the photo below shows the parts i have from the stock air cleaner assembly which i removed when i bought the 73. last fall i got the cowl door working simply by instaling a new wiring loon from the sylinoid to the jack in the headlight area. the previous owner had these parts installed on a 750 holly. he cut a small piece out in the rear of the cleaner base to allow it to seat on the 750, i removed this when i bought the car and have using a small chrome air cleaner which sucks up hot engine compartment air. my question is this,,, what purpose is the cowl induction(supposed to get cool air from windshield area) if there are 2 snorkle vents coming out of the base and there sucking the same hot air as i am using the chrome air cleaner??should i block off the 2 snorkle air intakes? i looked in the assembly manual and it does not show any plastic duct work coming from these snorkles to the radiator area where they would also pick up cold air.. seems like its deafeating the pourpose of the cold cowl system if 1/2 of the air its breathing is hot engine area air any imput on this..?? having trouble posting pic i think site is temp down will edit and add photo later-----

 

It is designed to open up the cowl only when you floor it. Otherwise, the two snorkel vents pull in your air. I pulled the flap out of my cowl vent so it works full-time. If you block off the two vents, I doubt the cowl vent alone has enough area to work at full throttle without choiking the engine... Neat system, only surpassed by the L88 hood system...

 

so it will work then, i realize now this wont be as effective, because when the door opens i still will be getting hot air mixed with cooler air. i highly doubt this will make much a difference in preformance

 

Running a aftermarket aircleaner (all air coming from hot engine area - compared to L-88 set-up will make, at least slow speeds, around 10% difference in HP

 

i saw that l-88 piece in the ecklers catalog. i wonder if it will mate up with a stock 73 hood.. it says in the catalog ''not for use with stock air cleaner- so is that the only part i would need? or is there some kinda rubber gasket to take up space/seal it up?? also ,,do you have to remove the cowl flapper door when using the l-88 style, so it can air at idle and low speeds,cuz that door only seems to open when i punch it!

 

I think to efficiently use the cowl induction you need the rubber gasket that fits between the air filter and the hood. This is the gasket that slips over the retaining ring that will then go on the air filter. That way there is a good seal to the hood allowing the cooler air to be forced to the carb.

I agree with you that when the cowl vent is closed, the only air that is coming in is from the dual snorkels. There isnt any type of induction system or tubes feeding air to this like the later C3 styles have.

My 73 has the cowl vent disabled and it is open all the time. I dont know what that does to the performance, but it saves me some money from trying to fix it and hunting down replacement parts. I am not sure what effect it actually has by being open all the time, but so far it does not appear to hurt anything.

kdf

 

do you feel it runs better with the cool air gettin in as oposed to just engine bay air?

 

Gotta remember that the base of the windsheild where the cowl induction is ducted is a high pressure area. The snorkles on the air cleaner inside the engine compartment are at atmospheric pressure. So when the flapper on the cowl induction is open the cool air under pressure is what is flowing into the air cleaner.

Here's some pics from a couple years ago when I installed a Holley 650 cfm Model 4165 with a Weiand Action Plus manifold. The stock air cleaner worked with no mods.

 

I think the got it backwards for performance driving, the snorkels should be closed until needed and the cowl should be always open.

 

That's all very nice theory, but how often is the car going fast enough to create enough 'high pressure air' to actually make a difference? Insiders at GM will tell you the system was there just for wizz-bang marketing reasons.

 

I added a bosch solenoid, SPST switch and LED to the wiring so that I can open the door whenever I want and the LED lets me know when it opens. The control switch is located at the base of the gas pedal. Mine had a piece of rubber tubing wrapped around the control rod. I suspect this was to open the door sooner. I left it on.

 

I'm calling on this one.

Here's a quote from a GM insider for ya, "...cold air to the carburetor is one of the most effective methods of salvaging lost power. And the new hood is highly effective. Duntov reports that compared to a non-ducted hood, it cuts 0-60 mph times by a full second in the standard engine Corvette."
"1973 Corvette Comparison Test" CAR and DRIVER December 1972

 

Cuts a second off 0-60? The car isn't going fast enough (even at 60) to build any pressure. Gimme a break here. My turn to call double

 

One reason for the flapper was that it eliminate the intake system noise when it was closed. Only for WOT did it open and then intake noise was a minor issue. As far as the pressure being forced downed to the carb, put your hand out the window and feel the wind pressure. Just as the wind pushes on your hand it pushes on the windshield and a high percentage of that pressure will push air to the intake carb. While not hundreds of psi it is better than the vaccum pulled through the daul snokels.
Roger

 

OK, if you aren't going to believe Zora, I don't imagine anybody else will change your mind either ... but here goes,

"Higher density inlet air improves the engines volumetric efficiency. Improving density with the use of cool inlet air gets the best HP from your engine.

Underhood temperature is not ideal for HP production because - even on a reasonably cool day - air reaching the carburetor inlet has been warmed by passing through the radiator and over the hot engine components. Underhood temperatures soar to 175°F or higher when the engine is turned off and the car is standing in the sun. An engine ingesting warm air will be down on power by more than you might imagine: Assume that the outside temperature is 70°F and the underhood temperature is 150°F. If the engine produces 300HP with 150°F air-inlet temperature, it can be expected to produce 322 HP with 70°F air-inlet temperature. So, the density increase afforded by using outside air is considerable.

Cold air gives more improvement than ram air because approximately 1% HP increase is gained for every 11°F drop in temperature.

...You'll be better off ducting cold air from the cowl just ahead of the windshield. This is a high-pressure area which will ensure a supply of cool outside air to the carburetor."
How to Hotrod Small-block Chevys by Bill Fisher and Bob Waar

 

Now for some real world stuff girls. Everybody run out to their 73-75 cars and look at the plenum at the base of the windshield. Is it sealed in any way so that the 'high pressure air' is forced solely into the engine inlet ducting? No, it's wide open at both ends. Where does the air go? Into the Astro Ventilation system. Is there high pressure air coming out of these said vents? No. One of the biggest failings of a non-AC shark. The inlet for the engine is at the 'top' of the plenum- where the pressure (if any) is lowest. It's even higher than the top of the plenum if you include the hood bulge.

Go back out to your car and drive around at 60 with your hand outside the window- not perpendicular to the airflow, but raked at the same angle as the windshield. Where's all the pressure now? Pushing your hand down while the air rushes upwards and over. Very little air is trying to 'stagnate' at the base of your hand to build static pressure.

Yes it's better than hot underhood air, but some outrageous expectations and claims are running around here

I've tried all sorts of tests on my '73 with the flapper open and closed and can state that there is no difference. I now call triple BS on the 1 second saving on the zero to 60 times.

 

I'd be interested in hearing more details of these "tests". And holding your hand outside the window at various angles doesn't qualify as a test either.

And I suppose that the L-88 hood is just more of GM's whiz-bang gadgets?

 

Yup, I'll buy those numbers. Now let's apply it to the actual car in question. (A 1973- not a 69 L-88.) Lets use the L-82 version for example. 250 HP, net. The article you quoted says 1% for every 11 degrees, and shows a temp delta of 80 degrees, so thats 8% (rounded up) of 250HP or 20 HP.
So, 20 extra HP applied to a 3400 lb car will drop 1 sec off the 0-60 times? I don't think so.

This 20 extra Hp assumes that the engine is breathing 100% cool outside air. The configuration of a '73-75 does not provide any means for the 'hot air' snorkels to be closed off to assure this- they're always open. If the car is static (or at realistic highway speeds) there is not enough ram air from the hood ducting to feed the engine at anywhere near 100%. Don't forget that the air rushing under the hood from the forward motion fan is ALSO being pushed into the hot air snorkels. Lets also add in the fact that the hood ducting tends to be above ambient temp from radiant heat, so that will heat the intake charge as well.

I would bet that the system is no more than 30% efficient at best, so the calculated 20 Hp is down to 7. Big deal. I can get more by bumping my timing to optimum once in a while.

As to what testing I've done. I've (inadvertantly) driven the car with the duct flapper working when I thought it was not and vice versa. This was on occasion when I was with other Corvettes engaging in 'spirited driving' on road trips. Having the duct working made no difference in performance one way or the other.

My car is far from stock, having been dynoed at 375 horse. I should be getting 30 HP extra at 100% efficiency, or 10HP at assumed 30% efficiency. I don't think many people can feel that small a difference.

If somebody wants to do extensive BLIND TESTING on a drag strip to prove me wrong, that's fine with me.