of the air flow, no static pressure increase. :nonod:

The back window of your friend's Toyota truck is experiencing a similar aerodynamic effect, but in reverse...the area behind the cab of your friend's truck creates a LOW PRESSURE area which draws air out of the cab in an attempt to fill the void left by the moving truck. The air inside the passenger cab is replenished through intentional ventilation openings, the air conditioning system, leaks around the windows, through the firewall, etc.


[Modified by Chuck Sangerhausen, 10:44 AM 8/8/2002]

 

:lol: :lol: :lol:

 

Well, yeah I get it
Laymans terms:
Air bounces off windshield, into cowl.

 

Chuck,

Why don't you explain it in aerodynamic terms: it's not the lift of the air UNDER the wings that makes an aircraft fly, it's the low pressure of the air going OVER the wings that literally 'sucks' the aircraft into the sky...

 

If that's true....:whoa.

 

It's true, so :whoa!! To quote Morphius, "Do you think that's air you're breathing?"

 

The funtion of the cowl induction hood is also related to lower the carb inlet air temperature to get free HP. The idea is that cooler outside air goes in the carb instead of the higher temp under hood air. Depending on ambient air temp, the cooler air is supposed to ad a few HP compared to a non cowl induction equipped car.

 

Your example is, of course, correct, but I thought I had been using aerodynamic terms all along. :D

Airplanes fly because of Bernoulli's principle; i.e. when a fluid is accelerated, like over the convex surface of an airfoil, the static pressure is decreased in that area. Perhaps it is the higher ambient pressure below the wing that pushes the wing "UP" into the low pressure area created by the accelerated flow over the upper surface.

Consider a soda straw: you create a small "vacuum" that causes the drink to flow up the straw into your mouth...are you "sucking" the drink up the straw? No, the ambient air pressure acting on the surface of the fluid in the cup is forcing the drink up the straw. If you don't believe me, arrange a simple experiment using one of those large plastic cups with the tight sealing lids and flip openings...if one of those vents aren't open to let atmosphereic pressure in, you will play heck sucking ANYTHING up that straw. Not aerodynamics, just basic science.

In this case, the static pressure under the hood is about the same as ambient conditions...nothing has been done under the hood to cause the static pressure to be lowered below ambient. Cooler air is not being "sucked" under the hood because the pressure is lower than ambient; if the carburetor draw was enough to create this effect, cowl induction wouldn't be necessary.

The air flow is being pressured under the hood because the outside static pressure is ABOVE ambient. The dynamic term of the total pressure equation, rho V squared/2, has been converted into a static pressure increase that makes the static pressure at the base of the windshield HIGHER than the ambient pressure under the hood. To say the flow is "bouncing off" the windshield into the carburetor is a simplistic analysis. :D

 

All of this theory is 100% on the mark, but in practice (at least on mid-70's Corvettes) it's all show but no go-
1) The flapper valve is only open at WOT- how often does someone do that?
2) How often does some one drive fast enough to actually create positive pressure at the base of the windshield? And at WOT?
3) The crossectional area of the duct couldn't feed a Chevette, never mind a Corvette
4) The plenum at the base of the windshield that the system feeds from is also vented to the interior of the car as part of the Ashore Ventilation system. Any pressure built in the plenum is bled into the cabin of the car.
5) The air cleaner outer housing has two large diameter snorkels that open directly into the engine compartment. Any pressure built in the duct would be relieved by these snorkels.

Talk about building a self defeating system.

One day, I'll block off the Ashore vents, remove the flapper valve, install one way doors on the snorkel and go for a midnight WOT drive on Route 30 between Ste. Julie and Sorel. I'll borrow a few pressure transducers and a data recorder from work and analyse the delta P trace on the way to jail. :smash:

 

Mikey —

Good Post.

That's why the flapper door on my rubber-nose sharks was always wired into a vacuum switch so that the flapper was only closed at idle. And I painted the flapper white so that I could see it open at off-idle.

But, I don't recollect those cars having the snorkels. Rather, I seem to 'member a rubber seal against the underside of the hood. Similar to an L-88 hood, but not so deep.

And, I don't think that the plenum was that limiting on airflow. The SB usually had a 600 CFM carb, so the max airflow in the scoop would be about 40 or 50 ft/sec. Doesn't seem so limiting to me.

Or, are you just trying to yank Chuckle's chain??

Harruuumph — Signiture is goofed up; must re-edit.

Geezer


[Modified by OlGeezer, 1:51 PM 8/23/2002]